Tribblix: manual page: CURLOPT

CURLOPT_READFUNCTION(3) Introduction to Library Functions

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. size is always 1.

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
signals end-of-file to the library and causes 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 would
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 is not
sent.

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 is considered the final packet by
the server end and the transfer ends there.

If you set this callback pointer to NULL, or do not set it at all,
the default internal read function is 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

fread(3)

PROTOCOLS

This functionality affects all supported protocols

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;
}

int main(int argc, char **argv)
{
FILE *file = fopen(argv[1], "rb");
CURLcode result;

CURL *curl = curl_easy_init();
if(curl) {
/* 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);
}
}

HISTORY

CURL_READFUNC_PAUSE return code was added in 7.18.0 and
CURL_READFUNC_ABORT was added in 7.12.1.

AVAILABILITY

Added in curl 7.1

RETURN VALUE

This returns CURLE_OK.