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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">
&mdash; 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 &lt;unistd.h&gt;
          #include &lt;sys/syscall.h&gt;
          #include &lt;errno.h&gt;
          
          ...
          
          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 &lt;sys/types.h&gt;
          #include &lt;sys/stat.h&gt;
          #include &lt;errno.h&gt;
          
          ...
          
          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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