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| <h3 class="section">25.5 System Calls</h3> |
| |
| <p><a name="index-system-call-3126"></a>A system call is a request for service that a program makes of the |
| kernel. The service is generally something that only the kernel has |
| the privilege to do, such as doing I/O. Programmers don't normally |
| need to be concerned with system calls because there are functions in |
| the GNU C library to do virtually everything that system calls do. |
| These functions work by making system calls themselves. For example, |
| there is a system call that changes the permissions of a file, but |
| you don't need to know about it because you can just use the GNU C |
| library's <code>chmod</code> function. |
| |
| <p><a name="index-kernel-call-3127"></a>System calls are sometimes called kernel calls. |
| |
| <p>However, there are times when you want to make a system call explicitly, |
| and for that, the GNU C library provides the <code>syscall</code> function. |
| <code>syscall</code> is harder to use and less portable than functions like |
| <code>chmod</code>, but easier and more portable than coding the system call |
| in assembler instructions. |
| |
| <p><code>syscall</code> is most useful when you are working with a system call |
| which is special to your system or is newer than the GNU C library you |
| are using. <code>syscall</code> is implemented in an entirely generic way; |
| the function does not know anything about what a particular system |
| call does or even if it is valid. |
| |
| <p>The description of <code>syscall</code> in this section assumes a certain |
| protocol for system calls on the various platforms on which the GNU C |
| library runs. That protocol is not defined by any strong authority, but |
| we won't describe it here either because anyone who is coding |
| <code>syscall</code> probably won't accept anything less than kernel and C |
| library source code as a specification of the interface between them |
| anyway. |
| |
| <p><code>syscall</code> is declared in <samp><span class="file">unistd.h</span></samp>. |
| |
| <!-- unistd.h --> |
| <!-- ??? --> |
| <div class="defun"> |
| — Function: long int <b>syscall</b> (<var>long int sysno, ...</var>)<var><a name="index-syscall-3128"></a></var><br> |
| <blockquote> |
| <p><code>syscall</code> performs a generic system call. |
| |
| <p><a name="index-system-call-number-3129"></a><var>sysno</var> is the system call number. Each kind of system call is |
| identified by a number. Macros for all the possible system call numbers |
| are defined in <samp><span class="file">sys/syscall.h</span></samp> |
| |
| <p>The remaining arguments are the arguments for the system call, in |
| order, and their meanings depend on the kind of system call. Each kind |
| of system call has a definite number of arguments, from zero to five. |
| If you code more arguments than the system call takes, the extra ones to |
| the right are ignored. |
| |
| <p>The return value is the return value from the system call, unless the |
| system call failed. In that case, <code>syscall</code> returns <code>-1</code> and |
| sets <code>errno</code> to an error code that the system call returned. Note |
| that system calls do not return <code>-1</code> when they succeed. |
| <a name="index-errno-3130"></a> |
| If you specify an invalid <var>sysno</var>, <code>syscall</code> returns <code>-1</code> |
| with <code>errno</code> = <code>ENOSYS</code>. |
| |
| <p>Example: |
| |
| <pre class="smallexample"> |
| #include <unistd.h> |
| #include <sys/syscall.h> |
| #include <errno.h> |
| |
| ... |
| |
| int rc; |
| |
| rc = syscall(SYS_chmod, "/etc/passwd", 0444); |
| |
| if (rc == -1) |
| fprintf(stderr, "chmod failed, errno = %d\n", errno); |
| |
| </pre> |
| <p>This, if all the compatibility stars are aligned, is equivalent to the |
| following preferable code: |
| |
| <pre class="smallexample"> |
| #include <sys/types.h> |
| #include <sys/stat.h> |
| #include <errno.h> |
| |
| ... |
| |
| int rc; |
| |
| rc = chmod("/etc/passwd", 0444); |
| if (rc == -1) |
| fprintf(stderr, "chmod failed, errno = %d\n", errno); |
| |
| </pre> |
| </blockquote></div> |
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