#include <ioctl.h>

int ioctl(int fildes, int request, ... /* arg */);


Partially implemented


IEEE Std 1003.1-2017


The ioctl() function shall perform a variety of control functions on devices. For fildes not referring to devices, the functions performed by this call are unspecified.

The fildes argument is an open file descriptor that refers to a device.

The request argument selects the control function to be performed and shall depend on the device being addressed.

The arg argument represents additional information that is needed by this specific device to perform the requested function. The type of arg depends upon the particular control request, but it shall be either an integer or a pointer to a device-specific data structure.

In order to define driver request one should use one of the macros presented below, which are available in the ioctl.h header file:

  • _IOC(inout,group,num,len) - macro which returns every possible request

    • inout - use this defines: IOC_VOID (no data), IOC_OUT (read), IOC_IN (write), IOC_INOUT (read/write)

    • group - describe group type (collecting the ioctl’s in groups for a common purpose or a common driver e.g. 'S' for sockets), 8-bit value

    • num - specifies serial number of request, 8 -bit value

    • len - how big is size of data, 14-bit value

  • _IOV(g,n) - macro which returns no data transmission request

  • _IOR(g,n,t) - macro which returns read request

  • _IOW(g,n,t) - macro which returns write request

  • _IOWR(g,n,t) - macro which returns read/write request

    • g - 8-bit group value

    • n - 8-bit serial number

    • t - type of data

There are also some checking macros:

  • OCPARM_LEN(x) - returns data length

  • IOCBASECMD(x) - returns base command value (2 least significant bytes describing group and serial number)

  • IOCGROUP(x) - returns group type value

The resulting request will be in such form: 2 bits describe direction (00: none, 10: write, 01: read, 11: read/write) followed by 14 size bits, followed by an 8-bit group type (collecting the ioctl’s in groups for a common purpose or a common driver) and an 8-bit serial number.

It is worth to note that to get data sent to driver inside message, one should use ioctl_unapck().

Also, if you are willing to pass structure with pointer in arg argument of ioctl() you should do custom pack to your message in ioctl_pack() (see ioctl_pack() implementation).

Return value#

Upon successful completion, ioctl() shall return a value other than -1 that depends on the device control function. Otherwise, it shall return -1 and set errno to indicate the error.


The ioctl() function may fail if:

  • EBADF - The fildes argument is not a valid open file descriptor.

  • ENOSYS - Couldn’t get information about calling process.

  • EINVAL - Tried to send message to invalid port.

Example usage#

#include <ioctl.h>
#include <fcntl.h>
#include <stdio.h>

#define REQUEST _IOC(IOC_IN, 'A', 0x01, int)

int main() {
    int fd;
    int data = 4;

    if (fd = open(PATH, O_WRONLY) < 0) {
        fprintf(stderr, "Cannot open special device file\n");
     * Send request with additional data to device server,
     * assuming device server successfully created and accept our request.
    if (ioctl(fd, REQUEST, &data) < 0) {
        fprintf(stderr, "Something went wrong\n");



Known bugs#


See Also#

  1. Standard library functions

  2. Table of Contents