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 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  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

       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)