Ubuntu Manpage: ioctl - control device

NAME

       ioctl - control device

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/ioctl.h>

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

DESCRIPTION

       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

       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

       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

       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

       None.

HISTORY

       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

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.