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
       *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 _READDATA&section=3">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 _easy_pause&section=3">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 _READDATA&section=3">CURLOPT_READDATA(3).

       You can set the total size of the data you are sending by using
       _INFILESIZE_LARGE&section=3">CURLOPT_INFILESIZE_LARGE(3) or _POSTFIELDSIZE_LARGE&section=3">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 7.77.0                 December 30, 2020       CURLOPT_READFUNCTION(3)