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<a name="Stubs"></a>
<div class="header">
<p>
Next: <a href="Reentrant-Syscalls.html#Reentrant-Syscalls" accesskey="n" rel="next">Reentrant Syscalls</a>, Up: <a href="Syscalls.html#Syscalls" accesskey="u" rel="up">Syscalls</a> &nbsp; [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Definitions-for-OS-interface"></a>
<h3 class="section">13.1 Definitions for OS interface</h3>
<a name="index-stubs"></a>

<a name="index-subroutines-for-OS-interface"></a>
<a name="index-OS-interface-subroutines"></a>
<p>This is the complete set of system definitions (primarily subroutines)
required; the examples shown implement the minimal functionality
required to allow <code>libc</code> to link, and fail gracefully where OS
services are not available.  
</p>
<p>Graceful failure is permitted by returning an error code.  A minor
complication arises here: the C library must be compatible with
development environments that supply fully functional versions of these
subroutines.  Such environments usually return error codes in a global
<code>errno</code>.  However, the Red Hat newlib C library provides a <em>macro</em>
definition for <code>errno</code> in the header file <samp>errno.h</samp>, as part
of its support for reentrant routines (see <a href="Reentrancy.html#Reentrancy">Reentrancy</a>).
</p>
<a name="index-errno-global-vs-macro"></a>
<p>The bridge between these two interpretations of <code>errno</code> is
straightforward: the C library routines with OS interface calls
capture the <code>errno</code> values returned globally, and record them in
the appropriate field of the reentrancy structure (so that you can query
them using the <code>errno</code> macro from <samp>errno.h</samp>).
</p>
<p>This mechanism becomes visible when you write stub routines for OS
interfaces.   You must include <samp>errno.h</samp>, then disable the macro,
like this:
</p>
<div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
</pre></div>

<p>The examples in this chapter include this treatment of <code>errno</code>.
</p>
<dl compact="compact">
<dt><code>_exit</code>
<a name="index-_005fexit"></a>
</dt>
<dd><p>Exit a program without cleaning up files.  If your system doesn&rsquo;t
provide this, it is best to avoid linking with subroutines that require
it (<code>exit</code>, <code>system</code>).
</p>
</dd>
<dt><code>close</code>
<a name="index-close"></a>
</dt>
<dd><p>Close a file.  Minimal implementation:
</p>
<div class="example">
<pre class="example">int close(int file) {
  return -1;
}
</pre></div>

</dd>
<dt><code>environ</code>
<a name="index-environ-1"></a>
</dt>
<dd><p>A pointer to a list of environment variables and their values.  For a
minimal environment, this empty list is adequate:
</p>
<div class="example">
<pre class="example">char *__env[1] = { 0 };
char **environ = __env;
</pre></div>

</dd>
<dt><code>execve</code>
<a name="index-execve"></a>
</dt>
<dd><p>Transfer control to a new process.  Minimal implementation (for a system
without processes):
</p>
<div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
int execve(char *name, char **argv, char **env) {
  errno = ENOMEM;
  return -1;
}
</pre></div>

</dd>
<dt><code>fork</code>
<a name="index-fork"></a>
</dt>
<dd><p>Create a new process.  Minimal implementation (for a system without processes):
</p>
<div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
int fork(void) {
  errno = EAGAIN;
  return -1;
}
</pre></div>

</dd>
<dt><code>fstat</code>
<a name="index-fstat"></a>
</dt>
<dd><p>Status of an open file.  For consistency with other minimal
implementations in these examples, all files are regarded as character
special devices.  The <samp>sys/stat.h</samp> header file required is
distributed in the <samp>include</samp> subdirectory for this C library.
</p>
<div class="example">
<pre class="example">#include &lt;sys/stat.h&gt;
int fstat(int file, struct stat *st) {
  st-&gt;st_mode = S_IFCHR;
  return 0;
}
</pre></div>

</dd>
<dt><code>getpid</code>
<a name="index-getpid"></a>
</dt>
<dd><p>Process-ID; this is sometimes used to generate strings unlikely to
conflict with other processes.  Minimal implementation, for a system
without processes:
</p>
<div class="example">
<pre class="example">int getpid(void) {
  return 1;
}
</pre></div>

</dd>
<dt><code>isatty</code>
<a name="index-isatty"></a>
</dt>
<dd><p>Query whether output stream is a terminal.   For consistency with the
other minimal implementations, which only support output to
<code>stdout</code>, this minimal implementation is suggested:
</p>
<div class="example">
<pre class="example">int isatty(int file) {
  return 1;
}
</pre></div>

</dd>
<dt><code>kill</code>
<a name="index-kill"></a>
</dt>
<dd><p>Send a signal.  Minimal implementation:
</p>
<div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
int kill(int pid, int sig) {
  errno = EINVAL;
  return -1;
}
</pre></div>

</dd>
<dt><code>link</code>
<a name="index-link"></a>
</dt>
<dd><p>Establish a new name for an existing file.  Minimal implementation:
</p>
<div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
int link(char *old, char *new) {
  errno = EMLINK;
  return -1;
}
</pre></div>

</dd>
<dt><code>lseek</code>
<a name="index-lseek"></a>
</dt>
<dd><p>Set position in a file.  Minimal implementation:
</p>
<div class="example">
<pre class="example">int lseek(int file, int ptr, int dir) {
  return 0;
}
</pre></div>

</dd>
<dt><code>open</code>
<a name="index-open"></a>
</dt>
<dd><p>Open a file.  Minimal implementation:
</p>
<div class="example">
<pre class="example">int open(const char *name, int flags, int mode) {
  return -1;
}
</pre></div>

</dd>
<dt><code>read</code>
<a name="index-read"></a>
</dt>
<dd><p>Read from a file.  Minimal implementation:
</p>
<div class="example">
<pre class="example">int read(int file, char *ptr, int len) {
  return 0;
}
</pre></div>

</dd>
<dt><code>sbrk</code>
<a name="index-sbrk"></a>
</dt>
<dd><p>Increase program data space.  As <code>malloc</code> and related functions
depend on this, it is useful to have a working implementation.  The
following suffices for a standalone system; it exploits the symbol
<code>_end</code> automatically defined by the GNU linker.
</p>
<div class="example">
<pre class="example">caddr_t sbrk(int incr) {
  extern char _end;		/* <span class="roman">Defined by the linker</span> */
  static char *heap_end;
  char *prev_heap_end;
 
  if (heap_end == 0) {
    heap_end = &amp;_end;
  }
  prev_heap_end = heap_end;
  if (heap_end + incr &gt; stack_ptr) {
    write (1, &quot;Heap and stack collision\n&quot;, 25);
    abort ();
  }

  heap_end += incr;
  return (caddr_t) prev_heap_end;
}
</pre></div>

</dd>
<dt><code>stat</code>
<a name="index-stat"></a>
</dt>
<dd><p>Status of a file (by name).  Minimal implementation:
</p>
<div class="example">
<pre class="example">int stat(char *file, struct stat *st) {
  st-&gt;st_mode = S_IFCHR;
  return 0;
}
</pre></div>

</dd>
<dt><code>times</code>
<a name="index-times"></a>
</dt>
<dd><p>Timing information for current process.  Minimal implementation:
</p>
<div class="example">
<pre class="example">int times(struct tms *buf) {
  return -1;
}
</pre></div>

</dd>
<dt><code>unlink</code>
<a name="index-unlink"></a>
</dt>
<dd><p>Remove a file&rsquo;s directory entry.  Minimal implementation:
</p>
<div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
int unlink(char *name) {
  errno = ENOENT;
  return -1; 
}
</pre></div>

</dd>
<dt><code>wait</code>
<a name="index-wait"></a>
</dt>
<dd><p>Wait for a child process.  Minimal implementation:
</p><div class="example">
<pre class="example">#include &lt;errno.h&gt;
#undef errno
extern int errno;
int wait(int *status) {
  errno = ECHILD;
  return -1;
}
</pre></div>

</dd>
<dt><code>write</code>
<a name="index-write"></a>
</dt>
<dd><p>Write to a file.  <samp>libc</samp> subroutines will use this
system routine for output to all files, <em>including</em>
<code>stdout</code>&mdash;so if you need to generate any output, for example to a
serial port for debugging, you should make your minimal <code>write</code>
capable of doing this.  The following minimal implementation is an
incomplete example; it relies on a <code>outbyte</code> subroutine (not
shown; typically, you must write this in assembler from examples
provided by your hardware manufacturer) to actually perform the output.
</p>
<div class="example">
<pre class="example">int write(int file, char *ptr, int len) {
  int todo;

  for (todo = 0; todo &lt; len; todo++) {
    outbyte (*ptr++);
  }
  return len;
}
</pre></div>

</dd>
</dl>

<hr>
<div class="header">
<p>
Next: <a href="Reentrant-Syscalls.html#Reentrant-Syscalls" accesskey="n" rel="next">Reentrant Syscalls</a>, Up: <a href="Syscalls.html#Syscalls" accesskey="u" rel="up">Syscalls</a> &nbsp; [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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