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.
       size is always 1.

       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  signals  end-of-file  to  the  library  and causes 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  have  said you would 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 is not sent.

       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 is
       considered the final packet by the server end and the transfer ends there.

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

       You can set the total  size  of  the  data  you  are  sending  by  using  CURLOPT_INFILESIZE_LARGE(3)  or
       CURLOPT_POSTFIELDSIZE_LARGE(3),  depending  on  the  type  of transfer. For some transfer types it may be
       required and it allows for better error checking.

DEFAULT

       fread(3)

PROTOCOLS

       This functionality affects all supported protocols

EXAMPLE

       size_t read_callback(char *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;
       }

       int main(int argc, char **argv)
       {
         FILE *file = fopen(argv[1], "rb");
         CURLcode result;

         CURL *curl = curl_easy_init();
         if(curl) {
           /* 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);
         }
       }

HISTORY

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

AVAILABILITY

       Added in curl 7.1

RETURN VALUE

       This returns CURLE_OK.

SEE ALSO

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