CURLOPT_WRITEFUNCTION(3) Library Functions Manual CURLOPT_WRITEFUNCTION(3)
NAME
CURLOPT_WRITEFUNCTION - callback for writing received data
SYNOPSIS
#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);
DESCRIPTION
Pass a pointer to your callback function, which should match the proto-
type 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. ptr
points to the delivered data, and the size of that data is nmemb; size
is always 1.
The data passed to this function is not null-terminated.
The callback function is 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 is passed to the write
callback is defined in the curl.h header file: CURL_MAX_WRITE_SIZE (the
usual default is 16K). If CURLOPT_HEADER(3) is enabled, which makes
header data get passed to the write callback, you can get up to
CURL_MAX_HTTP_HEADER 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.
Set the userdata argument with the CURLOPT_WRITEDATA(3) 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 signals an error condition to the library. This causes the transfer
to get aborted and the libcurl function used returns CURLE_WRITE_ERROR.
You can also abort the transfer by returning CURL_WRITEFUNC_ERROR (added
in 7.87.0), which makes CURLE_WRITE_ERROR get returned.
If the callback function returns CURL_WRITEFUNC_PAUSE it pauses this
transfer. See curl_easy_pause(3) for further details.
Set this option to NULL to get the internal default function used in-
stead of your callback. The internal default function writes the data to
the FILE * given with CURLOPT_WRITEDATA(3).
This option does not enable HSTS, you need to use CURLOPT_HSTS_CTRL(3)
to do that.
DEFAULT
fwrite(3)
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
#include <stdlib.h> /* for realloc */
#include <string.h> /* for memcpy */
struct memory {
char *response;
size_t size;
};
static size_t cb(char *data, size_t size, size_t nmemb, void *clientp)
{
size_t realsize = size * nmemb;
struct memory *mem = (struct memory *)clientp;
char *ptr = realloc(mem->response, mem->size + realsize + 1);
if(!ptr)
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;
}
int main(void)
{
struct memory chunk = {0};
CURLcode res;
CURL *curl = curl_easy_init();
if(curl) {
/* send all data to this function */
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, cb);
/* we pass our 'chunk' struct to the callback function */
curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)&chunk);
/* send a request */
res = curl_easy_perform(curl);
/* remember to free the buffer */
free(chunk.response);
curl_easy_cleanup(curl);
}
}
HISTORY
Support for the CURL_WRITEFUNC_PAUSE return code was added in version
7.18.0.
AVAILABILITY
Added in curl 7.1
RETURN VALUE
This returns CURLE_OK.
SEE ALSO
CURLOPT_HEADERFUNCTION(3), CURLOPT_READFUNCTION(3), CURLOPT_WRITEDATA(3)
libcurl 2026-07-12 CURLOPT_WRITEFUNCTION(3)
Generated by dwww version 1.16 on Sat Oct 3 05:00:29 CEST 2026.