Tribblix: manual page: CURLOPT

CURLOPT_SEEKFUNCTION(3) Introduction to Library Functions

NAME

CURLOPT_SEEKFUNCTION - user callback for seeking in input stream

SYNOPSIS

#include <curl/curl.h>

/* These are the return codes for the seek callbacks */
#define CURL_SEEKFUNC_OK 0
#define CURL_SEEKFUNC_FAIL 1 /* fail the entire transfer */
#define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking cannot be done, so
libcurl might try other means instead */

int seek_callback(void *clientp, curl_off_t offset, int origin);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_callback);

DESCRIPTION

Pass a pointer to your callback function, which should match the
prototype shown above.

This function gets called by libcurl to seek to a certain position in
the input stream and can be used to fast forward a file in a resumed
upload (instead of reading all uploaded bytes with the normal read
function/callback). It is also called to rewind a stream when data
has already been sent to the server and needs to be sent again. This
may happen when doing an HTTP PUT or POST with a multi-pass
authentication method, or when an existing HTTP connection is reused
too late and the server closes the connection. The function shall
work like fseek(3) or lseek(3) and it gets SEEK_SET, SEEK_CUR or
SEEK_END as argument for origin, although libcurl currently only
passes SEEK_SET.

clientp is the pointer you set with CURLOPT_SEEKDATA(3).

The callback function must return CURL_SEEKFUNC_OK on success,
CURL_SEEKFUNC_FAIL to cause the upload operation to fail or
CURL_SEEKFUNC_CANTSEEK to indicate that while the seek failed,
libcurl is free to work around the problem if possible. The latter
can sometimes be done by instead reading from the input or similar.

If you forward the input arguments directly to fseek(3) or lseek(3),
note that the data type for offset is not the same as defined for
curl_off_t on many systems.

DEFAULT

NULL

PROTOCOLS

This functionality affects ftp, http and sftp

EXAMPLE

#include <unistd.h> /* for lseek */

struct data {
int our_fd;
};
static int seek_cb(void *clientp, curl_off_t offset, int origin)
{
struct data *d = (struct data *)clientp;
lseek(d->our_fd, offset, origin);
return CURL_SEEKFUNC_OK;
}

int main(void)
{
struct data seek_data;
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_SEEKFUNCTION, seek_cb);
curl_easy_setopt(curl, CURLOPT_SEEKDATA, &seek_data);
}
}

AVAILABILITY

Added in curl 7.18.0

RETURN VALUE

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error
occurred, see libcurl-errors(3).