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<TITLE>The GNU C Library - Process Startup and Termination</TITLE>
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<H1><A NAME="SEC385" HREF="library_toc.html#SEC385" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC385">Process Startup and Termination</A></H1>
<A NAME="IDX1622"></A>
<P>
<DFN>Processes</DFN> are the primitive units for allocation of system
resources.  Each process has its own address space and (usually) one
thread of control.  A process executes a program; you can have multiple
processes executing the same program, but each process has its own copy
of the program within its own address space and executes it
independently of the other copies.
<P>
This chapter explains what your program should do to handle the startup
of a process, to terminate its process, and to receive information
(arguments and the environment) from the parent process.
<P>
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<H2><A NAME="SEC386" HREF="library_toc.html#SEC386" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC386">Program Arguments</A></H2>
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<P>
The system starts a C program by calling the function <CODE>main</CODE>.  It
is up to you to write a function named <CODE>main</CODE>---otherwise, you
won't even be able to link your program without errors.
<P>
You can define <CODE>main</CODE> either to take no arguments, or to take two
arguments that represent the command line arguments to the program, like
this:
<P>
<PRE>
int main (int <VAR>argc</VAR>, char *<VAR>argv</VAR>[])
</PRE>
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<A NAME="IDX1627"></A>
<P>
The command line arguments are the whitespace-separated tokens given in
the shell command used to invoke the program; thus, in <SAMP>`cat foo
bar'</SAMP>, the arguments are <SAMP>`foo'</SAMP> and <SAMP>`bar'</SAMP>.  The only way a
program can look at its command line arguments is via the arguments of
<CODE>main</CODE>.  If <CODE>main</CODE> doesn't take arguments, then you cannot get
at the command line.
<P>
The value of the <VAR>argc</VAR> argument is the number of command line
arguments.  The <VAR>argv</VAR> argument is a vector of C strings; its
elements are the individual command line argument strings.  The file
name of the program being run is also included in the vector as the
first element; the value of <VAR>argc</VAR> counts this element.  A null
pointer always follows the last element: <CODE><VAR>argv</VAR>[<VAR>argc</VAR>]</CODE>
is this null pointer.
<P>
For the command <SAMP>`cat foo bar'</SAMP>, <VAR>argc</VAR> is 3 and <VAR>argv</VAR> has
three elements, <CODE>"cat"</CODE>, <CODE>"foo"</CODE> and <CODE>"bar"</CODE>.
<P>
If the syntax for the command line arguments to your program is simple
enough, you can simply pick the arguments off from <VAR>argv</VAR> by hand.
But unless your program takes a fixed number of arguments, or all of the
arguments are interpreted in the same way (as file names, for example),
you are usually better off using <CODE>getopt</CODE> to do the parsing.
<P>
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<H3><A NAME="SEC387" HREF="library_toc.html#SEC387" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC387">Program Argument Syntax Conventions</A></H3>
<P>
POSIX recommends these conventions for command line arguments.
<CODE>getopt</CODE> (see section <A HREF="library_22.html#SEC388" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_22.html#SEC388">Parsing Program Options</A>) makes it easy to implement them.
<P>
<UL>
<LI>
Arguments are options if they begin with a hyphen delimiter (<SAMP>`-'</SAMP>).
<P>
<LI>
Multiple options may follow a hyphen delimiter in a single token if
the options do not take arguments.  Thus, <SAMP>`-abc'</SAMP> is equivalent to
<SAMP>`-a -b -c'</SAMP>.
<P>
<LI>
Option names are single alphanumeric characters (as for <CODE>isalnum</CODE>;
see section <A HREF="library_4.html#SEC55" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_4.html#SEC55">Classification of Characters</A>).
<P>
<LI>
Certain options require an argument.  For example, the <SAMP>`-o'</SAMP> command
of the <CODE>ld</CODE> command requires an argument--an output file name.
<P>
<LI>
An option and its argument may or may not appear as separate tokens.  (In
other words, the whitespace separating them is optional.)  Thus,
<SAMP>`-o foo'</SAMP> and <SAMP>`-ofoo'</SAMP> are equivalent.
<P>
<LI>
Options typically precede other non-option arguments.
<P>
The implementation of <CODE>getopt</CODE> in the GNU C library normally makes
it appear as if all the option arguments were specified before all the
non-option arguments for the purposes of parsing, even if the user of
your program intermixed option and non-option arguments.  It does this
by reordering the elements of the <VAR>argv</VAR> array.  This behavior is
nonstandard; if you want to suppress it, define the
<CODE>_POSIX_OPTION_ORDER</CODE> environment variable.  See section <A HREF="library_22.html#SEC394" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_22.html#SEC394">Standard Environment Variables</A>.
<P>
<LI>
The argument <SAMP>`--'</SAMP> terminates all options; any following arguments
are treated as non-option arguments, even if they begin with a hyphen.
<P>
<LI>
A token consisting of a single hyphen character is interpreted as an
ordinary non-option argument.  By convention, it is used to specify
input from or output to the standard input and output streams.
<P>
<LI>
Options may be supplied in any order, or appear multiple times.  The
interpretation is left up to the particular application program.
</UL>
<A NAME="IDX1631"></A>
<P>
GNU adds <DFN>long options</DFN> to these conventions.  Long options consist
of <SAMP>`--'</SAMP> followed by a name made of alphanumeric characters and
dashes.  Option names are typically one to three words long, with
hyphens to separate words.  Users can abbreviate the option names as
long as the abbreviations are unique.
<P>
To specify an argument for a long option, write
<SAMP>`--<VAR>name</VAR>=<VAR>value</VAR>'</SAMP>.  This syntax enables a long option to
accept an argument that is itself optional.
<P>
Eventually, the GNU system will provide completion for long option names
in the shell.
<P>
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<A NAME="IDX1633"></A>
<A NAME="IDX1634"></A>
<H3><A NAME="SEC388" HREF="library_toc.html#SEC388" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC388">Parsing Program Options</A></H3>
<P>
Here are the details about how to call the <CODE>getopt</CODE> function.  To
use this facility, your program must include the header file
<TT>`unistd.h'</TT>.
<A NAME="IDX1635"></A>
<P>
<A NAME="IDX1636"></A>
<U>Variable:</U> int <B>opterr</B><P>
If the value of this variable is nonzero, then <CODE>getopt</CODE> prints an
error message to the standard error stream if it encounters an unknown
option character or an option with a missing required argument.  This is
the default behavior.  If you set this variable to zero, <CODE>getopt</CODE>
does not print any messages, but it still returns the character <CODE>?</CODE>
to indicate an error.
<P>
<A NAME="IDX1637"></A>
<U>Variable:</U> int <B>optopt</B><P>
When <CODE>getopt</CODE> encounters an unknown option character or an option
with a missing required argument, it stores that option character in
this variable.  You can use this for providing your own diagnostic
messages.
<P>
<A NAME="IDX1638"></A>
<U>Variable:</U> int <B>optind</B><P>
This variable is set by <CODE>getopt</CODE> to the index of the next element
of the <VAR>argv</VAR> array to be processed.  Once <CODE>getopt</CODE> has found
all of the option arguments, you can use this variable to determine
where the remaining non-option arguments begin.  The initial value of
this variable is <CODE>1</CODE>.
<P>
<A NAME="IDX1639"></A>
<U>Variable:</U> char * <B>optarg</B><P>
This variable is set by <CODE>getopt</CODE> to point at the value of the
option argument, for those options that accept arguments.
<P>
<A NAME="IDX1640"></A>
<U>Function:</U> int <B>getopt</B> <I>(int <VAR>argc</VAR>, char **<VAR>argv</VAR>, const char *<VAR>options</VAR>)</I><P>
The <CODE>getopt</CODE> function gets the next option argument from the
argument list specified by the <VAR>argv</VAR> and <VAR>argc</VAR> arguments.
Normally these values come directly from the arguments received by
<CODE>main</CODE>.
<P>
The <VAR>options</VAR> argument is a string that specifies the option
characters that are valid for this program.  An option character in this
string can be followed by a colon (<SAMP>`:'</SAMP>) to indicate that it takes a
required argument.
<P>
If the <VAR>options</VAR> argument string begins with a hyphen (<SAMP>`-'</SAMP>), this
is treated specially.  It permits arguments that are not options to be
returned as if they were associated with option character <SAMP>`\0'</SAMP>.
<P>
The <CODE>getopt</CODE> function returns the option character for the next
command line option.  When no more option arguments are available, it
returns <CODE>-1</CODE>.  There may still be more non-option arguments; you
must compare the external variable <CODE>optind</CODE> against the <VAR>argc</VAR>
parameter to check this.
<P>
If the option has an argument, <CODE>getopt</CODE> returns the argument by
storing it in the varables <VAR>optarg</VAR>.  You don't ordinarily need to
copy the <CODE>optarg</CODE> string, since it is a pointer into the original
<VAR>argv</VAR> array, not into a static area that might be overwritten.
<P>
If <CODE>getopt</CODE> finds an option character in <VAR>argv</VAR> that was not
included in <VAR>options</VAR>, or a missing option argument, it returns
<SAMP>`?'</SAMP> and sets the external variable <CODE>optopt</CODE> to the actual
option character.  If the first character of <VAR>options</VAR> is a colon
(<SAMP>`:'</SAMP>), then <CODE>getopt</CODE> returns <SAMP>`:'</SAMP> instead of <SAMP>`?'</SAMP> to
indicate a missing option argument.  In addition, if the external
variable <CODE>opterr</CODE> is nonzero (which is the default), <CODE>getopt</CODE>
prints an error message.
<P>
<H3><A NAME="SEC389" HREF="library_toc.html#SEC389" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC389">Example of Parsing Arguments with <CODE>getopt</CODE></A></H3>
<P>
Here is an example showing how <CODE>getopt</CODE> is typically used.  The
key points to notice are:
<P>
<UL>
<LI>
Normally, <CODE>getopt</CODE> is called in a loop.  When <CODE>getopt</CODE> returns
<CODE>-1</CODE>, indicating no more options are present, the loop terminates.
<P>
<LI>
A <CODE>switch</CODE> statement is used to dispatch on the return value from
<CODE>getopt</CODE>.  In typical use, each case just sets a variable that
is used later in the program.
<P>
<LI>
A second loop is used to process the remaining non-option arguments.
</UL>
<P>
<PRE>
#include &#60;unistd.h&#62;
#include &#60;stdio.h&#62;

int 
main (int argc, char **argv)
{
  int aflag = 0;
  int bflag = 0;
  char *cvalue = NULL;
  int index;
  int c;

  opterr = 0;

  while ((c = getopt (argc, argv, "abc:")) != -1)
    switch (c)
      {
      case 'a':
        aflag = 1;
        break;
      case 'b':
        bflag = 1;
        break;
      case 'c':
        cvalue = optarg;
        break;
      case '?':
        if (isprint (optopt))
          fprintf (stderr, "Unknown option `-%c'.\n", optopt);
        else
          fprintf (stderr,
                   "Unknown option character `\\x%x'.\n",
                   optopt);
        return 1;
      default:
        abort ();
      }

  printf ("aflag = %d, bflag = %d, cvalue = %s\n", aflag, bflag, cvalue);

  for (index = optind; index &#60; argc; index++)
    printf ("Non-option argument %s\n", argv[index]);
  return 0;
}
</PRE>
<P>
Here are some examples showing what this program prints with different
combinations of arguments:
<P>
<PRE>
% testopt
aflag = 0, bflag = 0, cvalue = (null)

% testopt -a -b
aflag = 1, bflag = 1, cvalue = (null)

% testopt -ab
aflag = 1, bflag = 1, cvalue = (null)

% testopt -c foo
aflag = 0, bflag = 0, cvalue = foo

% testopt -cfoo
aflag = 0, bflag = 0, cvalue = foo

% testopt arg1
aflag = 0, bflag = 0, cvalue = (null)
Non-option argument arg1

% testopt -a arg1
aflag = 1, bflag = 0, cvalue = (null)
Non-option argument arg1

% testopt -c foo arg1
aflag = 0, bflag = 0, cvalue = foo
Non-option argument arg1

% testopt -a -- -b
aflag = 1, bflag = 0, cvalue = (null)
Non-option argument -b

% testopt -a -
aflag = 1, bflag = 0, cvalue = (null)