CURLOPT_SEEKFUNCTION(3) Library Functions Manual CURLOPT_SEEKFUNCTION(3)
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 proto-
type 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 up-
load (instead of reading all uploaded bytes with the normal read func-
tion/callback). It is also called to rewind a stream when data has al-
ready been sent to the server and needs to be sent again. This may hap-
pen 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 ori-
gin, 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_SEEK-
FUNC_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).
SEE ALSO
CURLOPT_DEBUGFUNCTION(3), CURLOPT_IOCTLFUNCTION(3), CURLOPT_SEEKDATA(3),
CURLOPT_STDERR(3)
libcurl 2026-07-12 CURLOPT_SEEKFUNCTION(3)
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