Software & Apps Linux The Linux Command: IOCTL Control the underlying hardware on your Linux system By Juergen Haas Writer Former Lifewire writer Juergen Haas is a software developer, data scientist, and a fan of the Linux operating system. our editorial process Juergen Haas Updated February 20, 2020 Linux Switching from Windows Tweet Share Email ioctl - control device Synopsis #include <sys/ ioctl.h> int ioctl(int d, int request, ...); vgajic/E+/Getty Images Description 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. Errors EBADF d is not a valid descriptor. EFAULT argp references an inaccessible memory area. ENOTTY d is not associated with a character special device. ENOTTY The specified request does not apply to the kind of object that the descriptor d references. EINVAL 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.