Advanced UNIX Programming

The getopt(3) Function

What helps to make UNIX commands consistent in their syntax is that most commands use the library function getopt(3). Its synopsis is as follows:

#include <unistd.h>

extern char *optarg;
extern int optind;    /* initialized to 1 */
extern int optopt;
extern int opterr;    /* initialized to 1 */
extern int optreset;  /* extension to IEEE Std1003.2 "POSIX.2" */

int getopt(int argc, char * const *argv, const char *optstring);

extern void getoptreset(void);            /* SGI IRIX 6.5 only */

The getopt(3) function returns the option letter that is parsed. Alternatively, -1 is returned when the end of the options has been reached. The value ? is returned when an unrecognized option character has been encountered. If the argument optstring