getopt(3) Library Functions Manual getopt(3)
NAME
getopt - Gets flag letters from the argument vector
LIBRARY
Standard C Library (libc)
SYNOPSIS
#include <unistd.h>
int getopt(
int argc,
char * const argv[],
const char *optstring);
extern char *optarg;
extern int optind;
extern int opterr;
extern int optopt;
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
getopt(): XSH5.0
Refer to the standards(5) reference page for more information about industry standards and associated tags.
PARAMETERS
Specifies the number of parameters passed to the routine. Points to an array of argc pointers to argument strings. Specifies a string of
recognized flag characters. If a character is followed by a : (colon), the flag is expected to take a parameter that may or may not be
separated from it by white space.
DESCRIPTION
The getopt() function parses argument lists. It returns the next flag character in the argv parameter list that matches a character in the
optstring parameter. If that flag takes an argument, the getopt() function has the optarg variable point to the flag argument according to
the following rules: If the flag is the last character pointed to by an argv element, optarg will contain argv's next element, and optind
is incremented by 2. The getopt() function returns an error if the resulting optind is greater than or equal to argc. If the flag is not
the last character, then the optarg variable points to the string after the flag character in the associated element of argv. The optind
variable is incremented by 1.
The optarg external variable is set to point to the start of the flag's parameter on return from the getopt() function.
The getopt() function places the argv index of the next argument to be processed in optind. The optind variable is externally initialized
to 1 before the first call to getopt() so that argv[0] is not processed. Error messages can be suppressed by providing a value of 0 (zero)
as the opterr parameter.
NOTES
[Tru64 UNIX] The external int optopt variable is set to the real flag found in the argv parameter. This is true whether the flag is in the
optstring parameter or not.
EXAMPLES
The following example shows a suggested way to use the getopt() function.
#include <unistd.h> main(argc, argv) int argc; char *argv[];
#define ARGS "r:w:f:s"
{
int c, errflg = 0;
int readers = 1, writers = 1;
int freeBufs = 1;
int doStats = FALSE;
optarg = NULL;
while (!errflg && ((c = getopt(argc, argv, ARGS)) != -1))
switch (c) {
case 'r' :
readers = atoi(optarg);
break;
case 'w' :
writers = atoi(optarg);
break;
case 'f' :
freeBufs = atoi(optarg);
break;
case 's' :
doStats = TRUE;
break;
default :
errflg++;
}
RETURN VALUES
Upon successful completion, the getopt() function returns the flag character that was detected. If the function encounters a flag that is
not included in the optstring parameter, or if the : (colon) character is used incorrectly, the getopt() function prints an error message
on stderr and returns a ? (question mark). If there is a missing flag, the getopt() function returns a : (colon) if optstring's first
character is a : (colon), and a ? (question mark) otherwise. In addition, the getopt() function sets the optopt variable to the flag char-
acter that caused one of these errors.
The getopt() function also displays a diagnostic message if the application did not set the opterr variable to 0 (zero), and optstring's
first character is not a : (colon).
When all flags have been processed (that is, up to the first nonflag argument), the getopt() function returns a value of -1. The special
flag -- (dash dash) can be used to delimit the end of the flags; -1 is returned, and the -- (dash dash) string is skipped.
The getopt() function does not change optind, and also returns a value of -1, if one of the following occurs: The argv[optind] result is
NULL. The *argv[optind] result is not the special - (dash) flag. The argv[optind] result points to the - (dash) string.
The getopt() function does increment optind if the result of argv[optind] points to the -- (dash dash) string.
RELATED INFORMATION
Commands: getopt(1)
Standards: standards(5) delim off
getopt(3)