size_t read_callback(char *buffer, size_t size, size_t nitems, void *userdata);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READFUNCTION, read_callback);
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'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).
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.
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);
}