1IOCTL(2)                   Linux Programmer's Manual                  IOCTL(2)
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NAME

6       ioctl - control device
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SYNOPSIS

9       #include <sys/ioctl.h>
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11       int ioctl(int fd, unsigned long request, ...);
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DESCRIPTION

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

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

36       EBADF  fd is not a valid file descriptor.
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38       EFAULT argp references an inaccessible memory area.
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40       EINVAL request or argp is not valid.
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42       ENOTTY fd is not associated with a character special device.
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44       ENOTTY The  specified request does not apply to the kind of object that
45              the file descriptor fd references.
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CONFORMING TO

48       No single standard.  Arguments, returns, and semantics of ioctl()  vary
49       according  to  the  device  driver  in  question (the call is used as a
50       catch-all for operations that don't cleanly fit  the  UNIX  stream  I/O
51       model).
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53       The ioctl() system call appeared in Version 7 AT&T UNIX.
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NOTES

56       In  order  to  use this call, one needs an open file descriptor.  Often
57       the open(2) call has unwanted side effects, that can be  avoided  under
58       Linux by giving it the O_NONBLOCK flag.
59
60   ioctl structure
61       Ioctl  command  values  are  32-bit constants.  In principle these con‐
62       stants are completely arbitrary, but people have tried  to  build  some
63       structure into them.
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65       The  old Linux situation was that of mostly 16-bit constants, where the
66       last byte is a serial number, and the preceding byte(s) give a type in‐
67       dicating the driver.  Sometimes the major number was used: 0x03 for the
68       HDIO_* ioctls, 0x06 for the LP* ioctls.   And  sometimes  one  or  more
69       ASCII  letters  were  used.   For example, TCGETS has value 0x00005401,
70       with 0x54 = 'T' indicating the terminal driver,  and  CYGETTIMEOUT  has
71       value  0x00435906,  with  0x43  0x59  = 'C' 'Y' indicating the cyclades
72       driver.
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74       Later (0.98p5) some more information was built into  the  number.   One
75       has  2  direction  bits (00: none, 01: write, 10: read, 11: read/write)
76       followed by 14 size bits (giving the size of the argument), followed by
77       an  8-bit type (collecting the ioctls in groups for a common purpose or
78       a common driver), and an 8-bit serial number.
79
80       The macros describing this structure  live  in  <asm/ioctl.h>  and  are
81       _IO(type,nr)    and    {_IOR,_IOW,_IOWR}(type,nr,size).     They    use
82       sizeof(size) so that size is a misnomer here: this third argument is  a
83       data type.
84
85       Note  that the size bits are very unreliable: in lots of cases they are
86       wrong, either because of buggy macros using sizeof(sizeof(struct)),  or
87       because of legacy values.
88
89       Thus,  it seems that the new structure only gave disadvantages: it does
90       not help in checking, but it causes varying values for the various  ar‐
91       chitectures.
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SEE ALSO

94       execve(2),   fcntl(2),  ioctl_console(2),  ioctl_fat(2),  ioctl_ficlon‐
95       erange(2), ioctl_fideduperange(2), ioctl_fslabel(2), ioctl_getfsmap(2),
96       ioctl_iflags(2),   ioctl_ns(2),   ioctl_tty(2),   ioctl_userfaultfd(2),
97       open(2), sd(4), tty(4)
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COLOPHON

100       This page is part of release 5.13 of the Linux  man-pages  project.   A
101       description  of  the project, information about reporting bugs, and the
102       latest    version    of    this    page,    can     be     found     at
103       https://www.kernel.org/doc/man-pages/.
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107Linux                             2021-03-22                          IOCTL(2)
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