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 have 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 will not 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 do not 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).

       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;
       }

       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),

libcurl 7.81.0                                  November 26, 2021                        CURLOPT_READFUNCTION(3)