1---
2c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
3SPDX-License-Identifier: curl
4Title: CURLOPT_WRITEFUNCTION
5Section: 3
6Source: libcurl
7See-also:
8  - CURLOPT_HEADERFUNCTION (3)
9  - CURLOPT_READFUNCTION (3)
10  - CURLOPT_WRITEDATA (3)
11---
12
13# NAME
14
15CURLOPT_WRITEFUNCTION - callback for writing received data
16
17# SYNOPSIS
18
19~~~c
20#include <curl/curl.h>
21
22size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata);
23
24CURLcode curl_easy_setopt(CURL *handle, CURLOPT_WRITEFUNCTION, write_callback);
25~~~
26
27# DESCRIPTION
28
29Pass a pointer to your callback function, which should match the prototype
30shown above.
31
32This callback function gets called by libcurl as soon as there is data
33received that needs to be saved. For most transfers, this callback gets called
34many times and each invoke delivers another chunk of data. *ptr* points to the
35delivered data, and the size of that data is *nmemb*; *size* is always 1.
36
37The data passed to this function is not null-terminated.
38
39The callback function is passed as much data as possible in all invokes, but
40you must not make any assumptions. It may be one byte, it may be
41thousands. The maximum amount of body data that is be passed to the write
42callback is defined in the curl.h header file: *CURL_MAX_WRITE_SIZE* (the
43usual default is 16K). If CURLOPT_HEADER(3) is enabled, which makes header
44data get passed to the write callback, you can get up to
45*CURL_MAX_HTTP_HEADER* bytes of header data passed into it. This usually means
46100K.
47
48This function may be called with zero bytes data if the transferred file is
49empty.
50
51Set the *userdata* argument with the CURLOPT_WRITEDATA(3) option.
52
53Your callback should return the number of bytes actually taken care of. If
54that amount differs from the amount passed to your callback function, it
55signals an error condition to the library. This causes the transfer to get
56aborted and the libcurl function used returns *CURLE_WRITE_ERROR*.
57
58You can also abort the transfer by returning CURL_WRITEFUNC_ERROR (added in
597.87.0), which makes *CURLE_WRITE_ERROR* get returned.
60
61If the callback function returns CURL_WRITEFUNC_PAUSE it pauses this
62transfer. See curl_easy_pause(3) for further details.
63
64Set this option to NULL to get the internal default function used instead of
65your callback. The internal default function writes the data to the FILE *
66given with CURLOPT_WRITEDATA(3).
67
68This option does not enable HSTS, you need to use CURLOPT_HSTS_CTRL(3) to
69do that.
70
71# DEFAULT
72
73libcurl uses 'fwrite' as a callback by default.
74
75# PROTOCOLS
76
77For all protocols
78
79# EXAMPLE
80
81~~~c
82#include <stdlib.h> /* for realloc */
83#include <string.h> /* for memcpy */
84
85struct memory {
86  char *response;
87  size_t size;
88};
89
90static size_t cb(void *data, size_t size, size_t nmemb, void *clientp)
91{
92  size_t realsize = size * nmemb;
93  struct memory *mem = (struct memory *)clientp;
94
95  char *ptr = realloc(mem->response, mem->size + realsize + 1);
96  if(!ptr)
97    return 0;  /* out of memory! */
98
99  mem->response = ptr;
100  memcpy(&(mem->response[mem->size]), data, realsize);
101  mem->size += realsize;
102  mem->response[mem->size] = 0;
103
104  return realsize;
105}
106
107int main(void)
108{
109  struct memory chunk = {0};
110  CURLcode res;
111  CURL *curl = curl_easy_init();
112  if(curl) {
113    /* send all data to this function  */
114    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, cb);
115
116    /* we pass our 'chunk' struct to the callback function */
117    curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)&chunk);
118
119    /* send a request */
120    res = curl_easy_perform(curl);
121
122    /* remember to free the buffer */
123    free(chunk.response);
124
125    curl_easy_cleanup(curl);
126  }
127}
128~~~
129
130# AVAILABILITY
131
132Support for the CURL_WRITEFUNC_PAUSE return code was added in version 7.18.0.
133
134# RETURN VALUE
135
136This returns CURLE_OK.
137