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 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:
       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 will not be 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  will  signal  an  error
       condition  to  the  library.  This  will cause the transfer to get aborted and the libcurl
       function used will return CURLE_WRITE_ERROR.

       You can also abort the transfer by returning CURL_WRITEFUNC_ERROR. (7.87.0)

       If your callback function returns CURL_WRITEFUNC_PAUSE it  will  cause  this  transfer  to
       become paused.  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 will write 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 will use 'fwrite' as a callback by default.

PROTOCOLS

       For all protocols

EXAMPLE

       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 == 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};
       CURLcode res;
       CURL *curl_handle = curl_easy_init();

       if (curl_handle)
       {
         /* 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);

         /* send a request */
         res = curl_easy_perform(curl_handle);

         /* remember to free the buffer */
         free(chunk.response);

         curl_easy_cleanup(curl_handle);
       }

AVAILABILITY

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

RETURN VALUE

       This will return CURLE_OK.

SEE ALSO

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