xref: /third_party/ltp/doc/c-test-api.txt

LTP C Test API
==============

NOTE: See also
      https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines[Test Writing Guidelines],
      https://github.com/linux-test-project/ltp/wiki/C-Test-Case-Tutorial[C Test Case Tutorial],
      https://github.com/linux-test-project/ltp/wiki/Shell-Test-API[Shell Test API].

1 Writing a test in C
---------------------

1.1 Basic test structure
~~~~~~~~~~~~~~~~~~~~~~~~

Let's start with an example, following code is a simple test for a 'getenv()'.

[source,c]
-------------------------------------------------------------------------------
/*
 * This is test for basic functionality of getenv().
 *
 *  - create an env variable and verify that getenv() can get get it
 *  - call getenv() with nonexisting variable name, check that it returns NULL
 */

#include "tst_test.h"

#define ENV1 "LTP_TEST_ENV"
#define ENV2 "LTP_TEST_THIS_DOES_NOT_EXIST"
#define ENV_VAL "val"

static void setup(void)
{
	if (setenv(ENV1, ENV_VAL, 1))
		tst_brk(TBROK | TERRNO, "setenv() failed");
}

static void test(void)
{
	char *ret;

	ret = getenv(ENV1);

	if (!ret) {
		tst_res(TFAIL, "getenv(" ENV1 ") = NULL");
		goto next;
	}

	if (!strcmp(ret, ENV_VAL)) {
		tst_res(TPASS, "getenv(" ENV1 ") = '"ENV_VAL "'");
	} else {
		tst_res(TFAIL, "getenv(" ENV1 ") = '%s', expected '"
		               ENV_VAL "'", ret);
	}

next:
	ret = getenv(ENV2);

	if (ret)
		tst_res(TFAIL, "getenv(" ENV2 ") = '%s'", ret);
	else
		tst_res(TPASS, "getenv(" ENV2 ") = NULL");
}

static struct tst_test test = {
	.test_all = test,
	.setup = setup,
};
-------------------------------------------------------------------------------

Each test includes the 'tst_test.h' header and must define the 'struct
tst_test test' structure.

The overall test initialization is done in the 'setup()' function.

The overall cleanup is done in a 'cleanup()' function. Here 'cleanup()' is
omitted as the test does not have anything to clean up. If cleanup is set in
the test structure it's called on test exit just before the test library
cleanup. That especially means that cleanup can be called at any point in a
test execution. For example even when a test setup step has failed, therefore
the 'cleanup()' function must be able to cope with unfinished initialization,
and so on.

The test itself is done in the 'test()' function. The test function must work
fine if called in a loop.

There are two types of a test function pointers in the test structure. The
first one is a '.test_all' pointer that is used when test is implemented as a
single function. Then there is a '.test' function along with the number of
tests '.tcnt' that allows for more detailed result reporting. If the '.test'
pointer is set the function is called '.tcnt' times with an integer parameter
in range of [0, '.tcnt' - 1].

IMPORTANT: Only one of '.test' and '.test_all' can be set at a time.

Each test has a default timeout set to 300s. The default timeout can be
overridden by setting '.timeout' in the test structure or by calling
'tst_set_timeout()' in the test 'setup()'. There are a few testcases whose run
time may vary arbitrarily, for these timeout can be disabled by setting it to
-1.

Test can find out how much time (in seconds) is remaining to timeout,
by calling 'tst_timeout_remaining()'.

LAPI headers
++++++++++++

Use our LAPI headers ('include "lapi/foo.h"') to keep compatibility with old
distributions. LAPI header should always include original header. Older linux
headers were problematic, therefore we preferred to use libc headers. There are
still some bugs when combining certain glibc headers with linux headers, see
https://sourceware.org/glibc/wiki/Synchronizing_Headers.

A word about the cleanup() callback
+++++++++++++++++++++++++++++++++++

There are a few rules that needs to be followed in order to write correct
cleanup() callback.

1. Free only resources that were initialized. Keep in mind that callback can
   be executed at any point in the test run.

2. Make sure to free resources in the reverse order they were
   initialized. (Some of the steps may not depend on others and everything
   will work if there were swapped but let's keep it in order.)

The first rule may seem complicated at first however, on the contrary, it's
quite easy. All you have to do is to keep track of what was already
initialized. For example file descriptors needs to be closed only if they were
assigned a valid file descriptor. For most of the things you need to create
extra flag that is set right after successful initialization though. Consider,
for example, test setup below.

We also prefer cleaning up resources that would otherwise be released on the
program exit. There are two main reasons for this decision. Resources such as
file descriptors and mmaped memory could block umounting a block device in
cases where the test library has mounted a filesystem for the test temporary
directory. Not freeing allocated memory would upset static analysis and tools
such as valgrind and produce false-positives when checking for leaks in the
libc and other low level libraries.

[source,c]
-------------------------------------------------------------------------------
static int fd0, fd1, mount_flag;

#define MNTPOINT "mntpoint"
#define FILE1 "mntpoint/file1"
#define FILE2 "mntpoint/file2"

static void setup(void)
{
	SAFE_MKDIR(MNTPOINT, 0777);
	SAFE_MKFS(tst_device->dev, tst_device->fs_type, NULL, NULL);
	SAFE_MOUNT(tst_device->dev, MNTPOINT, tst_device->fs_type, 0, 0);
	mount_flag = 1;

	fd0 = SAFE_OPEN(cleanup, FILE1, O_CREAT | O_RDWR, 0666);
	fd1 = SAFE_OPEN(cleanup, FILE2, O_CREAT | O_RDWR, 0666);
}
-------------------------------------------------------------------------------

In this case the 'cleanup()' function may be invoked when any of the 'SAFE_*'
macros has failed and therefore must be able to work with unfinished
initialization as well. Since global variables are initialized to zero we can
just check that fd > 0 before we attempt to close it. The mount function
requires extra flag to be set after device was successfully mounted.

[source,c]
-------------------------------------------------------------------------------
static void cleanup(void)
{
	if (fd1 > 0)
		SAFE_CLOSE(fd1);

	if (fd0 > 0)
		SAFE_CLOSE(fd0);

	if (mount_flag && tst_umouont(MNTPOINT))
		tst_res(TWARN | TERRNO, "umount(%s)", MNTPOINT);
}
-------------------------------------------------------------------------------

IMPORTANT: 'SAFE_MACROS()' used in cleanup *do not* exit the test. Failure
           only produces a warning and the 'cleanup()' carries on. This is
	   intentional as we want to execute as much 'cleanup()' as possible.

WARNING: Calling tst_brk() in test 'cleanup()' does not exit the test as well
         and 'TBROK' is converted to 'TWARN'.

NOTE: Creation and removal of the test temporary directory is handled in
      the test library and the directory is removed recursively. Therefore
      we do not have to remove files and directories in the test cleanup.

1.2 Basic test interface
~~~~~~~~~~~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
void tst_res(int ttype, char *arg_fmt, ...);
-------------------------------------------------------------------------------

Printf-like function to report test result, it's mostly used with ttype:

|==============================
| 'TPASS' | Test has passed.
| 'TFAIL' | Test has failed.
| 'TINFO' | General message.
| 'TWARN' | Something went wrong but we decided to continue. Mostly used in cleanup functions.
|==============================

The 'ttype' can be combined bitwise with 'TERRNO' or 'TTERRNO' to print
'errno', 'TST_ERR' respectively.

[source,c]
-------------------------------------------------------------------------------
void tst_brk(int ttype, char *arg_fmt, ...);
-------------------------------------------------------------------------------

Printf-like function to report error and exit the test, it can be used with ttype:

|============================================================
| 'TBROK' | Something has failed in test preparation phase.
| 'TCONF' | Test is not appropriate for current configuration
            (syscall not implemented, unsupported arch, ...)
|============================================================

The 'ttype' can be combined bitwise with 'TERRNO' or 'TTERRNO' to print
'errno', 'TST_ERR' respectively.

There are also 'TST_EXP_*()' macros that can simplify syscall unit tests to a
single line, use them whenever possible. These macros take a function call as
the first parameter and a printf-like format string and parameters as well.
These test macros then expand to a code that runs the call, checks the return
value and errno and reports the test result.

[source,c]
-------------------------------------------------------------------------------
static void test(void)
{
	...
	TST_EXP_PASS(stat(fname, &statbuf), "stat(%s, ...)", fname);

	if (!TST_PASS)
		return;
	...
}
-------------------------------------------------------------------------------

The 'TST_EXP_PASS()' can be used for calls that return -1 on failure and 0 on
success. It will check for the return value and reports failure if the return
value is not equal to 0. The call also sets the 'TST_PASS' variable to 1 if
the call succeeeded.

As seen above, this and similar macros take optional variadic arguments. These
begin with a format string and then appropriate values to be formatted.

[source,c]
-------------------------------------------------------------------------------
static void test(void)
{
	...
	TST_EXP_FD(open(fname, O_RDONLY), "open(%s, O_RDONLY)", fname);

	SAFE_CLOSE(TST_RET);
	...
}
-------------------------------------------------------------------------------

The 'TST_EXP_FD()' is the same as 'TST_EXP_PASS()' the only difference is that
the return value is expected to be a file descriptor so the call passes if
positive integer is returned.

[source,c]
-------------------------------------------------------------------------------
static void test(void)
{
	...
	TST_EXP_FAIL(stat(fname, &statbuf), ENOENT, "stat(%s, ...)", fname);
	...
}
-------------------------------------------------------------------------------

The 'TST_EXP_FAIL()' is similar to 'TST_EXP_PASS()' but it fails the test if
the call haven't failed with -1 and 'errno' wasn't set to the expected one
passed as the second argument.

[source,c]
-------------------------------------------------------------------------------
static void test(void)
{
	...
	TST_EXP_FAIL2(msgget(key, flags), EINVAL, "msgget(%i, %i)", key, flags);
	...
}
-------------------------------------------------------------------------------

The 'TST_EXP_FAIL2()' is the same as 'TST_EXP_FAIL()' except the return value is
expected to be non-negative integer if call passes. These macros build upon the
+TEST()+ macro and associated variables.

[source,c]
-------------------------------------------------------------------------------
TEST(socket(AF_INET, SOCK_RAW, 1));
if (TST_RET > -1) {
	tst_res(TFAIL, "Created raw socket");
	SAFE_CLOSE(TST_RET);
} else if (TST_ERR != EPERM) {
	tst_res(TFAIL | TTERRNO,
		"Failed to create socket for wrong reason");
} else {
	tst_res(TPASS | TTERRNO, "Didn't create raw socket");
}
-------------------------------------------------------------------------------

The +TEST+ macro sets +TST_RET+ to its argument's return value and +TST_ERR+ to
+errno+. The +TTERNO+ flag can be used to print the error number's symbolic
value.

No LTP library function or macro, except those in 'tst_test_macros.h', will
write to these variables (rule 'LTP-002'). So their values will not be changed
unexpectedly.

[source,c]
-------------------------------------------------------------------------------
TST_EXP_POSITIVE(wait(&status));

if (!TST_PASS)
	return;
-------------------------------------------------------------------------------

If the return value of 'wait' is positive. This macro will print a pass result
and set +TST_PASS+ appropriately. If the return value is zero or negative, then
it will print fail.  There are many similar macros to those shown here, please
see 'tst_test_macros.h'.

[source,c]
-------------------------------------------------------------------------------
const char *tst_strsig(int sig);
-------------------------------------------------------------------------------

Return the given signal number's corresponding string.

[source,c]
-------------------------------------------------------------------------------
const char *tst_strerrno(int err);
-------------------------------------------------------------------------------

Return the given errno number's corresponding string. Using this function to
translate 'errno' values to strings is preferred. You should not use the
'strerror()' function in the testcases.

[source,c]
-------------------------------------------------------------------------------
const char *tst_strstatus(int status);
-------------------------------------------------------------------------------

Returns string describing the status as returned by 'wait()'.

WARNING: This function is not thread safe.

[source,c]
-------------------------------------------------------------------------------
void tst_set_timeout(unsigned int timeout);
-------------------------------------------------------------------------------

Allows for setting timeout per test iteration dynamically in the test setup(),
the timeout is specified in seconds. There are a few testcases whose runtime
can vary arbitrarily, these can disable timeouts by setting it to -1.

[source,c]
-------------------------------------------------------------------------------
void tst_flush(void);
-------------------------------------------------------------------------------

Flush output streams, handling errors appropriately.

This function is rarely needed when you have to flush the output streams
before calling 'fork()' or 'clone()'. Note that the 'SAFE_FORK()' and 'SAFE_CLONE()'
calls this function automatically. See 2.4 FILE buffers and fork() for explanation
why is this needed.

1.3 Test temporary directory
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If '.needs_tmpdir' is set to '1' in the 'struct tst_test' unique test
temporary is created and it's set as the test working directory. Tests *MUST
NOT* create temporary files outside that directory. The flag is not needed to
be set when use these flags: '.all_filesystems', '.format_device', '.mntpoint',
'.mount_device' '.needs_checkpoints', '.needs_device', '.resource_file'
(these flags imply creating temporary directory).

IMPORTANT: Close all file descriptors (that point to files in test temporary
           directory, even the unlinked ones) either in the 'test()' function
	   or in the test 'cleanup()' otherwise the test may break temporary
	   directory removal on NFS (look for "NFS silly rename").

1.4 Safe macros
~~~~~~~~~~~~~~~

Safe macros aim to simplify error checking in test preparation. Instead of
calling system API functions, checking for their return value and aborting the
test if the operation has failed, you just use corresponding safe macro.

Use them whenever it's possible.

Instead of writing:

[source,c]
-------------------------------------------------------------------------------
	fd = open("/dev/null", O_RDONLY);
	if (fd < 0)
		tst_brk(TBROK | TERRNO, "opening /dev/null failed");
-------------------------------------------------------------------------------

You write just:

[source,c]
-------------------------------------------------------------------------------
	fd = SAFE_OPEN("/dev/null", O_RDONLY);
-------------------------------------------------------------------------------

IMPORTANT: The SAFE_CLOSE() function also sets the passed file descriptor to -1
           after it's successfully closed.

They can also simplify reading and writing of sysfs files, you can, for
example, do:

[source,c]
-------------------------------------------------------------------------------
	SAFE_FILE_SCANF("/proc/sys/kernel/pid_max", "%lu", &pid_max);
-------------------------------------------------------------------------------

See 'include/tst_safe_macros.h', 'include/tst_safe_stdio.h' and
'include/tst_safe_file_ops.h' and 'include/tst_safe_net.h' for a complete list.

1.5 Test specific command line options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
struct tst_option {
        char *optstr;
        char **arg;
        char *help;
};
-------------------------------------------------------------------------------

Test specific command line parameters can be passed with the 'NULL' terminated
array of 'struct tst_option'. The 'optstr' is the command line option i.e. "o"
or "o:" if option has a parameter. Only short options are supported. The 'arg'
is where 'optarg' is stored upon match. If option has no parameter it's set to
non-'NULL' value if option was present. The 'help' is a short help string.

NOTE: The test parameters must not collide with common test parameters defined
      in the library the currently used ones are +-i+, +-I+, +-C+, and +-h+.

[source,c]
-------------------------------------------------------------------------------
int tst_parse_int(const char *str, int *val, int min, int max);
int tst_parse_long(const char *str, long *val, long min, long max);
int tst_parse_float(const char *str, float *val, float min, float max);
int tst_parse_filesize(const char *str, long long *val, long long min, long long max);
-------------------------------------------------------------------------------

Helpers for parsing the strings returned in the 'struct tst_option'.

Helpers return zero on success and 'errno', mostly 'EINVAL' or 'ERANGE', on
failure.

Helpers functions are no-op if 'str' is 'NULL'.

The valid range for result includes both 'min' and 'max'.

In particular, 'tst_parse_filesize' function accepts prefix multiplies such as
"k/K for kilobytes, "m/M" for megabytes and "g/G" for gigabytes. For example,
10K are converted into 10240 bytes.

.Example Usage
[source,c]
-------------------------------------------------------------------------------
#include <limits.h>
#include "tst_test.h"

static char *str_threads;
static int threads = 10;

static struct tst_option options[] = {
	{"t:", &str_threads, "Number of threads (default 10)"},
	...
	{}
};

static void setup(void)
{
	if (tst_parse_int(str_threads, &threads, 1, INT_MAX))
		tst_brk(TBROK, "Invalid number of threads '%s'", str_threads);

	...
}

static void test_threads(void)
{
	...

	for (i = 0; i < threads; i++) {
		...
	}

	...
}

static struct tst_test test = {
	...
	.options = options,
	...
};
-------------------------------------------------------------------------------


1.6 Runtime kernel version detection
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Testcases for newly added kernel functionality require kernel newer than a
certain version to run. All you need to skip a test on older kernels is to
set the '.min_kver' string in the 'struct tst_test' to a minimal required
kernel version, e.g. '.min_kver = "2.6.30"'.

For more complicated operations such as skipping a test for a certain range
of kernel versions, following functions could be used:

[source,c]
-------------------------------------------------------------------------------
int tst_kvercmp(int r1, int r2, int r3);

struct tst_kern_exv {
        char *dist_name;
        char *extra_ver;
};

int tst_kvercmp2(int r1, int r2, int r3, struct tst_kern_exv *vers);
-------------------------------------------------------------------------------

These two functions are intended for runtime kernel version detection. They
parse the output from 'uname()' and compare it to the passed values.

The return value is similar to the 'strcmp()' function, i.e. zero means equal,
negative value means that the kernel is older than than the expected value and
positive means that it's newer.

The second function 'tst_kvercmp2()' allows for specifying per-vendor table of
kernel versions as vendors typically backport fixes to their kernels and the
test may be relevant even if the kernel version does not suggests so. See
'testcases/kernel/syscalls/inotify/inotify04.c' for example usage.

WARNING: The shell 'tst_kvercmp' maps the result into unsigned integer - the
         process exit value.

1.7 Fork()-ing
~~~~~~~~~~~~~~

Be wary that if the test forks and there were messages printed by the
'tst_*()' interfaces, the data may still be in libc/kernel buffers and these
*ARE NOT* flushed automatically.

This happens when 'stdout' gets redirected to a file. In this case, the
'stdout' is not line buffered, but block buffered. Hence after a fork content
of the buffers will be printed by the parent and each of the children.

To avoid that you should use 'SAFE_FORK()', 'SAFE_CLONE()' or 'tst_clone()'.

IMPORTANT: You have to set the '.forks_child' flag in the test structure
           if your testcase forks or calls 'SAFE_CLONE()'.

1.8 Doing the test in the child process
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Results reported by 'tst_res()' are propagated to the parent test process via
block of shared memory.

Calling 'tst_brk()' causes child process to exit with non-zero exit value.
Which means that it's safe to use 'SAFE_*()' macros in the child processes as
well.

Children that outlive the 'test()' function execution are waited for in the
test library. Unclean child exit (killed by signal, non-zero exit value, etc.)
will cause the main test process to exit with 'tst_brk()', which especially
means that 'TBROK' propagated from a child process will cause the whole test
to exit with 'TBROK'.

If a test needs a child that segfaults or does anything else that cause it to
exit uncleanly all you need to do is to wait for such children from the
'test()' function so that it's reaped before the main test exits the 'test()'
function.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

void tst_reap_children(void);
-------------------------------------------------------------------------------

The 'tst_reap_children()' function makes the process wait for all of its
children and exits with 'tst_brk(TBROK, ...)' if any of them returned
a non zero exit code.

When using 'SAFE_CLONE' or 'tst_clone', this may not work depending on
the parameters passed to clone. The following call to 'SAFE_CLONE' is
identical to 'fork()', so will work as expected.

[source,c]
--------------------------------------------------------------------------------
const struct tst_clone_args args = {
	.exit_signal = SIGCHLD,
};

SAFE_CLONE(&args);
--------------------------------------------------------------------------------

If 'exit_signal' is set to something else, then this will break
'tst_reap_children'. It's not expected that all parameters to clone will
work with the LTP library unless specific action is taken by the test code.

.Using 'tst_res()' from binaries started by 'exec()'
[source,c]
-------------------------------------------------------------------------------
/* test.c */
#define _GNU_SOURCE
#include <unistd.h>
#include "tst_test.h"

static void do_test(void)
{
	char *const argv[] = {"test_exec_child", NULL};
	char path[4096];

	if (tst_get_path("test_exec_child", path, sizeof(path)))
		tst_brk(TCONF, "Couldn't find test_exec_child in $PATH");

	execve(path, argv, environ);

	tst_res(TFAIL | TERRNO, "EXEC!");
}

static struct tst_test test = {
	.test_all = do_test,
	.child_needs_reinit = 1,
};

/* test_exec_child.c */
#define TST_NO_DEFAULT_MAIN
#include "tst_test.h"

int main(void)
{
	tst_reinit();
	tst_res(TPASS, "Child passed!");
	return 0;
}
-------------------------------------------------------------------------------

The 'tst_res()' function can be also used from binaries started by 'exec()',
the parent test process has to set the '.child_needs_reinit' flag so that the
library prepares for it and has to make sure the 'LTP_IPC_PATH' environment
variable is passed down, then the very fist thing the program has to call in
'main()' is 'tst_reinit()' that sets up the IPC.

1.9 Fork() and Parent-child synchronization
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As LTP tests are written for Linux, most of the tests involve fork()-ing and
parent-child process synchronization. LTP includes a checkpoint library that
provides wait/wake futex based functions.

In order to use checkpoints the '.needs_checkpoints' flag in the 'struct
tst_test' must be set to '1', this causes the test library to initialize
checkpoints before the 'test()' function is called.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

TST_CHECKPOINT_WAIT(id)

TST_CHECKPOINT_WAIT2(id, msec_timeout)

TST_CHECKPOINT_WAKE(id)

TST_CHECKPOINT_WAKE2(id, nr_wake)

TST_CHECKPOINT_WAKE_AND_WAIT(id)
-------------------------------------------------------------------------------

The checkpoint interface provides pair of wake and wait functions. The 'id' is
unsigned integer which specifies checkpoint to wake/wait for. As a matter of
fact it's an index to an array stored in a shared memory, so it starts on
'0' and there should be enough room for at least of hundred of them.

The 'TST_CHECKPOINT_WAIT()' and 'TST_CHECKPOINT_WAIT2()' suspends process
execution until it's woken up or until timeout is reached.

The 'TST_CHECKPOINT_WAKE()' wakes one process waiting on the checkpoint.
If no process is waiting the function retries until it success or until
timeout is reached.

If timeout has been reached process exits with appropriate error message (uses
'tst_brk()').

The 'TST_CHECKPOINT_WAKE2()' does the same as 'TST_CHECKPOINT_WAKE()' but can
be used to wake precisely 'nr_wake' processes.

The 'TST_CHECKPOINT_WAKE_AND_WAIT()' is a shorthand for doing wake and then
immediately waiting on the same checkpoint.

Child processes created via 'SAFE_FORK()' are ready to use the checkpoint
synchronization functions, as they inherited the mapped page automatically.

Child processes started via 'exec()', or any other processes not forked from
the test process must initialize the checkpoint by calling 'tst_reinit()'.

For the details of the interface, look into the 'include/tst_checkpoint.h'.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

/*
 * Waits for process state change.
 *
 * The state is one of the following:
 *
 * R - process is running
 * S - process is sleeping
 * D - process sleeping uninterruptibly
 * Z - zombie process
 * T - process is traced
 */
TST_PROCESS_STATE_WAIT(pid, state, msec_timeout)
-------------------------------------------------------------------------------

The 'TST_PROCESS_STATE_WAIT()' waits until process 'pid' is in requested
'state' or timeout is reached. The call polls +/proc/pid/stat+ to get this
information. A timeout of 0 will wait infinitely.

On timeout -1 is returned and errno set to ETIMEDOUT.

It's mostly used with state 'S' which means that process is sleeping in kernel
for example in 'pause()' or any other blocking syscall.

1.10 Signals and signal handlers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you need to use signal handlers, keep the code short and simple. Don't
forget that the signal handler is called asynchronously and can interrupt the
code execution at any place.

This means that problems arise when global state is changed both from the test
code and signal handler, which will occasionally lead to:

* Data corruption (data gets into inconsistent state), this may happen, for
  example, for any operations on 'FILE' objects.

* Deadlock, this happens, for example, if you call 'malloc(2)', 'free(2)',
  etc. from both the test code and the signal handler at the same time since
  'malloc' has global lock for it's internal data structures. (Be wary that
  'malloc(2)' is used by the libc functions internally too.)

* Any other unreproducible and unexpected behavior.

Quite common mistake is to call 'exit(3)' from a signal handler. Note that this
function is not signal-async-safe as it flushes buffers, etc. If you need to
exit a test immediately from a signal handler use '_exit(2)' instead.

TIP: See 'man 7 signal' for the list of signal-async-safe functions.

If a signal handler sets a variable, its declaration must be 'volatile',
otherwise compiler may misoptimize the code. This is because the variable may
not be changed in the compiler code flow analysis. There is 'sig_atomic_t'
type defined in C99 but this one *DOES NOT* imply 'volatile' (it's just a
'typedef' to 'int'). So the correct type for a flag that is changed from a
signal handler is either 'volatile int' or 'volatile sig_atomic_t'.

If a crash (e.g. triggered by signal SIGSEGV) is expected in testing, you
can avoid creation of core files by calling tst_no_corefile() function.
This takes effect for process (and its children) which invoked it, unless
they subsequently modify RLIMIT_CORE.

Note that LTP library will reap any processes that test didn't reap itself,
and report any non-zero exit code as failure.

1.11 Kernel Modules
~~~~~~~~~~~~~~~~~~~

There are certain cases where the test needs a kernel part and userspace part,
happily, LTP can build a kernel module and then insert it to the kernel on test
start for you. See 'testcases/kernel/device-drivers/block' for details.

1.12 Useful macros
~~~~~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
ARRAY_SIZE(arr)
-------------------------------------------------------------------------------

Returns the size of statically defined array, i.e.
'(sizeof(arr) / sizeof(*arr))'

[source,c]
-------------------------------------------------------------------------------
LTP_ALIGN(x, a)
-------------------------------------------------------------------------------

Aligns the x to be next multiple of a. The a must be power of 2.

1.13 Filesystem type detection and skiplist
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests are known to fail on certain filesystems (you cannot swap on TMPFS,
there are unimplemented 'fcntl()' etc.).

If your test needs to be skipped on certain filesystems use the
'.skip_filesystems' field in the tst_test structure as follows:

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static struct tst_test test = {
	...
        .skip_filesystems = (const char *const []) {
                "tmpfs",
                "ramfs",
                "nfs",
                NULL
        },
};
-------------------------------------------------------------------------------

When the '.all_filesystems' flag is set the '.skip_filesystems' list is passed
to the function that detects supported filesystems any listed filesystem is
not included in the resulting list of supported filesystems.

If test needs to adjust expectations based on filesystem type it's also
possible to detect filesystem type at the runtime. This is preferably used
when only subset of the test is not applicable for a given filesystem.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static void run(void)
{
	...

	switch ((type = tst_fs_type("."))) {
	case TST_NFS_MAGIC:
	case TST_TMPFS_MAGIC:
	case TST_RAMFS_MAGIC:
		tst_brk(TCONF, "Subtest not supported on %s",
		        tst_fs_type_name(type));
		return;
	break;
	}

	...
}
-------------------------------------------------------------------------------

1.14 Thread-safety in the LTP library
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is safe to use library 'tst_res()' function in multi-threaded tests.

Only the main thread must return from the 'test()' function to the test
library and that must be done only after all threads that may call any library
function has been terminated. That especially means that threads that may call
'tst_brk()' must terminate before the execution of the 'test()' function
returns to the library. This is usually done by the main thread joining all
worker threads at the end of the 'test()' function. Note that the main thread
will never get to the library code in a case that 'tst_brk()' was called from
one of the threads since it will sleep at least in 'pthread_join()' on the
thread that called the 'tst_brk()' till 'exit()' is called by 'tst_brk()'.

The test-supplied cleanup function runs *concurrently* to the rest of the
threads in a case that cleanup was entered from 'tst_brk()'. Subsequent
threads entering 'tst_brk()' must be suspended or terminated at the start of
the user supplied cleanup function. It may be necessary to stop or exit
the rest of the threads before the test cleans up as well. For example threads
that create new files should be stopped before temporary directory is be
removed.

Following code example shows thread safe cleanup function example using atomic
increment as a guard. The library calls its cleanup after the execution returns
from the user supplied cleanup and expects that only one thread returns from
the user supplied cleanup to the test library.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static void cleanup(void)
{
	static int flag;

	if (tst_atomic_inc(&flag) != 1)
		pthread_exit(NULL);

	/* if needed stop the rest of the threads here */

	...

	/* then do cleanup work */

	...

	/* only one thread returns to the library */
}
-------------------------------------------------------------------------------


1.15 Testing with a block device
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests needs a block device (inotify tests, syscall 'EROFS' failures,
etc.). LTP library contains a code to prepare a testing device.

If '.needs_device' flag in the 'struct tst_test' is set the 'tst_device'
structure is initialized with a path to a test device and default filesystem
to be used.

You can also request minimal device size in megabytes by setting
'.dev_min_size' the device is guaranteed to have at least the requested size
then.

If '.format_device' flag is set the device is formatted with a filesystem as
well. You can use '.dev_fs_type' to override the default filesystem type if
needed and pass additional options to mkfs via '.dev_fs_opts' and
'.dev_extra_opts' pointers. Note that '.format_device' implies '.needs_device'
there is no need to set both.

If '.mount_device' is set, the device is mounted at '.mntpoint' which is used
to pass a directory name that will be created and used as mount destination.
You can pass additional flags and data to the mount command via '.mnt_flags'
and '.mnt_data' pointers. Note that '.mount_device' implies '.needs_device'
and '.format_device' so there is no need to set the later two.

If '.needs_rofs' is set, read-only filesystem is mounted at '.mntpoint' this
one is supposed to be used for 'EROFS' tests.

If '.all_filesystems' is set the test function is executed for all supported
filesystems. Supported filesystems are detected based on existence of the
'mkfs.$fs' helper and on kernel support to mount it. For each supported
filesystem the 'tst_device.fs_type' is set to the currently tested fs type, if
'.format_device' is set the device is formatted as well, if '.mount_device' is
set it's mounted at '.mntpoint'. Also the test timeout is reset for each
execution of the test function. This flag is expected to be used for filesystem
related syscalls that are at least partly implemented in the filesystem
specific code e.g. 'fallocate()'.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

struct tst_device {
	const char *dev;
	const char *fs_type;
};

extern struct tst_device *tst_device;

int tst_umount(const char *path);
-------------------------------------------------------------------------------

In case that 'LTP_DEV' is passed to the test in an environment, the library
checks that the file exists and that it's a block device, if
'.device_min_size' is set the device size is checked as well. If 'LTP_DEV'
wasn't set or if size requirements were not met a temporary file is created
and attached to a free loop device.

If there is no usable device and loop device couldn't be initialized the test
exits with 'TCONF'.

The 'tst_umount()' function works exactly as 'umount(2)' but retries several
times on 'EBUSY'. This is because various desktop daemons (gvfsd-trash is known
for that) may be stupid enough to probe all newly mounted filesystem which
results in 'umount(2)' failing with 'EBUSY'.

IMPORTANT: All testcases should use 'tst_umount()' instead of 'umount(2)' to
           umount filesystems.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_find_free_loopdev(const char *path, size_t path_len);
-------------------------------------------------------------------------------

This function finds a free loopdev and returns the free loopdev minor (-1 for no
free loopdev). If path is non-NULL, it will be filled with free loopdev path.
If you want to use a customized loop device, we can call 'tst_find_free_loopdev(NULL, 0)'
in tests to get a free minor number and then mknod.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

unsigned long tst_dev_bytes_written(const char *dev);
-------------------------------------------------------------------------------

This function reads test block device stat file ('/sys/block/<device>/stat') and
returns the bytes written since the last invocation of this function. To avoid
FS deferred IO metadata/cache interference, we suggest doing "syncfs" before the
tst_dev_bytes_written first invocation. And an inline function named 'tst_dev_sync()'
is created for that intention.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

voud tst_find_backing_dev(const char *path, char *dev);
-------------------------------------------------------------------------------

This function finds the block dev that this path belongs to, it uses stat function
to get the major/minor number of the path. Then scan them in '/proc/self/mountinfo'
and list 2th column value after ' - ' string as its block dev if match succeeds.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

uint64_t tst_get_device_size(const char *dev_path);
-------------------------------------------------------------------------------

This function gets size of the given block device, it checks the 'dev_path' is
valid first, if yes, return the size in MB, otherwise return -1.

1.16 Formatting a device with a filesystem
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static void setup(void)
{
	...
	SAFE_MKFS(tst_device->dev, tst_device->fs_type, NULL, NULL);
	...
}
-------------------------------------------------------------------------------

This function takes a path to a device, filesystem type and an array of extra
options passed to mkfs.

The fs options 'fs_opts' should either be 'NULL' if there are none, or a
'NULL' terminated array of strings such as:
+const char *const opts[] = {"-b", "1024", NULL}+.

The extra options 'extra_opts' should either be 'NULL' if there are none, or a
'NULL' terminated array of strings such as +{"102400", NULL}+; 'extra_opts'
will be passed after device name. e.g: +mkfs -t ext4 -b 1024 /dev/sda1 102400+
in this case.

Note that perfer to store the options which can be passed before or after device
name by 'fs_opts' array.

1.17 Verifying a filesystem's free space
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests have size requirements for the filesystem's free space. If these
requirements are not satisfied, the tests should be skipped.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_fs_has_free(const char *path, unsigned int size, unsigned int mult);
-------------------------------------------------------------------------------

The 'tst_fs_has_free()' function returns 1 if there is enough space and 0 if
there is not.

The 'path' is the pathname of any directory/file within a filesystem.

The 'mult' is a multiplier, one of 'TST_BYTES', 'TST_KB', 'TST_MB' or 'TST_GB'.

The required free space is calculated by 'size * mult', e.g.
'tst_fs_has_free("/tmp/testfile", 64, TST_MB)' will return 1 if the
filesystem, which '"/tmp/testfile"' is in, has 64MB free space at least, and 0
if not.

1.18 Files, directories and fs limits
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests need to know the maximum count of links to a regular file or
directory, such as 'rename(2)' or 'linkat(2)' to test 'EMLINK' error.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_fs_fill_hardlinks(const char *dir);
-------------------------------------------------------------------------------

Try to get maximum count of hard links to a regular file inside the 'dir'.

NOTE: This number depends on the filesystem 'dir' is on.

This function uses 'link(2)' to create hard links to a single file until it
gets 'EMLINK' or creates 65535 links. If the limit is hit, the maximum number of
hardlinks is returned and the 'dir' is filled with hardlinks in format
"testfile%i", where i belongs to [0, limit) interval. If no limit is hit or if
'link(2)' failed with 'ENOSPC' or 'EDQUOT', zero is returned and previously
created files are removed.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_fs_fill_subdirs(const char *dir);
-------------------------------------------------------------------------------

Try to get maximum number of subdirectories in directory.

NOTE: This number depends on the filesystem 'dir' is on. For current kernel,
subdir limit is not available for all filesystems (available for ext2, ext3,
minix, sysv and more). If the test runs on some other filesystems, like ramfs,
tmpfs, it will not even try to reach the limit and return 0.

This function uses 'mkdir(2)' to create directories in 'dir' until it gets
'EMLINK' or creates 65535 directories. If the limit is hit, the maximum number
of subdirectories is returned and the 'dir' is filled with subdirectories in
format "testdir%i", where i belongs to [0, limit - 2) interval (because each
newly created dir has two links already - the '.' and the link from parent
dir). If no limit is hit or if 'mkdir(2)' failed with 'ENOSPC' or 'EDQUOT',
zero is returned and previously created directories are removed.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_dir_is_empty(const char *dir, int verbose);
-------------------------------------------------------------------------------

Returns non-zero if directory is empty and zero otherwise.

Directory is considered empty if it contains only '.' and '..'.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

void tst_purge_dir(const char *path);
-------------------------------------------------------------------------------

Deletes the contents of given directory but keeps the directory itself. Useful
for cleaning up the temporary directory and mount points between test cases or
test iterations. Terminates the program with 'TBROK' on error.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_fill_fd(int fd, char pattern, size_t bs, size_t bcount);
-------------------------------------------------------------------------------

Fill a file with specified pattern using file descriptor.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_prealloc_size_fd(int fd, size_t bs, size_t bcount);
-------------------------------------------------------------------------------

Preallocate the specified amount of space using 'fallocate()'. Falls back to
'tst_fill_fd()' if 'fallocate()' fails.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_fill_file(const char *path, char pattern, size_t bs, size_t bcount);
-------------------------------------------------------------------------------

Creates/overwrites a file with specified pattern using file path.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_prealloc_file(const char *path, size_t bs, size_t bcount);
-------------------------------------------------------------------------------

Create/overwrite a file and preallocate the specified amount of space for it.
The allocated space will not be initialized to any particular content.

1.19 Getting an unused PID number
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests require a 'PID', which is not used by the OS (does not belong to
any process within it). For example, kill(2) should set errno to 'ESRCH' if
it's passed such 'PID'.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

pid_t tst_get_unused_pid(void);
-------------------------------------------------------------------------------

Return a 'PID' value not used by the OS or any process within it.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_get_free_pids(void);
-------------------------------------------------------------------------------

Returns number of unused pids in the system. Note that this number may be
different once the call returns and should be used only for rough estimates.

1.20 Running executables
~~~~~~~~~~~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

int tst_cmd(const char *const argv[],
	        const char *stdout_path,
	        const char *stderr_path,
	        enum tst_cmd_flags flags);
-------------------------------------------------------------------------------

'tst_cmd()' is a wrapper for 'vfork() + execvp()' which provides a way
to execute an external program.

'argv[]' is a 'NULL' terminated array of strings starting with the program name
which is followed by optional arguments.

'TST_CMD_PASS_RETVAL' enum 'tst_cmd_flags' makes 'tst_cmd()'
return the program exit code to the caller, otherwise 'tst_cmd()' exit the
tests on failure. 'TST_CMD_TCONF_ON_MISSING' check for program in '$PATH' and exit
with 'TCONF' if not found.

In case that 'execvp()' has failed and the enum 'TST_CMD_PASS_RETVAL' flag was set, the
return value is '255' if 'execvp()' failed with 'ENOENT' and '254' otherwise.

'stdout_path' and 'stderr_path' determine where to redirect the program
stdout and stderr I/O streams.

The 'SAFE_CMD()' macro can be used automatic handling non-zero exits (exits
with 'TBROK') and 'ENOENT' (exits with 'TCONF').

.Example
[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

const char *const cmd[] = { "ls", "-l", NULL };

...
	/* Store output of 'ls -l' into log.txt */
	tst_cmd(cmd, "log.txt", NULL, 0);
...
-------------------------------------------------------------------------------

1.21 Measuring elapsed time and helper functions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
#include "tst_timer.h"

void tst_timer_check(clockid_t clk_id);

void tst_timer_start(clockid_t clk_id);

void tst_timer_stop(void);

struct timespec tst_timer_elapsed(void);

long long tst_timer_elapsed_ms(void);

long long tst_timer_elapsed_us(void);

int tst_timer_expired_ms(long long ms);
-------------------------------------------------------------------------------

The 'tst_timer_check()' function checks if specified 'clk_id' is suppored and
exits the test with 'TCONF' otherwise. It's expected to be used in test
'setup()' before any resources that needs to be cleaned up are initialized,
hence it does not include a cleanup function parameter.

The 'tst_timer_start()' marks start time and stores the 'clk_id' for further
use.

The 'tst_timer_stop()' marks the stop time using the same 'clk_id' as last
call to 'tst_timer_start()'.

The 'tst_timer_elapsed*()' returns time difference between the timer start and
last timer stop in several formats and units.

The 'tst_timer_expired_ms()' function checks if the timer started by
'tst_timer_start()' has been running longer than ms milliseconds. The function
returns non-zero if timer has expired and zero otherwise.

IMPORTANT: The timer functions use 'clock_gettime()' internally which needs to
           be linked with '-lrt' on older glibc. Please do not forget to add
	   'LDLIBS+=-lrt' in Makefile.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"
#include "tst_timer.h"

static void setup(void)
{
	...
	tst_timer_check(CLOCK_MONOTONIC);
	...
}

static void run(void)
{
	...
	tst_timer_start(CLOCK_MONOTONIC);
	...
	while (!tst_timer_expired_ms(5000)) {
		...
	}
	...
}

struct tst_test test = {
	...
	.setup = setup,
	.test_all = run,
	...
};
-------------------------------------------------------------------------------

Expiration timer example usage.

[source,c]
-------------------------------------------------------------------------------
long long tst_timespec_to_us(struct timespec t);
long long tst_timespec_to_ms(struct timespec t);

struct timeval tst_us_to_timeval(long long us);
struct timeval tst_ms_to_timeval(long long ms);

int tst_timespec_lt(struct timespec t1, struct timespec t2);

struct timespec tst_timespec_add_us(struct timespec t, long long us);

struct timespec tst_timespec_diff(struct timespec t1, struct timespec t2);
long long tst_timespec_diff_us(struct timespec t1, struct timespec t2);
long long tst_timespec_diff_ms(struct timespec t1, struct timespec t2);

struct timespec tst_timespec_abs_diff(struct timespec t1, struct timespec t2);
long long tst_timespec_abs_diff_us(struct timespec t1, struct timespec t2);
long long tst_timespec_abs_diff_ms(struct timespec t1, struct timespec t2);
-------------------------------------------------------------------------------

The first four functions are simple inline conversion functions.

The 'tst_timespec_lt()' function returns non-zero if 't1' is earlier than
't2'.

The 'tst_timespec_add_us()' function adds 'us' microseconds to the timespec
't'. The 'us' is expected to be positive.

The 'tst_timespec_diff*()' functions returns difference between two times, the
't1' is expected to be later than 't2'.

The 'tst_timespec_abs_diff*()' functions returns absolute value of difference
between two times.

NOTE: All conversions to ms and us rounds the value.

1.22 Datafiles
~~~~~~~~~~~~~~

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static const char *const res_files[] = {
	"foo",
	"bar",
	NULL
};

static struct tst_test test = {
	...
	.resource_files = res_files,
	...
}
-------------------------------------------------------------------------------

If the test needs additional files to be copied to the test temporary
directory all you need to do is to list their filenames in the
'NULL' terminated array '.resource_files' in the tst_test structure.

When resource files is set test temporary directory is created automatically,
there is need to set '.needs_tmpdir' as well.

The test library looks for datafiles first, these are either stored in a
directory called +datafiles+ in the +$PWD+ at the start of the test or in
+$LTPROOT/testcases/data/${test_binary_name}+. If the file is not found the
library looks into +$LTPROOT/testcases/bin/+ and to +$PWD+ at the start of the
test. This ensures that the testcases can copy the file(s) effortlessly both
when test is started from the directory it was compiled in as well as when LTP
was installed.

The file(s) are copied to the newly created test temporary directory which is
set as the test working directory when the 'test()' functions is executed.

1.23 Code path tracing
~~~~~~~~~~~~~~~~~~~~~~

'tst_res' is a macro, so on when you define a function in one file:

[source,c]
-------------------------------------------------------------------------------
int do_action(int arg)
{
	...

	if (ok) {
		tst_res(TPASS, "check passed");
		return 0;
	} else {
		tst_res(TFAIL, "check failed");
		return -1;
	}
}
-------------------------------------------------------------------------------

and call it from another file, the file and line reported by 'tst_res' in this
function will be from the former file.

'TST_TRACE' can make the analysis of such situations easier. It's a macro which
inserts a call to 'tst_res(TINFO, ...)' in case its argument evaluates to
non-zero. In this call to 'tst_res(TINFO, ...)' the file and line will be
expanded using the actual location of 'TST_TRACE'.

For example, if this another file contains:

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

if (TST_TRACE(do_action(arg))) {
	...
}
-------------------------------------------------------------------------------

the generated output may look similar to:

-------------------------------------------------------------------------------
common.h:9: FAIL: check failed
test.c:8: INFO: do_action(arg) failed
-------------------------------------------------------------------------------

1.24 Tainted kernels
~~~~~~~~~~~~~~~~~~~~

If you need to detect whether a testcase triggers a kernel warning, bug or
oops, the following can be used to detect TAINT_W or TAINT_D:

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static struct tst_test test = {
	...
	.taint_check = TST_TAINT_W | TST_TAINT_D,
	...
};

void run(void)
{
	...
	if (tst_taint_check() != 0)
		tst_res(TFAIL, "kernel has issues");
	else
		tst_res(TPASS, "kernel seems to be fine");
}
-------------------------------------------------------------------------------

To initialize taint checks, you have to set the taint flags you want to test
for in the 'taint_check' attribute of the tst_test struct. LTP library will
then automatically call 'tst_taint_init()' during test setup. The function
will generate a 'TCONF' if the requested flags are not fully supported on the
running kernel, and 'TBROK' if the kernel is already tainted before executing
the test.

LTP library will then automatically check kernel taint at the end of testing.
If '.all_filesystems' is set in struct tst_test, taint check will be performed
after each file system and taint will abort testing early with 'TFAIL'. You
can optionally also call 'tst_taint_check()' during 'run()', which returns 0
or the tainted flags set in '/proc/sys/kernel/tainted' as specified earlier.

Depending on your kernel version, not all tainted-flags will be supported.

For reference to tainted kernels, see kernel documentation:
Documentation/admin-guide/tainted-kernels.rst or
https://www.kernel.org/doc/html/latest/admin-guide/tainted-kernels.html

1.25 Checksums
~~~~~~~~~~~~~~

CRC32c checksum generation is supported by LTP. In order to use it, the
test should include 'tst_checksum.h' header, then can call 'tst_crc32c()'.

1.26 Checking kernel for the driver support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests may need specific kernel drivers, either compiled in, or built
as a module. If '.needs_drivers' points to a 'NULL' terminated array of kernel
module names these are all checked and the test exits with 'TCONF' on the
first missing driver.

Since it relies on modprobe command, the check will be skipped if the command
itself is not available on the system.

1.27 Saving & restoring /proc|sys values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

LTP library can be instructed to save and restore value of specified
(/proc|sys) files. This is achieved by initialized tst_test struct
field 'save_restore'. It is a 'NULL' terminated array of strings where
each string represents a file, whose value is saved at the beginning
and restored at the end of the test. Only first line of a specified
file is saved and restored.

Pathnames can be optionally prefixed to specify how strictly (during
'store') are handled errors:

* (no prefix) - test ends with 'TCONF', if file doesn't exist
* '?'         - test prints info message and continues,
                if file doesn't exist or open/read fails
* '!'         - test ends with 'TBROK', if file doesn't exist

'restore' is always strict and will TWARN if it encounters any error.

[source,c]
-------------------------------------------------------------------------------
static const char *save_restore[] = {
	"/proc/sys/kernel/core_pattern",
	NULL,
};

static void setup(void)
{
	FILE_PRINTF("/proc/sys/kernel/core_pattern", "/mypath");
}

static struct tst_test test = {
	...
	.setup = setup,
	.save_restore = save_restore,
};
-------------------------------------------------------------------------------

1.28 Parsing kernel .config
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Generally testcases should attempt to autodetect as much kernel features as
possible based on the currently running kernel. We do have tst_check_driver()
to check if functionality that could be compiled as kernel module is present
on the system, disabled syscalls can be detected by checking for 'ENOSYS'
errno etc.

However in rare cases core kernel features couldn't be detected based on the
kernel userspace API and we have to resort to parse the kernel .config.

For this cases the test should set the 'NULL' terminated '.needs_kconfigs'
array of boolean expressions with constraints on the kconfig variables. The
boolean expression consits of variables, two binary operations '&' and '|',
negation '!' and correct sequence of parentesis '()'. Variables are expected
to be in a form of "CONFIG_FOO[=bar]".

The test will continue to run if all expressions are evaluated to 'True'.
Missing variable is mapped to 'False' as well as variable with different than
specified value, e.g. 'CONFIG_FOO=bar' will evaluate to 'False' if the value
is anything else but 'bar'. If config variable is specified as plain
'CONFIG_FOO' it's evaluated to true it's set to any value (typically =y or =m).

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static const char *kconfigs[] = {
	"CONFIG_X86_INTEL_UMIP | CONFIG_X86_UMIP",
	NULL
};

static struct tst_test test = {
	...
	.needs_kconfigs = kconfigs,
	...
};
-------------------------------------------------------------------------------

1.29 Changing the Wall Clock Time during test execution
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are some tests that, for different reasons, might need to change the
system-wide clock time. Whenever this happens, it is imperative that the clock
is restored, at the end of test's execution, taking in consideration the amount
of time elapsed during that test.

In order for that to happen, struct tst_test has a variable called
"restore_wallclock" that should be set to "1" so LTP knows it should: (1)
initialize a monotonic clock during test setup phase and (2) use that monotonic
clock to fix the system-wide clock time at the test cleanup phase.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static void setup(void)
{
	...
}

static void run(void)
{
	...
}

struct tst_test test = {
	...
	.setup = setup,
	.test_all = run,
	.restore_wallclock = 1,
	...
};
-------------------------------------------------------------------------------

1.30 Testing similar syscalls in one test
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In some cases kernel has several very similar syscalls that do either the same
or very similar job. This is most noticeable on i386 where we commonly have
two or three syscall versions. That is because i386 was first platform that
Linux was developed on and because of that most mistakes in API happened there
as well. However this is not limited to i386 at all, it's quite common that
version two syscall has added missing flags parameters or so.

In such cases it does not make much sense to copy&paste the test code over and
over, rather than that the test library provides support for test variants.
The idea behind test variants is simple, we run the test several times each
time with different syscall variant.

The implementation consist of test_variants integer that, if set, denotes number
of test variants. The test is then forked and executed test_variants times each
time with different value in global tst_variant variable.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static int do_foo(void)
{
	switch (tst_variant) {
	case 0:
		return foo();
	case 1:
		return syscall(__NR_foo);
	}

	return -1;
}

static void run(void)
{
	...

	TEST(do_foo);

	...
}

static void setup(void)
{
	switch (tst_variant) {
	case 0:
		tst_res(TINFO, "Testing foo variant 1");
	break;
	case 1:
		tst_res(TINFO, "Testing foo variant 2");
	break;
	}
}

struct tst_test test = {
	...
	.setup = setup,
	.test_all = run,
	.test_variants = 2,
	...
};
-------------------------------------------------------------------------------

1.31 Guarded buffers
~~~~~~~~~~~~~~~~~~~~

The test library supports guarded buffers, which are buffers allocated so
that:

* The end of the buffer is followed by a PROT_NONE page

* The remainder of the page before the buffer is filled with random canary
  data

Which means that the any access after the buffer will yield a Segmentation
fault or EFAULT depending on if the access happened in userspace or the kernel
respectively. The canary before the buffer will also catch any write access
outside of the buffer.

The purpose of the patch is to catch off-by-one bugs which happens when
buffers and structures are passed to syscalls. New tests should allocate
guarded buffers for all data passed to the tested syscall which are passed by
a pointer.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static struct foo *foo_ptr;
static struct iovec *iov;
static void *buf_ptr;
static char *id;
...

static void run(void)
{
	...

	foo_ptr->bar = 1;
	foo_ptr->buf = buf_ptr;

	...
}

static void setup(void)
{
	...

	id = tst_strdup(string);

	...
}

static struct tst_test test = {
	...
	.bufs = (struct tst_buffers []) {
		{&foo_ptr, .size = sizeof(*foo_ptr)},
		{&buf_ptr, .size = BUF_SIZE},
		{&iov, .iov_sizes = (int[]){128, 32, -1},
		{}
	}
};
-------------------------------------------------------------------------------

Guarded buffers can be allocated on runtime in a test setup() by a
'tst_alloc()' or by 'tst_strdup()' as well as by filling up the .bufs array in
the tst_test structure.

So far the tst_test structure supports allocating either a plain buffer by
setting up the size or struct iovec, which is allocated recursively including
the individual buffers as described by an '-1' terminated array of buffer
sizes.

1.32 Adding and removing capabilities
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests may require the presence or absence of particular
capabilities. Using the API provided by 'tst_capability.h' the test author can
try to ensure that some capabilities are either present or absent during the
test.

For example; below we try to create a raw socket, which requires
CAP_NET_ADMIN. During setup we should be able to do it, then during run it
should be impossible. The LTP capability library will check before setup that
we have this capability, then after setup it will drop it.

[source,c]
--------------------------------------------------------------------------------
#include "tst_test.h"
#include "tst_capability.h"
#include "tst_safe_net.h"

#include "lapi/socket.h"

static void run(void)
{
	TEST(socket(AF_INET, SOCK_RAW, 1));
	if (TST_RET > -1) {
		tst_res(TFAIL, "Created raw socket");
	} else if (TST_ERR != EPERM) {
		tst_res(TFAIL | TTERRNO,
			"Failed to create socket for wrong reason");
	} else {
		tst_res(TPASS | TTERRNO, "Didn't create raw socket");
	}
}

static void setup(void)
{
	TEST(socket(AF_INET, SOCK_RAW, 1));
	if (TST_RET < 0)
		tst_brk(TCONF | TTERRNO, "We don't have CAP_NET_RAW to begin with");

	SAFE_CLOSE(TST_RET);
}

static struct tst_test test = {
	.setup = setup,
	.test_all = run,
	.caps = (struct tst_cap []) {
		TST_CAP(TST_CAP_REQ, CAP_NET_RAW),
		TST_CAP(TST_CAP_DROP, CAP_NET_RAW),
		{}
	},
};
--------------------------------------------------------------------------------

Look at the test struct at the bottom. We have filled in the 'caps' field with
a 'NULL' terminated array containing two 'tst_cap' structs. 'TST_CAP_REQ'
actions are executed before setup and 'TST_CAP_DROP' are executed after
setup. This means it is possible to both request and drop a capability.

[source,c]
--------------------------------------------------------------------------------
static struct tst_test test = {
	.test_all = run,
	.caps = (struct tst_cap []) {
		TST_CAP(TST_CAP_REQ, CAP_NET_RAW),
		TST_CAP(TST_CAP_DROP, CAP_SYS_ADMIN),
		{}
	},
};
--------------------------------------------------------------------------------

Here we request 'CAP_NET_RAW', but drop 'CAP_SYS_ADMIN'. If the capability is
in the permitted set, but not the effective set, the library will try to
permit it. If it is not in the permitted set, then it will fail with 'TCONF'.

This API does not require 'libcap' to be installed. However it has limited
features relative to 'libcap'. It only tries to add or remove capabilities
from the effective set. This means that tests which need to spawn child
processes may have difficulties ensuring the correct capabilities are
available to the children (see the capabilities (7) manual pages).

However a lot of problems can be solved by using 'tst_cap_action(struct
tst_cap  *cap)' directly which can be called at any time. This also helps if
you wish to drop a capability at the begining of setup.

1.33 Reproducing race-conditions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If a bug is caused by two tasks in the kernel racing and you wish to create a
regression test (or bug-fix validation test) then the 'tst_fuzzy_sync.h'
library should be used.

It allows you to specify, in your code, two race windows. One window in each
thread's loop (triggering a race usually requires many iterations). These
windows show fuzzy-sync where the race can happen. They don't need to be
exact, hence the 'fuzzy' part. If the race condition is not immediately
triggered then the library will begin experimenting with different timings.

[source,c]
--------------------------------------------------------------------------------
#include "tst_fuzzy_sync.h"

static struct tst_fzsync_pair fzsync_pair;

static void setup(void)
{
        tst_fzsync_pair_init(&fzsync_pair);
}

static void cleanup(void)
{
	tst_fzsync_pair_cleanup(&fzsync_pair);
}

static void *thread_b(void *arg)
{
	while (tst_fzsync_run_b(&fzsync_pair)) {

		tst_fzsync_start_race_b(&fzsync_pair);

                /* This is the race window for thread B */

                tst_fzsync_end_race_b(&fzsync_pair);
	}

	return arg;
}

static void thread_a(void)
{
	tst_fzsync_pair_reset(&fzsync_pair, thread_b);

        while (tst_fzsync_run_a(&fzsync_pair)) {

		tst_fzsync_start_race_a(&fzsync_pair);

		/* This is the race window for thread A */

                tst_fzsync_end_race_a(&fzsync_pair);
	}
}

static struct tst_test test = {
	.test_all = thread_a,
	.setup = setup,
	.cleanup = cleanup,
};
--------------------------------------------------------------------------------

Above is a minimal template for a test using fuzzy-sync. In a simple case, you
just need to put the bits you want to race inbetween 'start_race' and
'end_race'. Meanwhile, any setup you need to do per-iteration goes outside the
windows.

Fuzzy sync synchronises 'run_a' and 'run_b', which act as barriers, so that
neither thread can progress until the other has caught up with it. There is
also the 'pair_wait' function which can be used to add barriers in other
locations. Of course 'start/end_race_a/b' are also a barriers.

The library decides how long the test should run for based on the timeout
specified by the user plus some other heuristics.

For full documentation see the comments in 'include/tst_fuzzy_sync.h'.

1.34 Reserving hugepages
~~~~~~~~~~~~~~~~~~~~~~~~

Many of the LTP tests need to use hugepage in their testing, this allows the
test can reserve hugepages from system only via '.request_hugepages = xx'.

If set non-zero number of 'request_hugepages', test will try to reserve the
expected number of hugepage for testing in setup phase. If system does not
have enough hpage for using, it will try the best to reserve 80% available
number of hpages. With success test stores the reserved hugepage number in
'tst_hugepages'. For the system without hugetlb supporting, variable
'tst_hugepages' will be set to 0. If the hugepage number needs to be set to 0
on supported hugetlb system, please use '.request_hugepages = TST_NO_HUGEPAGES'.

Also, we do cleanup and restore work for the hpages resetting automatically.

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static void run(void)
{
	...

	if (tst_hugepages == test.request_hugepages)
		TEST(do_hpage_test);
	else
		...
	...
}

struct tst_test test = {
	.test_all = run,
	.request_hugepages = 2,
	...
};
-------------------------------------------------------------------------------

or,

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static void run(void)
{
	...
}

static void setup(void)
{
        if (tst_hugepages != test.requested_hugepages)
                tst_brk(TCONF, "...");
}

struct tst_test test = {
	.test_all = run,
	.request_hugepages = 2,
	...
};
-------------------------------------------------------------------------------

1.35 Checking for required commands
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Required commands can be checked with '.needs_cmds', which points to a 'NULL'
terminated array of strings such as:

[source,c]
-------------------------------------------------------------------------------
.needs_cmds = (const char *const []) {
	"useradd",
	"userdel",
	NULL
},
-------------------------------------------------------------------------------

Also can check required command version whether is satisfied by using 'needs_cmds'
such as:

[source,c]
-------------------------------------------------------------------------------
.needs_cmds = (const char *const []) {
	"mkfs.ext4 >= 1.43.0",
	NULL
},
-------------------------------------------------------------------------------

Currently, we only support mkfs.ext4 command version check.
If you want to support more commands, please fill your own .parser and .table_get
method in the version_parsers structure of lib/tst_cmd.c.

1.36 Assert sys or proc file value
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Using TST_ASSERT_INT/STR(path, val) to assert that integer value or string stored in
the prefix field of file pointed by path equals to the value passed to this function.

Also having a similar api pair TST_ASSERT_FILE_INT/STR(path, prefix, val) to assert
the field value of file.

1.36 Using Control Group
~~~~~~~~~~~~~~~~~~~~~~~~

Some LTP tests need specific Control Group configurations. tst_cgroup.h provides
APIs to discover and use CGroups. There are many differences between CGroups API
V1 and V2. We encapsulate the details of configuring CGroups in high-level
functions which follow the V2 kernel API. Allowing one to use CGroups without
caring too much about the current system's configuration.

Also, the LTP library will automatically mount/umount and configure the CGroup
hierarchies if that is required (e.g. if you run the tests from init with no
system manager).

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"
#include "tst_cgroup.h"

static const struct tst_cgroup_group *cg;

static void run(void)
{
	...
	// do test under cgroup
	...
}

static void setup(void)
{
	tst_cgroup_require("memory", NULL);
	cg = tst_cgroup_get_test_group();
	SAFE_CGROUP_PRINTF(cg, "cgroup.procs", "%d", getpid());
	SAFE_CGROUP_PRINTF(cg, "memory.max", "%lu", MEMSIZE);
	if (SAFE_CGROUP_HAS(cg, "memory.swap.max"))
		SAFE_CGROUP_PRINTF(cg, "memory.swap.max", "%zu", memsw);
}

static void cleanup(void)
{
	tst_cgroup_cleanup();
}

struct tst_test test = {
	.setup = setup,
	.test_all = run,
	.cleanup = cleanup,
	...
};
-------------------------------------------------------------------------------

Above, we first ensure the memory controller is available on the
test's CGroup with 'tst_cgroup_require'. We then get a structure,
'cg', which represents the test's CGroup. Note that
'tst_cgroup_get_test_group' should not be called many times, as it is
allocated in a guarded buffer (See section 2.2.31). Therefor it is
best to call it once in 'setup' and not 'run' because 'run' may be
repeated with the '-i' option.

We then write the current processes PID into 'cgroup.procs', which
moves the current process into the test's CGroup. After which we set
the maximum memory size by writing to 'memory.max'. If the memory
controller is mounted on CGroups V1 then the library will actually
write to 'memory.limit_in_bytes'. As a general rule, if a file exists
on both CGroup versions, then we use the V2 naming.

Some controller features, such as 'memory.swap', can be
disabled. Therefor we need to check if they exist before accessing
them. This can be done with 'SAFE_CGROUP_HAS' which can be called on
any control file or feature.

Most tests only require setting a few limits similar to the above. In
such cases the differences between V1 and V2 are hidden. Setup and
cleanup is also mostly hidden. However things can get much worse.

[source,c]
-------------------------------------------------------------------------------
static const struct tst_cgroup_group *cg;
static const struct tst_cgroup_group *cg_drain;
static struct tst_cgroup_group *cg_child;

static void run(void)
{
	char buf[BUFSIZ];
	size_t mem = 0;

	cg_child = tst_cgroup_group_mk(cg, "child");
	SAFE_CGROUP_PRINTF(cg_child, "cgroup.procs", "%d", getpid());

	if (!TST_CGROUP_VER_IS_V1(cg, "memory"))
		SAFE_CGROUP_PRINT(cg, "cgroup.subtree_control", "+memory");
	if (!TST_CGROUP_VER_IS_V1(cg, "cpuset"))
		SAFE_CGROUP_PRINT(cg, "cgroup.subtree_control", "+cpuset");

	if (!SAFE_FORK()) {
		SAFE_CGROUP_PRINTF(cg_child, "cgroup.procs", "%d", getpid());

		if (SAFE_CGROUP_HAS(cg_child, "memory.swap"))
			SAFE_CGROUP_SCANF(cg_child, "memory.swap.current", "%zu", &mem);
		SAFE_CGROUP_READ(cg_child, "cpuset.mems", buf, sizeof(buf));

		// Do something with cpuset.mems and memory.current values
		...

		exit(0);
	}

	tst_reap_children();
	SAFE_CGROUP_PRINTF(cg_drain, "cgroup.procs", "%d", getpid());
	cg_child = tst_cgroup_group_rm(cg_child);
}

static void setup(void)
{
	tst_cgroup_require("memory", NULL);
	tst_cgroup_require("cpuset", NULL);

	cg = tst_cgroup_get_test_group();
	cg_drain = tst_cgroup_get_drain_group();
}

static void cleanup(void)
{
	if (cg_child) {
		SAFE_CGROUP_PRINTF(cg_drain, "cgroup.procs", "%d", getpid());
		cg_child = tst_cgroup_group_rm(cg_child);
	}

	tst_cgroup_cleanup();
}

struct tst_test test = {
	.setup = setup,
	.test_all = run,
	.cleanup = cleanup,
	...
};
-------------------------------------------------------------------------------

Starting with setup; we can see here that we also fetch the 'drain'
CGroup. This is a shared group (between parallel tests) which may
contain processes from other tests. It should have default settings and
these should not be changed by the test. It can be used to remove
processes from other CGroups incase the hierarchy root is not
accessible.

In 'run', we first create a child CGroup with 'tst_cgroup_mk'. As we
create this CGroup in 'run' we should also remove it at the end of
run. We also need to check if it exists and remove it in cleanup as
well. Because there are 'SAFE_' functions which may jump to cleanup.

We then move the main test process into the child CGroup. This is
important as it means that before we destroy the child CGroup we have
to move the main test process elsewhere. For that we use the 'drain'
group.

Next we enable the memory and cpuset controller configuration on the
test CGroup's descendants (i.e. 'cg_child'). This allows each child to
have its own settings. The file 'cgroup.subtree_control' does not
exist on V1. Because it is possible to have both V1 and V2 active at
the same time. We can not simply check if 'subtree_control' exists
before writing to it. We have to check if a particular controller is
on V2 before trying to add it to 'subtree_control'. Trying to add a V1
controller will result in 'ENOENT'.

We then fork a child process and add this to the child CGroup. Within
the child process we try to read 'memory.swap.current'. It is possible
that the memory controller was compiled without swap support, so it is
necessary to check if 'memory.swap' is enabled. That is unless the
test will never reach the point where 'memory.swap.*' are used without
swap support.

The parent process waits for the child process to be reaped before
destroying the child CGroup. So there is no need to transfer the child
to drain. However the parent process must be moved otherwise we will
get 'EBUSY' when trying to remove the child CGroup.

Another example of an edge case is the following.

[source,c]
-------------------------------------------------------------------------------
	if (TST_CGROUP_VER_IS_V1(cg, "memory"))
		SAFE_CGROUP_PRINTF(cg, "memory.swap.max", "%lu", ~0UL);
	else
		SAFE_CGROUP_PRINT(cg, "memory.swap.max", "max");
-------------------------------------------------------------------------------

CGroups V2 introduced a feature where 'memory[.swap].max' could be set to
"max". This does not appear to work on V1 'limit_in_bytes' however. For most
tests, simply using a large number is sufficient and there is no need to use
"max". Importantly though, one should be careful to read both the V1 and V2
kernel docs. The LTP library can not handle all edge cases. It does the minimal
amount of work to make testing on both V1 and V2 feasible.

1.37 Require minimum numbers of CPU for a testcase
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some tests require more than specific number of CPU. It can be defined with
`.min_cpus = N`.

1.38 Test tags
~~~~~~~~~~~~~~

Test tags are name-value pairs that can hold any test metadata.

We have additional support for CVE entries, git commit in mainline kernel,
stable kernel or glibc git repository.  If a test is a regression test it
should include these tags.  They are printed when test fails and exported
into documentation.

CVE, mainline and stable kernel git commits in a regression test for a kernel bug:
[source,c]
-------------------------------------------------------------------------------
struct tst_test test = {
	...
	.tags = (const struct tst_tag[]) {
		{"linux-git", "9392a27d88b9"},
		{"linux-git", "ff002b30181d"},
		{"known-fail", "ustat() is known to fail with EINVAL on Btrfs"},
		{"linux-stable-git", "c4a23c852e80"},
		{"CVE", "2020-29373"},
		{}
	}
};
-------------------------------------------------------------------------------

NOTE: We don't track all backports to stable kernel but just those which are
      stable branch specific (unique), i.e. no commit in mainline. Example of
      commits: c4a23c852e80, cac68d12c531.

Glibc git commit in a regression test for a glibc bug:
[source,c]
-------------------------------------------------------------------------------
struct tst_test test = {
	...
	.tags = (const struct tst_tag[]) {
		{"glibc-git", "574500a108be"},
		{}
	}
};
-------------------------------------------------------------------------------

1.39 Testing on the specific architecture
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Testcases for specific arch should be limited on that only being supported
platform to run, we now involve a .supported_archs to achieve this feature
in LTP library. All you need to run a test on the expected arch is to set
the '.supported_archs' array in the 'struct tst_test' to choose the required
arch list. e.g.

    .supported_archs = (const char *const []){"x86_64", "ppc64", NULL}

This helps move the TCONF info from code to tst_test metadata as well.

And, we also export a struct tst_arch to save the system architecture for
using in the whole test cases.

    extern const struct tst_arch {
             char name[16];
             enum tst_arch_type type;
    } tst_arch;

[source,c]
-------------------------------------------------------------------------------
#include "tst_test.h"

static struct tst_test test = {
       ...
       .setup = setup,
       .supported_archs = (const char *const []) {
                 "x86_64",
                 "ppc64",
                 "s390x",
                 NULL
       },
};
-------------------------------------------------------------------------------

2. Common problems
------------------

This chapter describes common problems/misuses and less obvious design patters
(quirks) in UNIX interfaces. Read it carefully :)

2.1 umask()
~~~~~~~~~~~

I've been hit by this one several times already... When you create files
with 'open()' or 'creat()' etc, the mode specified as the last parameter *is
not* the mode the file is created with. The mode depends on current 'umask()'
settings which may clear some of the bits. If your test depends on specific
file permissions you need either to change umask to 0 or 'chmod()' the file
afterwards or use 'SAFE_TOUCH()' that does the 'chmod()' for you.

2.2 access()
~~~~~~~~~~~~

If 'access(some_file, W_OK)' is executed by root, it will return success even
if the file doesn't have write permission bits set (the same holds for R_OK
too). For sysfs files you can use 'open()' as a workaround to check file
read/write permissions. It might not work for other filesystems, for these you
have to use 'stat()', 'lstat()' or 'fstat()'.

2.3 umount() EBUSY
~~~~~~~~~~~~~~~~~~

Various desktop daemons (gvfsd-trash is known for that) may be stupid enough
to probe all newly mounted filesystem which results in 'umount(2)' failing
with 'EBUSY'; use 'tst_umount()' described in 1.19 that retries in this case
instead of plain 'umount(2)'.

2.4 FILE buffers and fork()
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Be vary that if a process calls 'fork(2)' the child process inherits open
descriptors as well as copy of the parent memory so especially if there are
any open 'FILE' buffers with a data in them they may be written both by the
parent and children resulting in corrupted/duplicated data in the resulting
files.

Also open 'FILE' streams are flushed and closed at 'exit(3)' so if your
program works with 'FILE' streams, does 'fork(2)', and the child may end up
calling 'exit(3)' you will likely end up with corrupted files.

The solution to this problem is either simply call 'fflush(NULL)' that flushes
all open output 'FILE' streams just before doing 'fork(2)'. You may also use
'_exit(2)' in child processes which does not flush 'FILE' buffers and also
skips 'atexit(3)' callbacks.