ioctl(2) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON

ioctl(2)                   System Calls Manual                  ioctl(2)

NAME         top

       ioctl - control device

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/ioctl.h>

       int ioctl(int fd, unsigned long op, ...);  /* glibc, BSD */
       int ioctl(int fd, int op, ...);            /* musl, other UNIX */

DESCRIPTION         top

       The ioctl() 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() operations.  The argument fd must be
       an open file descriptor.

       The second argument is a device-dependent operation 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() op 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() op are
       located in the file <sys/ioctl.h>.  See NOTES.

RETURN VALUE         top

       Usually, on success zero is returned.  A few ioctl() operations
       use the return value as an output parameter and return a
       nonnegative value on success.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       EBADF  fd is not a valid file descriptor.

       EFAULT argp references an inaccessible memory area.

       EINVAL op or argp is not valid.

       ENOTTY fd is not associated with a character special device.

       ENOTTY The specified operation does not apply to the kind of
              object that the file descriptor fd references.

VERSIONS         top

       Arguments, returns, and semantics of ioctl() 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).

STANDARDS         top

       None.

HISTORY         top

       Version 7 AT&T UNIX has
           ioctl(int fildes, int op, struct sgttyb *argp);
       (where struct sgttyb has historically been used by stty(2) and
       gtty(2), and is polymorphic by operation type (like a void *
       would be, if it had been available)).

       SysIII documents arg without a type at all.

       4.3BSD has
           ioctl(int d, unsigned long op, char *argp);
       (with char * similarly in for void *).

       SysVr4 has
           int ioctl(int fildes, int op, ... /* arg */);

NOTES         top

       In order to use this call, one needs an open file descriptor.
       Often the open(2) call has unwanted side effects, that can be
       avoided under Linux by giving it the O_NONBLOCK flag.

   ioctl structure
       Ioctl op values are 32-bit constants.  In principle these
       constants are completely arbitrary, but people have tried to
       build some structure into them.

       The old Linux situation was that of mostly 16-bit constants,
       where the last byte is a serial number, and the preceding byte(s)
       give a type indicating the driver.  Sometimes the major number
       was used: 0x03 for the HDIO_* ioctls, 0x06 for the LP* ioctls.
       And sometimes one or more ASCII letters were used.  For example,
       TCGETS has value 0x00005401, with 0x54 = 'T' indicating the
       terminal driver, and CYGETTIMEOUT has value 0x00435906, with 0x43
       0x59 = 'C' 'Y' indicating the cyclades driver.

       Later (0.98p5) some more information was built into the number.
       One has 2 direction bits (00: none, 01: write, 10: read, 11:
       read/write) followed by 14 size bits (giving the size of the
       argument), followed by an 8-bit type (collecting the ioctls in
       groups for a common purpose or a common driver), and an 8-bit
       serial number.

       The macros describing this structure live in <asm/ioctl.h> and
       are _IO(type,nr) and {_IOR,_IOW,_IOWR}(type,nr,size).  They use
       sizeof(size) so that size is a misnomer here: this third argument
       is a data type.

       Note that the size bits are very unreliable: in lots of cases
       they are wrong, either because of buggy macros using
       sizeof(sizeof(struct)), or because of legacy values.

       Thus, it seems that the new structure only gave disadvantages: it
       does not help in checking, but it causes varying values for the
       various architectures.

SEE ALSO         top

       execve(2), fcntl(2), ioctl_console(2), ioctl_fat(2), ioctl_fs(2),
       ioctl_fsmap(2), ioctl_nsfs(2), ioctl_tty(2),
       ioctl_userfaultfd(2), ioctl_eventpoll(2), open(2), sd(4), tty(4)

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

Linux man-pages 6.9.1          2024-06-13                       ioctl(2)

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