getopt()--Get Flag Letters from Argument Vector

 #include <unistd.h>

 int getopt(int argc, char * const argv[],  
      const char *optionstring);

The getopt() function returns the next flag letter in the argv list that matches a letter in optionstring. The optarg external variable is set to point to the start of the flag's parameter on return from getopt()

getopt() places the argv index of the next argument to be processed in optind. The optind variable is external. It is initialized to 1 before the first call to getopt().

getopt() can be used to help a program interpret command line flags that are passed to it.

Parameters

argc

(Input) The number of parameters passed by the function.

argv

(Input) The list of parameters passed to the function.

optionstring

(Input) A string of flag letters. The string must contain the flag letters that the program using getopt() recognizes. If a letter is followed by a colon, the flag is expected to have an argument or group of arguments, which can be separated from it by blank spaces.

The special flag "--" (two hyphens) can be used to delimit the end of the options. When this flag is encountered, the "--" is skipped and EOF is returned. This flag is useful in delimiting arguments beginning with a hyphen that are not options.

Authorities

None.

Return Value

Error Conditions

The getopt() function does not return an error.

Example

See Code disclaimer information for information pertaining to code examples.

The following example processes the flags for a command that can take the mutually exclusive flags a and b, and the flags f and o, both of which require parameters.

#include <unistd.h>

int main( int argc, char *argv[] )
{
 int c;
 extern int optind;
 extern char *optarg;
 .
 .
 .
 while ((c = getopt(argc, argv, "abf:o:")) != EOF)
 {
  switch (c)
  {
   case 'a':
    if (bflg)
     errflg++;
    else
     aflg++;
    break;
   case 'b':
    if (aflg)
     errflg++;
    else
     bflg++;
    break;
   case 'f':
    ifile = optarg;
    break;
   case 'o':
    ofile = optarg;
    break;
   case '?':
    errflg++;
  } /* case */
  if (errflg)
  {
   fprintf(stderr, "usage: . . . ");
   exit(2);
  }
 } /* while */
 for ( ; optind < argc; optind++)
 {
  if (access(argv[optind], R_OK))
  {
   .
   .
   .
  }
 } /* for */
}   /* main */