CURLOPT_READFUNCTION(3)    curl_easy_setopt options    CURLOPT_READFUNCTION(3)

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
       *instream);

       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
       nmemb number of bytes by your function.

       Your function must then return the  actual  number  of  bytes  that  it
       stored  in that memory area. 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
       Here's an example setting a read callback for reading that to upload to
       an FTP site: https://curl.haxx.se/libcurl/c/ftpupload.html

AVAILABILITY
       CURL_READFUNC_PAUSE  return  code  was  added  in 7.18.0 and CURL_READ-
       FUNC_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),

libcurl 7.54.0                 February 03, 2016       CURLOPT_READFUNCTION(3)