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| <h4 class="subsection">14.9.5 The Mode Bits for Access Permission</h4> |
| |
| <p>The <dfn>file mode</dfn>, stored in the <code>st_mode</code> 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 <a href="Testing-File-Type.html#Testing-File-Type">Testing File Type</a>, for information about the file type code. |
| |
| <p>All of the symbols listed in this section are defined in the header file |
| <samp><span class="file">sys/stat.h</span></samp>. |
| <a name="index-sys_002fstat_002eh-1538"></a> |
| <a name="index-file-permission-bits-1539"></a>These symbolic constants are defined for the file mode bits that control |
| access permission for the file: |
| |
| <dl> |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <dt><code>S_IRUSR</code><dd><a name="index-S_005fIRUSR-1540"></a><!-- sys/stat.h --> |
| <!-- BSD --> |
| <dt><code>S_IREAD</code><dd><a name="index-S_005fIREAD-1541"></a>Read permission bit for the owner of the file. On many systems this bit |
| is 0400. <code>S_IREAD</code> is an obsolete synonym provided for BSD |
| compatibility. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IWUSR</code><dd><a name="index-S_005fIWUSR-1542"></a><!-- sys/stat.h --> |
| <!-- BSD --> |
| <dt><code>S_IWRITE</code><dd><a name="index-S_005fIWRITE-1543"></a>Write permission bit for the owner of the file. Usually 0200. |
| <code>S_IWRITE</code><!-- /@w --> is an obsolete synonym provided for BSD compatibility. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IXUSR</code><dd><a name="index-S_005fIXUSR-1544"></a><!-- sys/stat.h --> |
| <!-- BSD --> |
| <dt><code>S_IEXEC</code><dd><a name="index-S_005fIEXEC-1545"></a>Execute (for ordinary files) or search (for directories) permission bit |
| for the owner of the file. Usually 0100. <code>S_IEXEC</code> is an obsolete |
| synonym provided for BSD compatibility. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IRWXU</code><dd><a name="index-S_005fIRWXU-1546"></a>This is equivalent to ‘<samp><span class="samp">(S_IRUSR | S_IWUSR | S_IXUSR)</span></samp>’. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IRGRP</code><dd><a name="index-S_005fIRGRP-1547"></a>Read permission bit for the group owner of the file. Usually 040. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IWGRP</code><dd><a name="index-S_005fIWGRP-1548"></a>Write permission bit for the group owner of the file. Usually 020. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IXGRP</code><dd><a name="index-S_005fIXGRP-1549"></a>Execute or search permission bit for the group owner of the file. |
| Usually 010. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IRWXG</code><dd><a name="index-S_005fIRWXG-1550"></a>This is equivalent to ‘<samp><span class="samp">(S_IRGRP | S_IWGRP | S_IXGRP)</span></samp>’. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IROTH</code><dd><a name="index-S_005fIROTH-1551"></a>Read permission bit for other users. Usually 04. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IWOTH</code><dd><a name="index-S_005fIWOTH-1552"></a>Write permission bit for other users. Usually 02. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IXOTH</code><dd><a name="index-S_005fIXOTH-1553"></a>Execute or search permission bit for other users. Usually 01. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>S_IRWXO</code><dd><a name="index-S_005fIRWXO-1554"></a>This is equivalent to ‘<samp><span class="samp">(S_IROTH | S_IWOTH | S_IXOTH)</span></samp>’. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX --> |
| <br><dt><code>S_ISUID</code><dd><a name="index-S_005fISUID-1555"></a>This is the set-user-ID on execute bit, usually 04000. |
| See <a href="How-Change-Persona.html#How-Change-Persona">How Change Persona</a>. |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX --> |
| <br><dt><code>S_ISGID</code><dd><a name="index-S_005fISGID-1556"></a>This is the set-group-ID on execute bit, usually 02000. |
| See <a href="How-Change-Persona.html#How-Change-Persona">How Change Persona</a>. |
| |
| <p><a name="index-sticky-bit-1557"></a><!-- sys/stat.h --> |
| <!-- BSD --> |
| <br><dt><code>S_ISVTX</code><dd><a name="index-S_005fISVTX-1558"></a>This is the <dfn>sticky</dfn> bit, usually 01000. |
| |
| <p>For a directory it gives permission to delete a file in that directory |
| only if you own that file. Ordinarily, a user can either delete all the |
| files in a directory or cannot delete any of them (based on whether the |
| user has write permission for the directory). The same restriction |
| applies—you must have both 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 <samp><span class="file">/tmp</span></samp> directory, where |
| anyone may create files but not delete files created by other users. |
| |
| <p>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. |
| |
| <p>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, <code>chmod</code> fails with <code>EFTYPE</code>; |
| see <a href="Setting-Permissions.html#Setting-Permissions">Setting Permissions</a>. |
| |
| <p>Some systems (particularly SunOS) have yet another use for the sticky |
| bit. If the sticky bit is set on a file that is <em>not</em> 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). |
| |
| <p>This bit is only available on BSD systems (and those derived from |
| them). Therefore one has to use the <code>_BSD_SOURCE</code> feature select |
| macro to get the definition (see <a href="Feature-Test-Macros.html#Feature-Test-Macros">Feature Test Macros</a>). |
| </dl> |
| |
| <p>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. |
| |
| <p><strong>Warning:</strong> Writing explicit numbers for file permissions is bad |
| practice. Not only is it not portable, it also requires everyone who |
| reads your program to remember what the bits mean. To make your program |
| clean use the symbolic names. |
| |
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