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

       The default internal read callback is fread().

PROTOCOLS

       This is used for all protocols when doing uploads.

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);
         }
       }

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 returns CURLE_OK.

SEE ALSO

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