CURLOPT_IOCTLFUNCTION(3) Library Functions Manual CURLOPT_IOCTLFUNCTION(3)
NAME
CURLOPT_IOCTLFUNCTION - callback for I/O operations
SYNOPSIS
#include <curl/curl.h>
typedef enum {
CURLIOE_OK, /* I/O operation successful */
CURLIOE_UNKNOWNCMD, /* command was unknown to callback */
CURLIOE_FAILRESTART, /* failed to restart the read */
CURLIOE_LAST /* never use */
} curlioerr;
typedef enum {
CURLIOCMD_NOP, /* no operation */
CURLIOCMD_RESTARTREAD, /* restart the read stream from start */
CURLIOCMD_LAST /* never use */
} curliocmd;
curlioerr ioctl_callback(CURL *handle, int cmd, void *clientp);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_IOCTLFUNCTION, ioctl_callback);
DESCRIPTION
Pass a pointer to your callback function, which should match the proto-
type shown above.
This callback function gets called by libcurl when something special
I/O-related needs to be done that the library cannot do by itself. For
now, rewinding the read data stream is the only action it can request.
The rewinding of the read data stream may be necessary when doing an
HTTP PUT or POST with a multi-pass authentication method.
The callback MUST return CURLIOE_UNKNOWNCMD if the input cmd is not
CURLIOCMD_RESTARTREAD.
The clientp argument to the callback is set with the CURLOPT_IOCTL-
DATA(3) option.
This option is deprecated. Do not use it. Use CURLOPT_SEEKFUNCTION(3)
instead to provide seeking. If CURLOPT_SEEKFUNCTION(3) is set, this pa-
rameter is ignored when seeking.
DEFAULT
NULL
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
#include <unistd.h> /* for lseek */
struct data {
int fd; /* our file descriptor */
};
static curlioerr ioctl_callback(CURL *handle, int cmd, void *clientp)
{
struct data *io = (struct data *)clientp;
if(cmd == CURLIOCMD_RESTARTREAD) {
lseek(io->fd, 0, SEEK_SET);
return CURLIOE_OK;
}
return CURLIOE_UNKNOWNCMD;
}
int main(void)
{
struct data ioctl_data;
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_IOCTLFUNCTION, ioctl_callback);
curl_easy_setopt(curl, CURLOPT_IOCTLDATA, &ioctl_data);
}
}
DEPRECATED
Deprecated since 7.18.0.
AVAILABILITY
Added in curl 7.12.3
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_IOCTLDATA(3), CURLOPT_SEEKFUNCTION(3)
libcurl 2026-07-12 CURLOPT_IOCTLFUNCTION(3)
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