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| <h4 class="subsection">14.9.7 Assigning File Permissions</h4> |
| |
| <p><a name="index-file-creation-mask-1562"></a><a name="index-umask-1563"></a>The primitive functions for creating files (for example, <code>open</code> or |
| <code>mkdir</code>) take a <var>mode</var> argument, which specifies the file |
| permissions to give the newly created file. This mode is modified by |
| the process's <dfn>file creation mask</dfn>, or <dfn>umask</dfn>, before it is |
| used. |
| |
| <p>The bits that are set in the file creation mask identify permissions |
| that are always to be disabled for newly created files. For example, if |
| you set all the “other” access bits in the mask, then newly created |
| files are not accessible at all to processes in the “other” category, |
| even if the <var>mode</var> argument passed to the create function would |
| permit such access. In other words, the file creation mask is the |
| complement of the ordinary access permissions you want to grant. |
| |
| <p>Programs that create files typically specify a <var>mode</var> argument that |
| includes all the permissions that make sense for the particular file. |
| For an ordinary file, this is typically read and write permission for |
| all classes of users. These permissions are then restricted as |
| specified by the individual user's own file creation mask. |
| |
| <p><a name="index-chmod-1564"></a>To change the permission of an existing file given its name, call |
| <code>chmod</code>. This function uses the specified permission bits and |
| ignores the file creation mask. |
| |
| <p><a name="index-umask-1565"></a>In normal use, the file creation mask is initialized by the user's login |
| shell (using the <code>umask</code> shell command), and inherited by all |
| subprocesses. Application programs normally don't need to worry about |
| the file creation mask. It will automatically do what it is supposed to |
| do. |
| |
| <p>When your program needs to create a file and bypass the umask for its |
| access permissions, the easiest way to do this is to use <code>fchmod</code> |
| after opening the file, rather than changing the umask. In fact, |
| changing the umask is usually done only by shells. They use the |
| <code>umask</code> function. |
| |
| <p>The functions in this section are declared in <samp><span class="file">sys/stat.h</span></samp>. |
| <a name="index-sys_002fstat_002eh-1566"></a> |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Function: mode_t <b>umask</b> (<var>mode_t mask</var>)<var><a name="index-umask-1567"></a></var><br> |
| <blockquote><p>The <code>umask</code> function sets the file creation mask of the current |
| process to <var>mask</var>, and returns the previous value of the file |
| creation mask. |
| |
| <p>Here is an example showing how to read the mask with <code>umask</code> |
| without changing it permanently: |
| |
| <pre class="smallexample"> mode_t |
| read_umask (void) |
| { |
| mode_t mask = umask (0); |
| umask (mask); |
| return mask; |
| } |
| </pre> |
| <p class="noindent">However, it is better to use <code>getumask</code> if you just want to read |
| the mask value, because it is reentrant (at least if you use the GNU |
| operating system). |
| </p></blockquote></div> |
| |
| <!-- sys/stat.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: mode_t <b>getumask</b> (<var>void</var>)<var><a name="index-getumask-1568"></a></var><br> |
| <blockquote><p>Return the current value of the file creation mask for the current |
| process. This function is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- sys/stat.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>chmod</b> (<var>const char *filename, mode_t mode</var>)<var><a name="index-chmod-1569"></a></var><br> |
| <blockquote><p>The <code>chmod</code> function sets the access permission bits for the file |
| named by <var>filename</var> to <var>mode</var>. |
| |
| <p>If <var>filename</var> is a symbolic link, <code>chmod</code> changes the |
| permissions of the file pointed to by the link, not those of the link |
| itself. |
| |
| <p>This function returns <code>0</code> if successful and <code>-1</code> if not. In |
| addition to the usual file name errors (see <a href="File-Name-Errors.html#File-Name-Errors">File Name Errors</a>), the following <code>errno</code> error conditions are defined for |
| this function: |
| |
| <dl> |
| <dt><code>ENOENT</code><dd>The named file doesn't exist. |
| |
| <br><dt><code>EPERM</code><dd>This process does not have permission to change the access permissions |
| of this file. Only the file's owner (as judged by the effective user ID |
| of the process) or a privileged user can change them. |
| |
| <br><dt><code>EROFS</code><dd>The file resides on a read-only file system. |
| |
| <br><dt><code>EFTYPE</code><dd><var>mode</var> has the <code>S_ISVTX</code> bit (the “sticky bit”) set, |
| and the named file is not a directory. Some systems do not allow setting the |
| sticky bit on non-directory files, and some do (and only some of those |
| assign a useful meaning to the bit for non-directory files). |
| |
| <p>You only get <code>EFTYPE</code> on systems where the sticky bit has no useful |
| meaning for non-directory files, so it is always safe to just clear the |
| bit in <var>mode</var> and call <code>chmod</code> again. See <a href="Permission-Bits.html#Permission-Bits">Permission Bits</a>, |
| for full details on the sticky bit. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- sys/stat.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>fchmod</b> (<var>int filedes, int mode</var>)<var><a name="index-fchmod-1570"></a></var><br> |
| <blockquote><p>This is like <code>chmod</code>, except that it changes the permissions of the |
| currently open file given by <var>filedes</var>. |
| |
| <p>The return value from <code>fchmod</code> is <code>0</code> on success and <code>-1</code> |
| on failure. The following <code>errno</code> error codes are defined for this |
| function: |
| |
| <dl> |
| <dt><code>EBADF</code><dd>The <var>filedes</var> argument is not a valid file descriptor. |
| |
| <br><dt><code>EINVAL</code><dd>The <var>filedes</var> argument corresponds to a pipe or socket, or something |
| else that doesn't really have access permissions. |
| |
| <br><dt><code>EPERM</code><dd>This process does not have permission to change the access permissions |
| of this file. Only the file's owner (as judged by the effective user ID |
| of the process) or a privileged user can change them. |
| |
| <br><dt><code>EROFS</code><dd>The file resides on a read-only file system. |
| </dl> |
| </p></blockquote></div> |
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