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 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 youve 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 wont 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 dont 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).