1ioctl(2)                      System Calls Manual                     ioctl(2)
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3
4

NAME

6       ioctl - control device
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <sys/ioctl.h>
13
14       int ioctl(int fd, unsigned long request, ...);
15

DESCRIPTION

17       The ioctl() system call manipulates the underlying device parameters of
18       special files.  In particular, many operating characteristics of  char‐
19       acter  special  files  (e.g., terminals) may be controlled with ioctl()
20       requests.  The argument fd must be an open file descriptor.
21
22       The second argument is a device-dependent request code.  The third  ar‐
23       gument  is an untyped pointer to memory.  It's traditionally char *argp
24       (from the days before void * was valid C), and will  be  so  named  for
25       this discussion.
26
27       An  ioctl() request has encoded in it whether the argument is an in pa‐
28       rameter or out parameter, and the size of the argument argp  in  bytes.
29       Macros and defines used in specifying an ioctl() request are located in
30       the file <sys/ioctl.h>.  See NOTES.
31

RETURN VALUE

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

39       EBADF  fd is not a valid file descriptor.
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41       EFAULT argp references an inaccessible memory area.
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43       EINVAL request or argp is not valid.
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45       ENOTTY fd is not associated with a character special device.
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47       ENOTTY The  specified request does not apply to the kind of object that
48              the file descriptor fd references.
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VERSIONS

51       Arguments, returns, and semantics of ioctl() vary according to the  de‐
52       vice driver in question (the call is used as a catch-all for operations
53       that don't cleanly fit the UNIX stream I/O model).
54

STANDARDS

56       None.
57

HISTORY

59       Version 7 AT&T UNIX.
60

NOTES

62       In order to use this call, one needs an open  file  descriptor.   Often
63       the  open(2)  call has unwanted side effects, that can be avoided under
64       Linux by giving it the O_NONBLOCK flag.
65
66   ioctl structure
67       Ioctl command values are 32-bit constants.   In  principle  these  con‐
68       stants  are  completely  arbitrary, but people have tried to build some
69       structure into them.
70
71       The old Linux situation was that of mostly 16-bit constants, where  the
72       last byte is a serial number, and the preceding byte(s) give a type in‐
73       dicating the driver.  Sometimes the major number was used: 0x03 for the
74       HDIO_*  ioctls,  0x06  for  the  LP* ioctls.  And sometimes one or more
75       ASCII letters were used.  For example,  TCGETS  has  value  0x00005401,
76       with  0x54  =  'T' indicating the terminal driver, and CYGETTIMEOUT has
77       value 0x00435906, with 0x43 0x59 =  'C'  'Y'  indicating  the  cyclades
78       driver.
79
80       Later  (0.98p5)  some  more information was built into the number.  One
81       has 2 direction bits (00: none, 01: write, 10:  read,  11:  read/write)
82       followed by 14 size bits (giving the size of the argument), followed by
83       an 8-bit type (collecting the ioctls in groups for a common purpose  or
84       a common driver), and an 8-bit serial number.
85
86       The  macros  describing  this  structure  live in <asm/ioctl.h> and are
87       _IO(type,nr)    and    {_IOR,_IOW,_IOWR}(type,nr,size).     They    use
88       sizeof(size)  so that size is a misnomer here: this third argument is a
89       data type.
90
91       Note that the size bits are very unreliable: in lots of cases they  are
92       wrong,  either because of buggy macros using sizeof(sizeof(struct)), or
93       because of legacy values.
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95       Thus, it seems that the new structure only gave disadvantages: it  does
96       not  help in checking, but it causes varying values for the various ar‐
97       chitectures.
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SEE ALSO

100       execve(2), fcntl(2), ioctl_console(2), ioctl_fat(2),  ioctl_ficlone(2),
101       ioctl_ficlonerange(2),     ioctl_fideduperange(2),    ioctl_fslabel(2),
102       ioctl_getfsmap(2),    ioctl_iflags(2),    ioctl_ns(2),    ioctl_tty(2),
103       ioctl_userfaultfd(2), open(2), sd(4), tty(4)
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107Linux man-pages 6.05              2023-03-30                          ioctl(2)
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