docs/libcurl/opts/CURLOPT_WRITEFUNCTION.3

.\" **************************************************************************
.\" *                                  _   _ ____  _
.\" *  Project                     ___| | | |  _ \| |
.\" *                             / __| | | | |_) | |
.\" *                            | (__| |_| |  _ <| |___
.\" *                             \___|\___/|_| \_\_____|
.\" *
.\" * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
.\" *
.\" * This software is licensed as described in the file COPYING, which
.\" * you should have received as part of this distribution. The terms
.\" * are also available at https://curl.se/docs/copyright.html.
.\" *
.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
.\" * copies of the Software, and permit persons to whom the Software is
.\" * furnished to do so, under the terms of the COPYING file.
.\" *
.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
.\" * KIND, either express or implied.
.\" *
.\" **************************************************************************
.\"
.TH CURLOPT_WRITEFUNCTION 3 "June 09, 2021" "libcurl 7.78.0" "curl_easy_setopt options"

.SH NAME
CURLOPT_WRITEFUNCTION \- set callback for writing received data
.SH SYNOPSIS
.nf
#include <curl/curl.h>

size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_WRITEFUNCTION, write_callback);
.SH DESCRIPTION
Pass a pointer to your callback function, which should match the prototype
shown above.

This callback function gets called by libcurl as soon as there is data
received that needs to be saved. For most transfers, this callback gets called
many times and each invoke delivers another chunk of data. \fIptr\fP points to
the delivered data, and the size of that data is \fInmemb\fP; \fIsize\fP is
always 1.

The callback function will be passed as much data as possible in all invokes,
but you must not make any assumptions. It may be one byte, it may be
thousands. The maximum amount of body data that will be passed to the write
callback is defined in the curl.h header file: \fICURL_MAX_WRITE_SIZE\fP (the
usual default is 16K). If \fICURLOPT_HEADER(3)\fP is enabled, which makes
header data get passed to the write callback, you can get up to
\fICURL_MAX_HTTP_HEADER\fP bytes of header data passed into it. This usually
means 100K.

This function may be called with zero bytes data if the transferred file is
empty.

The data passed to this function will not be null-terminated!

Set the \fIuserdata\fP argument with the \fICURLOPT_WRITEDATA(3)\fP option.

Your callback should return the number of bytes actually taken care of. If
that amount differs from the amount passed to your callback function, it'll
signal an error condition to the library. This will cause the transfer to get
aborted and the libcurl function used will return \fICURLE_WRITE_ERROR\fP.

If your callback function returns CURL_WRITEFUNC_PAUSE it will cause this
transfer to become paused.  See \fIcurl_easy_pause(3)\fP for further details.

Set this option to NULL to get the internal default function used instead of
your callback. The internal default function will write the data to the FILE *
given with \fICURLOPT_WRITEDATA(3)\fP.

This option doesn't enable HSTS, you need to use \fICURLOPT_HSTS_CTRL(3)\fP to
do that.
.SH DEFAULT
libcurl will use 'fwrite' as a callback by default.
.SH PROTOCOLS
For all protocols
.SH AVAILABILITY
Support for the CURL_WRITEFUNC_PAUSE return code was added in version 7.18.0.
.SH RETURN VALUE
This will return CURLE_OK.
.SH EXAMPLE
.nf
 struct memory {
   char *response;
   size_t size;
 };

 static size_t cb(void *data, size_t size, size_t nmemb, void *userp)
 {
   size_t realsize = size * nmemb;
   struct memory *mem = (struct memory *)userp;

   char *ptr = realloc(mem->response, mem->size + realsize + 1);
   if(ptr == NULL)
     return 0;  /* out of memory! */

   mem->response = ptr;
   memcpy(&(mem->response[mem->size]), data, realsize);
   mem->size += realsize;
   mem->response[mem->size] = 0;

   return realsize;
 }

 struct memory chunk = {0};

 /* send all data to this function  */
 curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, cb);

 /* we pass our 'chunk' struct to the callback function */
 curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);
.fi
.SH "SEE ALSO"
.BR CURLOPT_WRITEDATA "(3), " CURLOPT_READFUNCTION "(3), "
.BR CURLOPT_HEADERFUNCTION "(3), "