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<title>A Common Error Description Library for UNIX: 5. Run-time support routines</title>
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<a name="Run_002dtime-support-routines"></a>
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<a name="Run_002dtime-support-routines-1"></a>
<h2 class="section">5. Run-time support routines</h2>
<p>Any source file which uses the routines supplied with or produced by the
com_err package should include the header file ‘<tt><com_err.h></tt>’. It
contains declarations and definitions which may be needed on some systems.
(Some functions cannot be referenced properly without the return type
declarations in this file. Some functions may work properly on most
architectures even without the header file, but relying on this is not
recommended.)
</p>
<p>The run-time support routines and variables provided via this package
include the following:
</p>
<table><tr><td> </td><td><pre class="example">void initialize_<var>xxxx</var>_error_table (void);
</pre></td></tr></table>
<p>One of these routines is built by the error compiler for each error table.
It makes the <var>xxxx</var> error table “known” to the error reporting
system. By convention, this routine should be called in the initialization
routine of the <var>xxxx</var> library. If the library has no initialization
routine, some combination of routines which form the core of the library
should ensure that this routine is called. It is not advised to leave it
the caller to make this call.
</p>
<p>There is no harm in calling this routine more than once.
</p>
<table><tr><td> </td><td><pre class="example">#define ERROR_TABLE_BASE_<var>xxxx</var> <var>nnnnn</var>L
</pre></td></tr></table>
<p>This symbol contains the value of the first error code entry in the
specified table.
This rarely needs be used by the
programmer.
</p>
<dl>
<dt><a name="index-char"></a><u>Function:</u> const <b>char</b><i> *error_message (long <var>code</var>);</i></dt>
<dd>
<p>This routine returns the character string error message associated
with <code>code</code>; if this is associated with an unknown error table, or
if the code is associated with a known error table but the code is not
in the table, a string of the form ‘<samp>Unknown code <var>xxxx nn</var></samp>’ is
returned, where <var>xxxx</var> is the error table name produced by
reversing the compaction performed on the error table number implied
by that error code, and <var>nn</var> is the offset from that base value.
</p>
<p>Although this routine is available for use when needed, its use should be
left to circumstances which render <code>com_err</code> (below) unusable.
</p>
</dd></dl>
<dl>
<dt><a name="index-"></a><u>Function:</u></dt>
<dd><p>void com_err (const char *<var>whoami</var>, long <var>error_code</var>,
const char *<var>format</var>, ...);
</p>
<p>This routine provides an alternate way to print error messages to
standard error; it allows the error message to be passed in as a
parameter, rather than in an external variable. <em>Provide grammatical
context for “message.”</em>
</p>
<p>The module reporting the error should be passed in via <var>whoami</var>.
If <var>format</var> is <code>(char *)NULL</code>, the formatted message will not be
printed. <var>format</var> may not be omitted.
</p>
</dd></dl>
<dl>
<dt><a name="index--1"></a><u>Function:</u></dt>
<dd><p>void com_err_va (const char *<var>whoami</var>, long <var>error_code</var>, const char *<var>format</var>, va_list <var>args</var>);
</p>
<p>This routine provides an interface, equivalent to <code>com_err</code> above,
which may be used by higher-level variadic functions (functions which
accept variable numbers of arguments).
</p>
</dd></dl>
<dl>
<dt><a name="index-_0028"></a><u>Function:</u> void <b>(</b><i>*set_com_err_hook (void (*<var>proc</var>) (const char *<var>whoami</var>, long <var>error_code</var>, va_list <var>args</var>))) (const char *<var>whoami</var>, long <var>error_code</var>, va_list <var>args</var>);</i></dt>
<dt><a name="index-reset_005fcom_005ferr_005fhook"></a><u>Function:</u> void <b>reset_com_err_hook</b><i> ();</i></dt>
<dd>
<p>These two routines allow a routine to be dynamically substituted for
‘<samp>com_err</samp>’. After ‘<samp>set_com_err_hook</samp>’ has been called,
calls to ‘<samp>com_err</samp>’ will turn into calls to the new hook routine.
‘<samp>reset_com_err_hook</samp>’ turns off this hook. This may intended to
be used in daemons (to use a routine which calls <cite>syslog(3)</cite>), or
in a window system application (which could pop up a dialogue box).
</p>
<p>If a program is to be used in an environment in which simply printing
messages to the <code>stderr</code> stream would be inappropriate (such as in a
daemon program which runs without a terminal attached),
<code>set_com_err_hook</code> may be used to redirect output from <code>com_err</code>.
The following is an example of an error handler which uses <cite>syslog(3)</cite>
as supplied in BSD 4.3:
</p>
<table><tr><td> </td><td><pre class="example">#include <stdio.h>
#include <stdarg.h>
#include <syslog.h>
/* extern openlog (const char * name, int logopt, int facility); */
/* extern syslog (int priority, char * message, ...); */
void hook (const char * whoami, long code,
const char * format, va_list args)
{
char buffer[BUFSIZ];
static int initialized = 0;
if (!initialized) {
openlog (whoami,
LOG_NOWAIT|LOG_CONS|LOG_PID|LOG_NDELAY,
LOG_DAEMON);
initialized = 1;
}
vsprintf (buffer, format, args);
syslog (LOG_ERR, "%s %s", error_message (code), buffer);
}
</pre></td></tr></table>
<p>After making the call
<code>set_com_err_hook (hook);</code>,
any calls to <code>com_err</code> will result in messages being sent to the
<var>syslogd</var> daemon for logging.
The name of the program, ‘<samp>whoami</samp>’, is supplied to the
‘<samp>openlog()</samp>’ call, and the message is formatted into a buffer and
passed to <code>syslog</code>.
</p>
<p>Note that since the extra arguments to <code>com_err</code> are passed by
reference via the <code>va_list</code> value <code>args</code>, the hook routine may
place any form of interpretation on them, including ignoring them. For
consistency, <code>printf</code>-style interpretation is suggested, via
<code>vsprintf</code> (or <code>_doprnt</code> on BSD systems without full support for
the ANSI C library).
</p>
</dd></dl>
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