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

6       ioctl - control device
7

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.
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19       The second argument is a  device-dependent  request  code.   The  third
20       argument  is  an  untyped  pointer  to memory.  It's traditionally char
21       *argp (from the days before void * was valid C), and will be  so  named
22       for this discussion.
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24       An  ioctl()  request  has  encoded  in it whether the argument is an in
25       parameter 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 appropriately.
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ERRORS

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

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

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

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

COLOPHON

99       This  page  is  part of release 5.07 of the Linux man-pages project.  A
100       description of the project, information about reporting bugs,  and  the
101       latest     version     of     this    page,    can    be    found    at
102       https://www.kernel.org/doc/man-pages/.
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106Linux                             2020-04-11                          IOCTL(2)
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