CURLOPT_READFUNCTION man page
CURLOPT_READFUNCTION — read callback for data uploads
size_t read_callback(char *buffer, size_t size, size_t nitems, void *instream);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READFUNCTION, read_callback);
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.
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).
The default internal read callback is fread().
This is used for all protocols when doing uploads.
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
CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT was added in 7.12.1.
This will return CURLE_OK.
CURLOPT_READDATA(3), CURLOPT_WRITEFUNCTION(3), CURLOPT_SEEKFUNCTION(3), CURLOPT_UPLOAD(3), CURLOPT_POST(3),
curl_easy_pause(3), curl_easy_setopt(3), curl_formadd(3), CURLOPT_POST(3), CURLOPT_POSTFIELDS(3), CURLOPT_READDATA(3), CURLOPT_UPLOAD(3), CURLOPT_WRITEFUNCTION(3).