ioctl - control device
int ioctl(int fd, unsigned long request,
() 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 second argument is a device-dependent request code. The third argument is an
untyped pointer to memory. It's traditionally char *argp
the days before void *
was valid C), and will be so named for this
has encoded in it whether the argument is an
parameter or out
parameter, and the size of the argument
in bytes. Macros and defines used in specifying an ioctl
are located in the file <sys/ioctl.h>
Usually, on success zero is returned. A few ioctl
() requests use the
return value as an output parameter and return a nonnegative value on success.
On error, -1 is returned, and errno
is set appropriately.
- fd is not a valid file descriptor.
- argp references an inaccessible memory area.
- request or argp is not valid.
- fd is not associated with a character special
- The specified request does not apply to the kind of object
that the file descriptor fd references.
No single standard. Arguments, returns, and semantics of ioctl
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). See
for a list of many of the known ioctl
() calls. The
() system call appeared in Version 7 AT&T UNIX.
In order to use this call, one needs an open file descriptor. Often the
call has unwanted side effects, that can be avoided under Linux
by giving it the O_NONBLOCK
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