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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 prototype
     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_IOCTLDATA(3)
     option.

     This  option  is deprecated. Do not use it. Use CURLOPT_SEEKFUNCTION(3) in-
     stead to provide seeking. If CURLOPT_SEEKFUNCTION(3) is set, this parameter
     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 			   2026-06-11		CURLOPT_IOCTLFUNCTION(3)

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