The Linux Command: IOCTL

Control the underlying hardware on your Linux system

ioctl - control device  


#include <sys/ ioctl.h>

int ioctl(int d, int request, ...);  

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The ioctl function manipulates the underlying device parameters of special files. In particular, many operating characteristics of character special files (e.g. terminals) may be controlled with ioctl requests. The argument d must be an open file descriptor.

The second argument is a device-dependent request code. The third argument is an untyped pointer to memory. It's traditionally char *argp (from the days before void * was valid C), and will be so named for this discussion.

An ioctl request has encoded in it whether the argument is an in parameter or out parameter, and the size of the argument argp in bytes. Macros and defines used in specifying an ioctl request are located in the file <sys/ioctl.h>

Return Value

Usually, on success zero is returned. A few ioctls use the return value as an output parameter and return a non-negative value on success. On error, -1 is returned, and errno is set appropriately.  



d is not a valid descriptor.


argp references an inaccessible memory area.


d is not associated with a character special device.


The specified request does not apply to the kind of object that the descriptor d references.


Request or argp is not valid.

Conforming To

No single standard. Arguments, returns, and semantics of ioctl(2) vary according to the device driver in question (the call is used as a catch-all for operations that don't cleanly fit the Unix stream I/O model). The ioctl function call appeared in Version 7 AT&T Unix.  

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