xref: /third_party/python/Modules/_multiprocessing/semaphore.c

/*
 * A type which wraps a semaphore
 *
 * semaphore.c
 *
 * Copyright (c) 2006-2008, R Oudkerk
 * Licensed to PSF under a Contributor Agreement.
 */

#include "multiprocessing.h"

#ifdef HAVE_MP_SEMAPHORE

enum { RECURSIVE_MUTEX, SEMAPHORE };

typedef struct {
    PyObject_HEAD
    SEM_HANDLE handle;
    unsigned long last_tid;
    int count;
    int maxvalue;
    int kind;
    char *name;
} SemLockObject;

/*[python input]
class SEM_HANDLE_converter(CConverter):
    type = "SEM_HANDLE"
    format_unit = '"F_SEM_HANDLE"'

[python start generated code]*/
/*[python end generated code: output=da39a3ee5e6b4b0d input=3e0ad43e482d8716]*/

/*[clinic input]
module _multiprocessing
class _multiprocessing.SemLock "SemLockObject *" "&_PyMp_SemLockType"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=935fb41b7d032599]*/

#include "clinic/semaphore.c.h"

#define ISMINE(o) (o->count > 0 && PyThread_get_thread_ident() == o->last_tid)


#ifdef MS_WINDOWS

/*
 * Windows definitions
 */

#define SEM_FAILED NULL

#define SEM_CLEAR_ERROR() SetLastError(0)
#define SEM_GET_LAST_ERROR() GetLastError()
#define SEM_CREATE(name, val, max) CreateSemaphore(NULL, val, max, NULL)
#define SEM_CLOSE(sem) (CloseHandle(sem) ? 0 : -1)
#define SEM_GETVALUE(sem, pval) _GetSemaphoreValue(sem, pval)
#define SEM_UNLINK(name) 0

static int
_GetSemaphoreValue(HANDLE handle, long *value)
{
    long previous;

    switch (WaitForSingleObjectEx(handle, 0, FALSE)) {
    case WAIT_OBJECT_0:
        if (!ReleaseSemaphore(handle, 1, &previous))
            return MP_STANDARD_ERROR;
        *value = previous + 1;
        return 0;
    case WAIT_TIMEOUT:
        *value = 0;
        return 0;
    default:
        return MP_STANDARD_ERROR;
    }
}

/*[clinic input]
_multiprocessing.SemLock.acquire

    block as blocking: bool(accept={int}) = True
    timeout as timeout_obj: object = None

Acquire the semaphore/lock.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock_acquire_impl(SemLockObject *self, int blocking,
                                      PyObject *timeout_obj)
/*[clinic end generated code: output=f9998f0b6b0b0872 input=86f05662cf753eb4]*/
{
    double timeout;
    DWORD res, full_msecs, nhandles;
    HANDLE handles[2], sigint_event;

    /* calculate timeout */
    if (!blocking) {
        full_msecs = 0;
    } else if (timeout_obj == Py_None) {
        full_msecs = INFINITE;
    } else {
        timeout = PyFloat_AsDouble(timeout_obj);
        if (PyErr_Occurred())
            return NULL;
        timeout *= 1000.0;      /* convert to millisecs */
        if (timeout < 0.0) {
            timeout = 0.0;
        } else if (timeout >= 0.5 * INFINITE) { /* 25 days */
            PyErr_SetString(PyExc_OverflowError,
                            "timeout is too large");
            return NULL;
        }
        full_msecs = (DWORD)(timeout + 0.5);
    }

    /* check whether we already own the lock */
    if (self->kind == RECURSIVE_MUTEX && ISMINE(self)) {
        ++self->count;
        Py_RETURN_TRUE;
    }

    /* check whether we can acquire without releasing the GIL and blocking */
    if (WaitForSingleObjectEx(self->handle, 0, FALSE) == WAIT_OBJECT_0) {
        self->last_tid = GetCurrentThreadId();
        ++self->count;
        Py_RETURN_TRUE;
    }

    /* prepare list of handles */
    nhandles = 0;
    handles[nhandles++] = self->handle;
    if (_PyOS_IsMainThread()) {
        sigint_event = _PyOS_SigintEvent();
        assert(sigint_event != NULL);
        handles[nhandles++] = sigint_event;
    }
    else {
        sigint_event = NULL;
    }

    /* do the wait */
    Py_BEGIN_ALLOW_THREADS
    if (sigint_event != NULL)
        ResetEvent(sigint_event);
    res = WaitForMultipleObjectsEx(nhandles, handles, FALSE, full_msecs, FALSE);
    Py_END_ALLOW_THREADS

    /* handle result */
    switch (res) {
    case WAIT_TIMEOUT:
        Py_RETURN_FALSE;
    case WAIT_OBJECT_0 + 0:
        self->last_tid = GetCurrentThreadId();
        ++self->count;
        Py_RETURN_TRUE;
    case WAIT_OBJECT_0 + 1:
        errno = EINTR;
        return PyErr_SetFromErrno(PyExc_OSError);
    case WAIT_FAILED:
        return PyErr_SetFromWindowsErr(0);
    default:
        PyErr_Format(PyExc_RuntimeError, "WaitForSingleObject() or "
                     "WaitForMultipleObjects() gave unrecognized "
                     "value %u", res);
        return NULL;
    }
}

/*[clinic input]
_multiprocessing.SemLock.release

Release the semaphore/lock.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock_release_impl(SemLockObject *self)
/*[clinic end generated code: output=b22f53ba96b0d1db input=ba7e63a961885d3d]*/
{
    if (self->kind == RECURSIVE_MUTEX) {
        if (!ISMINE(self)) {
            PyErr_SetString(PyExc_AssertionError, "attempt to "
                            "release recursive lock not owned "
                            "by thread");
            return NULL;
        }
        if (self->count > 1) {
            --self->count;
            Py_RETURN_NONE;
        }
        assert(self->count == 1);
    }

    if (!ReleaseSemaphore(self->handle, 1, NULL)) {
        if (GetLastError() == ERROR_TOO_MANY_POSTS) {
            PyErr_SetString(PyExc_ValueError, "semaphore or lock "
                            "released too many times");
            return NULL;
        } else {
            return PyErr_SetFromWindowsErr(0);
        }
    }

    --self->count;
    Py_RETURN_NONE;
}

#else /* !MS_WINDOWS */

/*
 * Unix definitions
 */

#define SEM_CLEAR_ERROR()
#define SEM_GET_LAST_ERROR() 0
#define SEM_CREATE(name, val, max) sem_open(name, O_CREAT | O_EXCL, 0600, val)
#define SEM_CLOSE(sem) sem_close(sem)
#define SEM_GETVALUE(sem, pval) sem_getvalue(sem, pval)
#define SEM_UNLINK(name) sem_unlink(name)

/* OS X 10.4 defines SEM_FAILED as -1 instead of (sem_t *)-1;  this gives
   compiler warnings, and (potentially) undefined behaviour. */
#ifdef __APPLE__
#  undef SEM_FAILED
#  define SEM_FAILED ((sem_t *)-1)
#endif

#ifndef HAVE_SEM_UNLINK
#  define sem_unlink(name) 0
#endif

#ifndef HAVE_SEM_TIMEDWAIT
#  define sem_timedwait(sem,deadline) sem_timedwait_save(sem,deadline,_save)

static int
sem_timedwait_save(sem_t *sem, struct timespec *deadline, PyThreadState *_save)
{
    int res;
    unsigned long delay, difference;
    struct timeval now, tvdeadline, tvdelay;

    errno = 0;
    tvdeadline.tv_sec = deadline->tv_sec;
    tvdeadline.tv_usec = deadline->tv_nsec / 1000;

    for (delay = 0 ; ; delay += 1000) {
        /* poll */
        if (sem_trywait(sem) == 0)
            return 0;
        else if (errno != EAGAIN)
            return MP_STANDARD_ERROR;

        /* get current time */
        if (gettimeofday(&now, NULL) < 0)
            return MP_STANDARD_ERROR;

        /* check for timeout */
        if (tvdeadline.tv_sec < now.tv_sec ||
            (tvdeadline.tv_sec == now.tv_sec &&
             tvdeadline.tv_usec <= now.tv_usec)) {
            errno = ETIMEDOUT;
            return MP_STANDARD_ERROR;
        }

        /* calculate how much time is left */
        difference = (tvdeadline.tv_sec - now.tv_sec) * 1000000 +
            (tvdeadline.tv_usec - now.tv_usec);

        /* check delay not too long -- maximum is 20 msecs */
        if (delay > 20000)
            delay = 20000;
        if (delay > difference)
            delay = difference;

        /* sleep */
        tvdelay.tv_sec = delay / 1000000;
        tvdelay.tv_usec = delay % 1000000;
        if (select(0, NULL, NULL, NULL, &tvdelay) < 0)
            return MP_STANDARD_ERROR;

        /* check for signals */
        Py_BLOCK_THREADS
        res = PyErr_CheckSignals();
        Py_UNBLOCK_THREADS

        if (res) {
            errno = EINTR;
            return MP_EXCEPTION_HAS_BEEN_SET;
        }
    }
}

#endif /* !HAVE_SEM_TIMEDWAIT */

/*[clinic input]
_multiprocessing.SemLock.acquire

    block as blocking: bool(accept={int}) = True
    timeout as timeout_obj: object = None

Acquire the semaphore/lock.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock_acquire_impl(SemLockObject *self, int blocking,
                                      PyObject *timeout_obj)
/*[clinic end generated code: output=f9998f0b6b0b0872 input=86f05662cf753eb4]*/
{
    int res, err = 0;
    struct timespec deadline = {0};

    if (self->kind == RECURSIVE_MUTEX && ISMINE(self)) {
        ++self->count;
        Py_RETURN_TRUE;
    }

    int use_deadline = (timeout_obj != Py_None);
    if (use_deadline) {
        double timeout = PyFloat_AsDouble(timeout_obj);
        if (PyErr_Occurred()) {
            return NULL;
        }
        if (timeout < 0.0) {
            timeout = 0.0;
        }

        struct timeval now;
        if (gettimeofday(&now, NULL) < 0) {
            PyErr_SetFromErrno(PyExc_OSError);
            return NULL;
        }
        long sec = (long) timeout;
        long nsec = (long) (1e9 * (timeout - sec) + 0.5);
        deadline.tv_sec = now.tv_sec + sec;
        deadline.tv_nsec = now.tv_usec * 1000 + nsec;
        deadline.tv_sec += (deadline.tv_nsec / 1000000000);
        deadline.tv_nsec %= 1000000000;
    }

    /* Check whether we can acquire without releasing the GIL and blocking */
    do {
        res = sem_trywait(self->handle);
        err = errno;
    } while (res < 0 && errno == EINTR && !PyErr_CheckSignals());
    errno = err;

    if (res < 0 && errno == EAGAIN && blocking) {
        /* Couldn't acquire immediately, need to block */
        do {
            Py_BEGIN_ALLOW_THREADS
            if (!use_deadline) {
                res = sem_wait(self->handle);
            }
            else {
                res = sem_timedwait(self->handle, &deadline);
            }
            Py_END_ALLOW_THREADS
            err = errno;
            if (res == MP_EXCEPTION_HAS_BEEN_SET)
                break;
        } while (res < 0 && errno == EINTR && !PyErr_CheckSignals());
    }

    if (res < 0) {
        errno = err;
        if (errno == EAGAIN || errno == ETIMEDOUT)
            Py_RETURN_FALSE;
        else if (errno == EINTR)
            return NULL;
        else
            return PyErr_SetFromErrno(PyExc_OSError);
    }

    ++self->count;
    self->last_tid = PyThread_get_thread_ident();

    Py_RETURN_TRUE;
}

/*[clinic input]
_multiprocessing.SemLock.release

Release the semaphore/lock.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock_release_impl(SemLockObject *self)
/*[clinic end generated code: output=b22f53ba96b0d1db input=ba7e63a961885d3d]*/
{
    if (self->kind == RECURSIVE_MUTEX) {
        if (!ISMINE(self)) {
            PyErr_SetString(PyExc_AssertionError, "attempt to "
                            "release recursive lock not owned "
                            "by thread");
            return NULL;
        }
        if (self->count > 1) {
            --self->count;
            Py_RETURN_NONE;
        }
        assert(self->count == 1);
    } else {
#ifdef HAVE_BROKEN_SEM_GETVALUE
        /* We will only check properly the maxvalue == 1 case */
        if (self->maxvalue == 1) {
            /* make sure that already locked */
            if (sem_trywait(self->handle) < 0) {
                if (errno != EAGAIN) {
                    PyErr_SetFromErrno(PyExc_OSError);
                    return NULL;
                }
                /* it is already locked as expected */
            } else {
                /* it was not locked so undo wait and raise  */
                if (sem_post(self->handle) < 0) {
                    PyErr_SetFromErrno(PyExc_OSError);
                    return NULL;
                }
                PyErr_SetString(PyExc_ValueError, "semaphore "
                                "or lock released too many "
                                "times");
                return NULL;
            }
        }
#else
        int sval;

        /* This check is not an absolute guarantee that the semaphore
           does not rise above maxvalue. */
        if (sem_getvalue(self->handle, &sval) < 0) {
            return PyErr_SetFromErrno(PyExc_OSError);
        } else if (sval >= self->maxvalue) {
            PyErr_SetString(PyExc_ValueError, "semaphore or lock "
                            "released too many times");
            return NULL;
        }
#endif
    }

    if (sem_post(self->handle) < 0)
        return PyErr_SetFromErrno(PyExc_OSError);

    --self->count;
    Py_RETURN_NONE;
}

#endif /* !MS_WINDOWS */

/*
 * All platforms
 */

static PyObject *
newsemlockobject(PyTypeObject *type, SEM_HANDLE handle, int kind, int maxvalue,
                 char *name)
{
    SemLockObject *self = (SemLockObject *)type->tp_alloc(type, 0);
    if (!self)
        return NULL;
    self->handle = handle;
    self->kind = kind;
    self->count = 0;
    self->last_tid = 0;
    self->maxvalue = maxvalue;
    self->name = name;
    return (PyObject*)self;
}

/*[clinic input]
@classmethod
_multiprocessing.SemLock.__new__

    kind: int
    value: int
    maxvalue: int
    name: str
    unlink: bool(accept={int})

[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock_impl(PyTypeObject *type, int kind, int value,
                              int maxvalue, const char *name, int unlink)
/*[clinic end generated code: output=30727e38f5f7577a input=b378c3ee27d3a0fa]*/
{
    SEM_HANDLE handle = SEM_FAILED;
    PyObject *result;
    char *name_copy = NULL;

    if (kind != RECURSIVE_MUTEX && kind != SEMAPHORE) {
        PyErr_SetString(PyExc_ValueError, "unrecognized kind");
        return NULL;
    }

    if (!unlink) {
        name_copy = PyMem_Malloc(strlen(name) + 1);
        if (name_copy == NULL) {
            return PyErr_NoMemory();
        }
        strcpy(name_copy, name);
    }

    SEM_CLEAR_ERROR();
    handle = SEM_CREATE(name, value, maxvalue);
    /* On Windows we should fail if GetLastError()==ERROR_ALREADY_EXISTS */
    if (handle == SEM_FAILED || SEM_GET_LAST_ERROR() != 0)
        goto failure;

    if (unlink && SEM_UNLINK(name) < 0)
        goto failure;

    result = newsemlockobject(type, handle, kind, maxvalue, name_copy);
    if (!result)
        goto failure;

    return result;

  failure:
    if (handle != SEM_FAILED)
        SEM_CLOSE(handle);
    PyMem_Free(name_copy);
    if (!PyErr_Occurred()) {
        _PyMp_SetError(NULL, MP_STANDARD_ERROR);
    }
    return NULL;
}

/*[clinic input]
@classmethod
_multiprocessing.SemLock._rebuild

    handle: SEM_HANDLE
    kind: int
    maxvalue: int
    name: str(accept={str, NoneType})
    /

[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock__rebuild_impl(PyTypeObject *type, SEM_HANDLE handle,
                                       int kind, int maxvalue,
                                       const char *name)
/*[clinic end generated code: output=2aaee14f063f3bd9 input=f7040492ac6d9962]*/
{
    char *name_copy = NULL;

    if (name != NULL) {
        name_copy = PyMem_Malloc(strlen(name) + 1);
        if (name_copy == NULL)
            return PyErr_NoMemory();
        strcpy(name_copy, name);
    }

#ifndef MS_WINDOWS
    if (name != NULL) {
        handle = sem_open(name, 0);
        if (handle == SEM_FAILED) {
            PyMem_Free(name_copy);
            return PyErr_SetFromErrno(PyExc_OSError);
        }
    }
#endif

    return newsemlockobject(type, handle, kind, maxvalue, name_copy);
}

static void
semlock_dealloc(SemLockObject* self)
{
    if (self->handle != SEM_FAILED)
        SEM_CLOSE(self->handle);
    PyMem_Free(self->name);
    Py_TYPE(self)->tp_free((PyObject*)self);
}

/*[clinic input]
_multiprocessing.SemLock._count

Num of `acquire()`s minus num of `release()`s for this process.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock__count_impl(SemLockObject *self)
/*[clinic end generated code: output=5ba8213900e517bb input=36fc59b1cd1025ab]*/
{
    return PyLong_FromLong((long)self->count);
}

/*[clinic input]
_multiprocessing.SemLock._is_mine

Whether the lock is owned by this thread.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock__is_mine_impl(SemLockObject *self)
/*[clinic end generated code: output=92dc98863f4303be input=a96664cb2f0093ba]*/
{
    /* only makes sense for a lock */
    return PyBool_FromLong(ISMINE(self));
}

/*[clinic input]
_multiprocessing.SemLock._get_value

Get the value of the semaphore.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock__get_value_impl(SemLockObject *self)
/*[clinic end generated code: output=64bc1b89bda05e36 input=cb10f9a769836203]*/
{
#ifdef HAVE_BROKEN_SEM_GETVALUE
    PyErr_SetNone(PyExc_NotImplementedError);
    return NULL;
#else
    int sval;
    if (SEM_GETVALUE(self->handle, &sval) < 0)
        return _PyMp_SetError(NULL, MP_STANDARD_ERROR);
    /* some posix implementations use negative numbers to indicate
       the number of waiting threads */
    if (sval < 0)
        sval = 0;
    return PyLong_FromLong((long)sval);
#endif
}

/*[clinic input]
_multiprocessing.SemLock._is_zero

Return whether semaphore has value zero.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock__is_zero_impl(SemLockObject *self)
/*[clinic end generated code: output=815d4c878c806ed7 input=294a446418d31347]*/
{
#ifdef HAVE_BROKEN_SEM_GETVALUE
    if (sem_trywait(self->handle) < 0) {
        if (errno == EAGAIN)
            Py_RETURN_TRUE;
        return _PyMp_SetError(NULL, MP_STANDARD_ERROR);
    } else {
        if (sem_post(self->handle) < 0)
            return _PyMp_SetError(NULL, MP_STANDARD_ERROR);
        Py_RETURN_FALSE;
    }
#else
    int sval;
    if (SEM_GETVALUE(self->handle, &sval) < 0)
        return _PyMp_SetError(NULL, MP_STANDARD_ERROR);
    return PyBool_FromLong((long)sval == 0);
#endif
}

/*[clinic input]
_multiprocessing.SemLock._after_fork

Rezero the net acquisition count after fork().
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock__after_fork_impl(SemLockObject *self)
/*[clinic end generated code: output=718bb27914c6a6c1 input=190991008a76621e]*/
{
    self->count = 0;
    Py_RETURN_NONE;
}

/*[clinic input]
_multiprocessing.SemLock.__enter__

Enter the semaphore/lock.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock___enter___impl(SemLockObject *self)
/*[clinic end generated code: output=beeb2f07c858511f input=c5e27d594284690b]*/
{
    return _multiprocessing_SemLock_acquire_impl(self, 1, Py_None);
}

/*[clinic input]
_multiprocessing.SemLock.__exit__

    exc_type: object = None
    exc_value: object = None
    exc_tb: object = None
    /

Exit the semaphore/lock.
[clinic start generated code]*/

static PyObject *
_multiprocessing_SemLock___exit___impl(SemLockObject *self,
                                       PyObject *exc_type,
                                       PyObject *exc_value, PyObject *exc_tb)
/*[clinic end generated code: output=3b37c1a9f8b91a03 input=7d644b64a89903f8]*/
{
    return _multiprocessing_SemLock_release_impl(self);
}

/*
 * Semaphore methods
 */

static PyMethodDef semlock_methods[] = {
    _MULTIPROCESSING_SEMLOCK_ACQUIRE_METHODDEF
    _MULTIPROCESSING_SEMLOCK_RELEASE_METHODDEF
    _MULTIPROCESSING_SEMLOCK___ENTER___METHODDEF
    _MULTIPROCESSING_SEMLOCK___EXIT___METHODDEF
    _MULTIPROCESSING_SEMLOCK__COUNT_METHODDEF
    _MULTIPROCESSING_SEMLOCK__IS_MINE_METHODDEF
    _MULTIPROCESSING_SEMLOCK__GET_VALUE_METHODDEF
    _MULTIPROCESSING_SEMLOCK__IS_ZERO_METHODDEF
    _MULTIPROCESSING_SEMLOCK__REBUILD_METHODDEF
    _MULTIPROCESSING_SEMLOCK__AFTER_FORK_METHODDEF
    {NULL}
};

/*
 * Member table
 */

static PyMemberDef semlock_members[] = {
    {"handle", T_SEM_HANDLE, offsetof(SemLockObject, handle), READONLY,
     ""},
    {"kind", T_INT, offsetof(SemLockObject, kind), READONLY,
     ""},
    {"maxvalue", T_INT, offsetof(SemLockObject, maxvalue), READONLY,
     ""},
    {"name", T_STRING, offsetof(SemLockObject, name), READONLY,
     ""},
    {NULL}
};

/*
 * Semaphore type
 */

PyTypeObject _PyMp_SemLockType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    /* tp_name           */ "_multiprocessing.SemLock",
    /* tp_basicsize      */ sizeof(SemLockObject),
    /* tp_itemsize       */ 0,
    /* tp_dealloc        */ (destructor)semlock_dealloc,
    /* tp_vectorcall_offset */ 0,
    /* tp_getattr        */ 0,
    /* tp_setattr        */ 0,
    /* tp_as_async       */ 0,
    /* tp_repr           */ 0,
    /* tp_as_number      */ 0,
    /* tp_as_sequence    */ 0,
    /* tp_as_mapping     */ 0,
    /* tp_hash           */ 0,
    /* tp_call           */ 0,
    /* tp_str            */ 0,
    /* tp_getattro       */ 0,
    /* tp_setattro       */ 0,
    /* tp_as_buffer      */ 0,
    /* tp_flags          */ Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
    /* tp_doc            */ "Semaphore/Mutex type",
    /* tp_traverse       */ 0,
    /* tp_clear          */ 0,
    /* tp_richcompare    */ 0,
    /* tp_weaklistoffset */ 0,
    /* tp_iter           */ 0,
    /* tp_iternext       */ 0,
    /* tp_methods        */ semlock_methods,
    /* tp_members        */ semlock_members,
    /* tp_getset         */ 0,
    /* tp_base           */ 0,
    /* tp_dict           */ 0,
    /* tp_descr_get      */ 0,
    /* tp_descr_set      */ 0,
    /* tp_dictoffset     */ 0,
    /* tp_init           */ 0,
    /* tp_alloc          */ 0,
    /* tp_new            */ _multiprocessing_SemLock,
};

/*
 * Function to unlink semaphore names
 */

PyObject *
_PyMp_sem_unlink(const char *name)
{
    if (SEM_UNLINK(name) < 0) {
        _PyMp_SetError(NULL, MP_STANDARD_ERROR);
        return NULL;
    }

    Py_RETURN_NONE;
}

#endif // HAVE_MP_SEMAPHORE