faqcatea63.html

this is a mirrored site c-faq. thought might need offline

</p><p>See also question <a href="faqcat1d60.html?sec=stdio#constrpath">12.28b</a>.
<hr><hr><hr>
<a name="systemret">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/systemret.html"><!-- qtag -->Question 19.29</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How do I get an accurate error status return from <TT>system</TT> on MS-DOS?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You can't;
COMMAND.COM

doesn't

tend to
provide one.
If you don't need
COMMAND.COM's

services
(i.e. if you're just trying to invoke a simple program,
without I/O redirection
and such)
try
one of the <TT>spawn</TT> routines, instead.
<hr><hr><hr>
<a name="popen">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/popen.html"><!-- qtag -->Question 19.30</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I invoke
another program
or command
and
trap
its output?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Unix
and some other systems provide a <TT>popen</TT>
function,
which sets up
a stdio stream
on a pipe
connected to the process
running a command,
so that the
calling program can read the output
(or
alternatively
supply the input).
Using <TT>popen</TT>,
the last example from
question <a href="faqcatea63.html?sec=osdep#system2">19.28</a>
would look like
<pre>
	extern FILE *popen();

	sprintf(cmdbuf, "sort &lt; %s", datafile);

	fp = popen(cmdbuf, "r");

	/* ...now read sorted data from fp... */

	pclose(fp);
</pre>

(Do be sure
to call <TT>pclose</TT>,
as shown;
leaving it out will seem to work at first
but may eventually run you out of processes




or
file descriptors.)
</p><p>If you can't use <TT>popen</TT>,
you may be able to use <TT>system</TT>,
with the output going to a file
which you then open and read,
as the code in question <a href="faqcatea63.html?sec=osdep#system2">19.28</a> was doing already.
<a href="../../osdep/concur.html" rel=subdocument>[footnote]</a>
</p><p>If you're using Unix and <TT>popen</TT> isn't sufficient,
you can learn about
<TT>pipe</TT>,
<TT>dup</TT>, 
<TT>fork</TT>,
and
<TT>exec</TT>.
</p><p>(One thing that
probably would <em>not</em> work,
by the way,
would be
to use <TT>freopen</TT>.)


</p>


<p>References:

PCS Sec. 11 p. 169
<hr><hr><hr>
<a name="exepath">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/exepath.html"><!-- qtag -->Question 19.31</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can my program discover the complete pathname to the executable
from
which it was invoked?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
<TT>argv[0]</TT> may contain all or part of the
pathname,
or it may contain nothing.
You may
be able to duplicate the command language interpreter's
search path logic to locate the
executable if the name in <TT>argv[0]</TT>
is
present but
incomplete.
However, there is no guaranteed
solution.
</p>



<p>References:

K&amp;R1 Sec. 5.11 p. 111
<br>
K&amp;R2 Sec. 5.10 p. 115
<br>
ISO Sec. 5.1.2.2.1
<br>
H&amp;S Sec. 20.1 p. 416
<hr><hr><hr>
<a name="auxfiles">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/auxfiles.html"><!-- qtag -->Question 19.32</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I automatically locate
a program's
configuration files
in the same directory as
the
executable?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It's hard,
in general;

it's an equivalent problem to the one in
question <a href="faqcatea63.html?sec=osdep#exepath">19.31</a>.

Even if you can
figure out a workable way to do it,
you might want to consider making the program's
auxiliary (library) directory configurable,
perhaps with an environment variable.
(It's especially important to allow variable placement
of a program's configuration files
when the program will be used by several people,
e.g. on a multiuser system.)
<hr><hr><hr>
<a name="setenv">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/setenv.html"><!-- qtag -->Question 19.33</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can a process change an environment variable in its caller?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It may or may not be possible to do so at all.
Different operating systems implement
global
name/value functionality
similar to the
Unix
environment
in different ways.
Whether the ``environment'' can be usefully altered by a
running program, and if so, how, is system-dependent.
</p><p>Under
Unix,
a process can modify its own environment
(some systems provide <TT>setenv</TT>
or <TT>putenv</TT> functions
for the purpose),
and the
modified environment is
generally
passed on to
child processes,
but it is
<em>not</em>
propagated back to the parent process.
(The environment of the parent process can only be altered if the
parent is explicitly set up
to listen for some kind of change requests.
The conventional execution of the BSD ``tset'' program in .profile
and .login files effects such a scheme.)
Under MS-DOS, it's possible to
manipulate the
master copy of the environment,

but the required techniques are arcane.
(See an MS-DOS FAQ list.)

<hr><hr><hr>
<a name="argv">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/argv.html"><!-- qtag -->Question 19.34</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I open files mentioned on the command line,
and parse option flags?
</p>
<p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcat38c2.html?sec=misc#argv">20.3</a>.
<hr><hr><hr>
<a name="exitvsreturn">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/exitvsreturn.html"><!-- qtag -->Question 19.35</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Is <TT>exit(status)</TT> truly equivalent
to returning the same <TT>status</TT> from <TT>main</TT>?
</p>
<p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcat7d4b.html?sec=ansi#exitvsreturn">11.16</a>.
<hr><hr><hr>
<a name="dynlink">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/dynlink.html"><!-- qtag -->Question 19.36</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I read in an object file and jump to
locations
in it?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You want a dynamic linker or loader.
It may be possible to <TT>malloc</TT> some space
and read in object files,
but you have to know an awful lot
about object file formats, relocation, etc.,



and this approach can't work if code and data
reside in separate address spaces
or
if code is otherwise privileged.
</p><p>
Under BSD
Unix,
you could use <TT>system</TT> and <TT>ld&nbsp;-A</TT>
to do the linking for you.


Many

versions of SunOS and System V
have the -ldl library
containing routines like
<TT>dlopen</TT> and
<TT>dlsym</TT>
which
allow
object files to be dynamically
loaded.



Under VMS, use LIB$FIND_IMAGE_SYMBOL.
GNU
has
a
package called ``dld''.
See also question
<a href="faqcat744e.html?sec=varargs#invvarargs">15.13</a>.
<hr><hr><hr>
<a name="subsecond">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/subsecond.html"><!-- qtag -->Question 19.37</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I implement a delay,
or time a user's response,
with sub-second resolution?

</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Unfortunately, there is no portable way.
Routines
you might look for on your system include
<TT>clock</TT>,



<TT>delay</TT>,
<TT>ftime</TT>,
<TT>gettimeofday</TT>,
<TT>msleep</TT>,
<TT>nap</TT>,



<TT>napms</TT>,




<TT>nanosleep</TT>,
<TT>setitimer</TT>,
<TT>sleep</TT>,




<TT>Sleep</TT>,
<TT>times</TT>,
and
<TT>usleep</TT>.
(A function called <TT>wait</TT>,
however,
is at least under Unix <em>not</em> what you want.)
The <TT>select</TT> and <TT>poll</TT> calls
(if available)
can be pressed into
service to implement simple delays.
On MS-DOS machines, it is possible to reprogram the system timer
and timer interrupts.

</p><p>Of these,
only
<TT>clock</TT> is part of the ANSI Standard.
The difference between two calls to <TT>clock</TT>

gives elapsed



execution
time,
and
may even
have subsecond resolution,
if CLOCKS_PER_SEC
is greater than 1.
However, <TT>clock</TT> gives
elapsed
processor time
used by the current program,
which
on a multitasking system
(or in a non-CPU-intensive program)
may
differ considerably from
real time.
</p><p>If you're trying to implement a delay
and all you have available
is a time-reporting function,
you can implement a
CPU-intensive
busy-wait,
but this is only
an
option on a single-user, single-tasking machine,
as it is terribly antisocial
to any other processes.
Under a multitasking operating system,
be sure to use
a
call
which puts your process to sleep for the duration,
such as <TT>sleep</TT> or <TT>select</TT>,
or <TT>pause</TT>
in conjunction with <TT>alarm</TT> or <TT>setitimer</TT>.
</p><p>For really brief delays,
it's tempting to use a do-nothing loop like
<pre>
	long int i;
	for(i = 0; i &lt; 1000000; i++)
		;
</pre>
but resist this temptation if at all possible!
For one thing,
your carefully-calculated delay loops
will
stop working
properly
next month when a faster processor comes out.
Perhaps
worse,

a
clever
compiler may notice that the loop does nothing
and optimize it away completely.
</p><p>Additional links:
A
<a href="../../osdep/sd25.html" rel=subdocument>technique involving I/O instructions</a>
described by Pedro Zorzenon Neto
</p>


<p>References:

H&amp;S Sec. 18.1 pp. 398-9
<br>
PCS Sec. 12 pp. 197-8,215-6
<br>
POSIX Sec. 4.5.2
<hr><hr><hr>
<a name="sigint">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/sigint.html"><!-- qtag -->Question 19.38</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I trap or ignore
keyboard interrupts
like
control-C?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The basic step is to call <TT>signal</TT>,
either as
<pre>
	#include &lt;signal.h&gt;
	signal(SIGINT, SIG_IGN);
</pre>
to ignore
the interrupt signal,
or as
<pre>
	extern void func(int);
	signal(SIGINT, func);
</pre>
to cause control to transfer to function <TT>func</TT>
on receipt of
an interrupt signal.
<a href="../../osdep/keybintr.html" rel=subdocument>[footnote]</a>
</p><p>On a multi-tasking system such as Unix,
it's best to use a slightly more
involved
technique:
<pre>
	extern void func(int);
	if(signal(SIGINT, SIG_IGN) != SIG_IGN)
		signal(SIGINT, func);
</pre>
The test and extra call

ensure that a
keyboard interrupt
typed in the foreground won't 
inadvertently interrupt a program running in the background
(and it doesn't hurt to code calls to <TT>signal</TT> this way 
on any system).
<a href="../../osdep/pgrps.html" rel=subdocument>[footnote]</a>
</p><p>On some systems,
keyboard interrupt
handling is also a function of
the mode of
the terminal-input subsystem;
see question <a href="faqcatea63.html?sec=osdep#cbreak">19.1</a>.
On some
systems,

checking for
keyboard interrupts
is only performed when the program is 
reading input,
and
keyboard interrupt
handling may therefore depend on which input 
routines are being called
(and <em>whether</em> any input routines are active at all).

On MS-DOS systems, <TT>setcbrk</TT>
or <TT>ctrlbrk</TT>

functions
may also be involved.
</p>



<p>References:

ISO Secs. 7.7,7.7.1
<br>
H&amp;S Sec. 19.6 pp. 411-3
<br>
PCS Sec. 12 pp. 210-2
<br>
POSIX Secs. 3.3.1,3.3.4
<hr><hr><hr>
<a name="fpexcepts">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/fpexcepts.html"><!-- qtag -->Question 19.39</a>
</h1>
<p>
<f