NAME

       CURLOPT_READFUNCTION - read callback for data uploads

SYNOPSIS

       #include <curl/curl.h>

       size_t read_callback(char *buffer, size_t size, size_t nitems, void *userdata);

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READFUNCTION, read_callback);

DESCRIPTION

       Pass a pointer to your callback function, as the prototype shows above.

       This callback function gets called by libcurl as soon as it needs to read data in order to send it to the
       peer - like if you ask it to upload or post data to the server. The data area pointed at by  the  pointer
       buffer should be filled up with at most size multiplied with nitems number of bytes by your function.

       Set the userdata argument with the CURLOPT_READDATA(3) option.

       Your  function  must  return the actual number of bytes that it stored in the data area pointed at by the
       pointer buffer. Returning 0 will signal end-of-file to the library and  cause  it  to  stop  the  current
       transfer.

       If  you  stop the current transfer by returning 0 "pre-maturely" (i.e before the server expected it, like
       when you've said you will upload N bytes and you upload less than N bytes), you may experience  that  the
       server "hangs" waiting for the rest of the data that won't come.

       The  read callback may return CURL_READFUNC_ABORT to stop the current operation immediately, resulting in
       a CURLE_ABORTED_BY_CALLBACK error code from the transfer.

       The callback can return  CURL_READFUNC_PAUSE  to  cause  reading  from  this  connection  to  pause.  See
       curl_easy_pause(3) for further details.

       Bugs:  when  doing  TFTP uploads, you must return the exact amount of data that the callback wants, or it
       will be considered the final packet by the server end and the transfer will end there.

       If you set this callback pointer to NULL, or don't set it at all, the default internal read function will
       be used. It is doing an fread() on the FILE * userdata set with CURLOPT_READDATA(3).

DEFAULT

       The default internal read callback is fread().

PROTOCOLS

       This is used for all protocols when doing uploads.

EXAMPLE

       size_t read_callback(void *ptr, size_t size, size_t nmemb, void *userdata)
       {
         FILE *readhere = (FILE *)userdata;
         curl_off_t nread;

         /* copy as much data as possible into the 'ptr' buffer, but no more than
            'size' * 'nmemb' bytes! */
         size_t retcode = fread(ptr, size, nmemb, readhere);

         nread = (curl_off_t)retcode;

         fprintf(stderr, "*** We read %" CURL_FORMAT_CURL_OFF_T
                 " bytes from file\n", nread);
         return retcode;
       }

       void setup(char *uploadthis)
       {
         FILE *file = fopen(uploadthis, "rb");
         CURLcode result;

         /* set callback to use */
         curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);

         /* pass in suitable argument to callback */
         curl_easy_setopt(curl, CURLOPT_READDATA, (void *)file);

         result = curl_easy_perform(curl);
       }

AVAILABILITY

       CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT was added in 7.12.1.

RETURN VALUE

       This will return CURLE_OK.

SEE ALSO

       CURLOPT_READDATA(3),      CURLOPT_WRITEFUNCTION(3),      CURLOPT_SEEKFUNCTION(3),      CURLOPT_UPLOAD(3),
       CURLOPT_POST(3), CURLOPT_UPLOAD_BUFFERSIZE(3),