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 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.
       ptr points to the delivered data, and the size of that data is nmemb; size is always 1.

       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  be  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.

       The data passed to this function is not null-terminated!

       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 instead 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

       libcurl uses 'fwrite' as a callback by default.

PROTOCOLS

       For all protocols

EXAMPLE

       #include <stdlib.h> /* for realloc */
       #include <string.h> /* for memcpy */

       struct memory {
         char *response;
         size_t size;
       };

       static size_t cb(void *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);
         }
       }

AVAILABILITY

       Support for the CURL_WRITEFUNC_PAUSE return code was added in version 7.18.0.

RETURN VALUE

       This returns CURLE_OK.

SEE ALSO

       CURLOPT_WRITEDATA(3), CURLOPT_READFUNCTION(3), CURLOPT_HEADERFUNCTION(3)

ibcurl 8.5.0                                    December 04, 2023                       CURLOPT_WRITEFUNCTION(3)