comp.lang.c FAQ list · Question 20.3

Q: How can I open files mentioned on the command line, and parse option flags?

A: Here is a skeleton which implements a traditional Unix-style argv parse, handling option flags beginning with -, and optional filenames. (The two flags accepted by this example are -a and -b; -b takes an argument.)

#include <stdio.h>
#include <string.h>
#include <errno.h>

main(int argc, char *argv[])
{
	int argi;
	int aflag = 0;
	char *bval = NULL;

	for(argi = 1; argi < argc && argv[argi][0] == '-'; argi++) {
		char *p;
		for(p = &argv[argi][1]; *p != '\0'; p++) {
			switch(*p) {
			case 'a':
				aflag = 1;
				printf("-a seen\n");
				break;

			case 'b':
				bval = argv[++argi];
				printf("-b seen (\"%s\")\n", bval);
				break;

			default:
				fprintf(stderr,
					"unknown option -%c\n", *p);
			}
		}
	}

	if(argi >= argc) {
		/* no filename arguments; process stdin */
		printf("processing standard input\n");
	} else {
		/* process filename arguments */

		for(; argi < argc; argi++) {
			FILE *ifp = fopen(argv[argi], "r");
			if(ifp == NULL) {
				fprintf(stderr, "can't open %s: %s\n",
					argv[argi], strerror(errno));
				continue;
			}

			printf("processing %s\n", argv[argi]);

			fclose(ifp);
		}
	}

	return 0;
}

There are several canned functions available for doing command line parsing in a standard way; the most popular one is getopt (see also question 18.16). Here is the above example, rewritten to use getopt:

extern char *optarg;
extern int optind;

main(int argc, char *argv[])
{
	int aflag = 0;
	char *bval = NULL;
	int c;

	while((c = getopt(argc, argv, "ab:")) != -1)
		switch(c) {
		case 'a':
			aflag = 1;
			printf("-a seen\n");
			break;

		case 'b':
			bval = optarg;
			printf("-b seen (\"%s\")\n", bval);
			break;
	}

	if(optind >= argc) {
		/* no filename arguments; process stdin */
		printf("processing standard input\n");
	} else {
		/* process filename arguments */

		for(; optind < argc; optind++) {
			FILE *ifp = fopen(argv[optind], "r");
			if(ifp == NULL) {
				fprintf(stderr, "can't open %s: %s\n",
					argv[optind], strerror(errno));
				continue;
			}

			printf("processing %s\n", argv[optind]);

			fclose(ifp);
		}
	}

	return 0;
}

The examples above overlook a number of nuances: a lone ``-'' is often taken to mean ``read standard input''; the marker ``--'' often signifies the end of the options (proper versions of getopt do handle this); it's traditional to print a usage message when a command is invoked with improper or missing arguments.

If you're wondering how argv is laid out in memory, it's actually a ``ragged array''; see the picture in question 20.2. See also questions 8.2, 13.7, and 19.20.

References: K&R1 Sec. 5.11 pp. 110-114
K&R2 Sec. 5.10 pp. 114-118
ISO Sec. 5.1.2.2.1
H&S Sec. 20.1 p. 416
PCS Sec. 5.6 pp. 81-2, Sec. 11 p. 159, pp. 339-40 Appendix F
Schumacher, ed., Software Solutions in C Sec. 4 pp. 75-85

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