diff options
| author | Michael Kerrisk <mtk.manpages@gmail.com> | 2020-04-16 08:57:18 +0200 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2020-06-09 09:40:08 +0200 |
| commit | 91b00e53b3b9846f46399bc1ac3424f1b92d601c (patch) | |
| tree | 4c030c8081a4a490cb9899d201fb0b0d91fd17f0 | |
| parent | 02e701cde37e3d01bd9ba4d615e56784498b7ef6 (diff) | |
| download | man-pages-91b00e53b3b9846f46399bc1ac3424f1b92d601c.tar.gz | |
ioctl.2: Move subsection on "ioctl structure" from ioctl_list(2) to ioctl(2)
In preparation for removing ioctl_list(2), let's preserve
some useful text that was added to ioctl_list(2)
by Andries Brouwer.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
| -rw-r--r-- | man2/ioctl.2 | 54 |
1 files changed, 54 insertions, 0 deletions
diff --git a/man2/ioctl.2 b/man2/ioctl.2 index 6957b80450..9f8f51828b 100644 --- a/man2/ioctl.2 +++ b/man2/ioctl.2 @@ -82,6 +82,7 @@ Macros and defines used in specifying an .I request are located in the file .IR <sys/ioctl.h> . +See NOTES. .SH RETURN VALUE Usually, on success zero is returned. A few @@ -139,6 +140,59 @@ call has unwanted side effects, that can be avoided under Linux by giving it the .B O_NONBLOCK flag. +.\" +.SS ioctl structure +.\" added two sections - aeb +Ioctl command values are 32-bit constants. +In principle these constants are completely arbitrary, but people have +tried to build some structure into them. +.PP +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 +.B HDIO_* +ioctls, 0x06 for the +.B LP* +ioctls. +And sometimes +one or more ASCII letters were used. +For example, +.B TCGETS +has value +0x00005401, with 0x54 = \(aqT\(aq indicating the terminal driver, and +.B CYGETTIMEOUT +has value 0x00435906, with 0x43 0x59 = \(aqC\(aq \(aqY\(aq +indicating the cyclades driver. +.PP +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. +.PP +The macros describing this structure live in +.I <asm/ioctl.h> +and are +.B _IO(type,nr) +and +.BR "{_IOR,_IOW,_IOWR}(type,nr,size)" . +They use +.I sizeof(size) +so that size is a +misnomer here: this third argument is a data type. +.PP +Note that the size bits are very unreliable: in lots of cases +they are wrong, either because of buggy macros using +.IR sizeof(sizeof(struct)) , +or because of legacy values. +.PP +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. .SH SEE ALSO .BR execve (2), .BR fcntl (2), |