## 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 prototype shown above.

This callback function gets called by libcurl when something special I/O-related needs to be done that the library can't 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 a 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 fUICURLOPT_IOCTLDATA(3) option.

This option is deprecated! Do not use it. Use CURLOPT_SEEKFUNCTION instead to provide seeking! If CURLOPT_SEEKFUNCTION is set, this parameter will be ignored when seeking.

## DEFAULT

By default, this parameter is set to NULL. Not used.

## PROTOCOLS

Used with HTTP

## EXAMPLE

TODO

## AVAILABILITY

Added in 7.12.3

## RETURN VALUE

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.

## SEE ALSO

CURLOPT_IOCTLDATA CURLOPT_SEEKFUNCTION

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