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| <h4 class="subsection">14.9.8 Testing Permission to Access a File</h4> |
| |
| <p><a name="index-testing-access-permission-1571"></a><a name="index-access_002c-testing-for-1572"></a><a name="index-setuid-programs-and-file-access-1573"></a> |
| In some situations it is desirable to allow programs to access files or |
| devices even if this is not possible with the permissions granted to the |
| user. One possible solution is to set the setuid-bit of the program |
| file. If such a program is started the <em>effective</em> user ID of the |
| process is changed to that of the owner of the program file. So to |
| allow write access to files like <samp><span class="file">/etc/passwd</span></samp>, which normally can |
| be written only by the super-user, the modifying program will have to be |
| owned by <code>root</code> and the setuid-bit must be set. |
| |
| <p>But beside the files the program is intended to change the user should |
| not be allowed to access any file to which s/he would not have access |
| anyway. The program therefore must explicitly check whether <em>the |
| user</em> would have the necessary access to a file, before it reads or |
| writes the file. |
| |
| <p>To do this, use the function <code>access</code>, which checks for access |
| permission based on the process's <em>real</em> user ID rather than the |
| effective user ID. (The setuid feature does not alter the real user ID, |
| so it reflects the user who actually ran the program.) |
| |
| <p>There is another way you could check this access, which is easy to |
| describe, but very hard to use. This is to examine the file mode bits |
| and mimic the system's own access computation. This method is |
| undesirable because many systems have additional access control |
| features; your program cannot portably mimic them, and you would not |
| want to try to keep track of the diverse features that different systems |
| have. Using <code>access</code> is simple and automatically does whatever is |
| appropriate for the system you are using. |
| |
| <p><code>access</code> is <em>only</em> only appropriate to use in setuid programs. |
| A non-setuid program will always use the effective ID rather than the |
| real ID. |
| |
| <p><a name="index-unistd_002eh-1574"></a>The symbols in this section are declared in <samp><span class="file">unistd.h</span></samp>. |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>access</b> (<var>const char *filename, int how</var>)<var><a name="index-access-1575"></a></var><br> |
| <blockquote><p>The <code>access</code> function checks to see whether the file named by |
| <var>filename</var> can be accessed in the way specified by the <var>how</var> |
| argument. The <var>how</var> argument either can be the bitwise OR of the |
| flags <code>R_OK</code>, <code>W_OK</code>, <code>X_OK</code>, or the existence test |
| <code>F_OK</code>. |
| |
| <p>This function uses the <em>real</em> user and group IDs of the calling |
| process, rather than the <em>effective</em> IDs, to check for access |
| permission. As a result, if you use the function from a <code>setuid</code> |
| or <code>setgid</code> program (see <a href="How-Change-Persona.html#How-Change-Persona">How Change Persona</a>), it gives |
| information relative to the user who actually ran the program. |
| |
| <p>The return value is <code>0</code> if the access is permitted, and <code>-1</code> |
| otherwise. (In other words, treated as a predicate function, |
| <code>access</code> returns true if the requested access is <em>denied</em>.) |
| |
| <p>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>EACCES</code><dd>The access specified by <var>how</var> is denied. |
| |
| <br><dt><code>ENOENT</code><dd>The file doesn't exist. |
| |
| <br><dt><code>EROFS</code><dd>Write permission was requested for a file on a read-only file system. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>These macros are defined in the header file <samp><span class="file">unistd.h</span></samp> for use |
| as the <var>how</var> argument to the <code>access</code> function. The values |
| are integer constants. |
| <a name="index-unistd_002eh-1576"></a> |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>R_OK</b><var><a name="index-R_005fOK-1577"></a></var><br> |
| <blockquote><p>Flag meaning test for read permission. |
| </p></blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>W_OK</b><var><a name="index-W_005fOK-1578"></a></var><br> |
| <blockquote><p>Flag meaning test for write permission. |
| </p></blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>X_OK</b><var><a name="index-X_005fOK-1579"></a></var><br> |
| <blockquote><p>Flag meaning test for execute/search permission. |
| </p></blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>F_OK</b><var><a name="index-F_005fOK-1580"></a></var><br> |
| <blockquote><p>Flag meaning test for existence of the file. |
| </p></blockquote></div> |
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