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CURLOPT_WRITEFUNCTION(3)            libcurl           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
       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),

ibcurl 8.1.2                    April 26, 2023        CURLOPT_WRITEFUNCTION(3)