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IOCTL(2) System Calls Manual IOCTL(2)


ioctlcontrol device


Standard C Library (libc, -lc)


#include <sys/ioctl.h>

ioctl(int fd, unsigned long request, ...);


The () system call 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 fd must be an open file descriptor.

The third argument to () is traditionally named char *argp. Most uses of ioctl(), however, require the third argument to be a caddr_t or an int.

An () request has encoded in it whether the argument is an “in” argument or “out” argument, 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>.


Some generic ioctls are not implemented for all types of file descriptors. These include:

Get the number of bytes that are immediately available for reading.
Get the number of bytes in the descriptor's send queue. These bytes are data which has been written to the descriptor but which are being held by the kernel for further processing. The nature of the required processing depends on the underlying device. For TCP sockets, these bytes have not yet been acknowledged by the other side of the connection.
Get the free space in the descriptor's send queue. This value is the size of the send queue minus the number of bytes being held in the queue. Note: while this value represents the number of bytes that may be added to the queue, other resource limitations may cause a write not larger than the send queue's space to be blocked. One such limitation would be a lack of network buffers for a write to a network connection.


If an error has occurred, a value of -1 is returned and errno is set to indicate the error.


The ioctl() system call will fail if:

The fd argument is not a valid descriptor.
The fd argument is not associated with a character special device.
The specified request does not apply to the kind of object that the descriptor fd references.
The request or argp argument is not valid.
The argp argument points outside the process's allocated address space.


execve(2), fcntl(2), intro(4), tty(4)


The ioctl() function appeared in Version 7 AT&T UNIX.

September 11, 2013 Debian