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_IRGRP
S_IWGRP
S_IXGRP
S_IRWXG
S_IROTH
S_IWOTH
S_IXOTH
S_IRWXO
S_ISUID
S_ISGID
S_ISVTX On a directory, it gives permission to delete a file in the directory only if you own that file. Ordinarily, a user either can delete all the files in the directory or cannot delete any of them (based on whether the user has write permission for the directory). The same restriction applies---you must both have write permission for the directory and own the file you want to delete. The one exception is that the owner of the directory can delete any file in the directory, no matter who owns it (provided the owner has given himself write permission for the directory). This is commonly used for the `/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 This usage is obsolete in modern systems.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. In this use the sticky bit also says that the filesystem may fail to record the file's modification time onto disk reliably (the idea being that noone cares for a swap file).
The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed.
Warning: Writing explicit numbers for file permissions is bad practice. It is not only nonportable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean, use the symbolic names.