Node:Permission Bits, Next:Access Permission, Previous:File Owner, Up:File Attributes
The file mode, stored in the st_mode
field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the access
permission bits, which control who can read or write the file.
See Testing File Type, for information about the file type code.
All of the symbols listed in this section are defined in the header file
sys/stat.h
.
These symbolic constants are defined for the file mode bits that control
access permission for the file:
S_IRUSR
S_IREAD
S_IREAD
is an obsolete synonym provided for BSD
compatibility.
S_IWUSR
S_IWRITE
S_IWRITE
is an obsolete synonym provided for BSD compatibility.
S_IXUSR
S_IEXEC
S_IEXEC
is an obsolete
synonym provided for BSD compatibility.
S_IRWXU
(S_IRUSR | S_IWUSR | S_IXUSR)
.
S_IRGRP
S_IWGRP
S_IXGRP
S_IRWXG
(S_IRGRP | S_IWGRP | S_IXGRP)
.
S_IROTH
S_IWOTH
S_IXOTH
S_IRWXO
(S_IROTH | S_IWOTH | S_IXOTH)
.
S_ISUID
S_ISGID
S_ISVTX
/tmp
directory, where
anyone may create files but not delete files created by other users.
Originally the sticky bit on an executable file modified the swapping
policies of the system. Normally, when a program terminated, its pages
in core were immediately freed and reused. If the sticky bit was set on
the executable file, the system kept the pages in core for a while as if
the program were still running. This was advantageous for a program
likely to be run many times in succession. This usage is obsolete in
modern systems. When a program terminates, its pages always remain in
core as long as there is no shortage of memory in the system. When the
program is next run, its pages will still be in core if no shortage
arose since the last run.
On some modern systems where the sticky bit has no useful meaning for an
executable file, you cannot set the bit at all for a non-directory.
If you try, chmod
fails with EFTYPE
;
see Setting Permissions.
Some systems (particularly SunOS) have yet another use for the sticky
bit. If the sticky bit is set on a file that is not executable,
it means the opposite: never cache the pages of this file at all. The
main use of this is for the files on an NFS server machine which are
used as the swap area of diskless client machines. The idea is that the
pages of the file will be cached in the client's memory, so it is a
waste of the server's memory to cache them a second time. With this
usage the sticky bit also implies that the filesystem may fail to record
the file's modification time onto disk reliably (the idea being that
no-one cares for a swap file).
This bit is only available on BSD systems (and those derived from
them). Therefore one has to use the _BSD_SOURCE
feature select
macro to get the definition (see Feature Test Macros).