comp.lang.c FAQ list · Question 13.6

Q: How can I split up a string into whitespace-separated fields?
How can I duplicate the process by which main() is handed argc and argv?

A: The only Standard function available for this kind of ``tokenizing'' is strtok, although it can be tricky to use [footnote] and it may not do everything you want it to. (For instance, it does not handle quoting.) Here is a usage example, which simply prints each field as it's extracted:

#include <stdio.h>
#include <string.h>
char string[] = "this is a test";	/* not char *; see Q 16.6 */
char *p;
for(p = strtok(string, " \t\n"); p != NULL;
			p = strtok(NULL, " \t\n"))
	printf("\"%s\"\n", p);

As an alternative, here is a routine I use for building an argv all at once:

#include <ctype.h>

int makeargv(char *string, char *argv[], int argvsize)
{
	char *p = string;
	int  i;
	int argc = 0;

	for(i = 0; i < argvsize; i++) {
		/* skip leading whitespace */
		while(isspace(*p))
			p++;

		if(*p != '\0')
			argv[argc++] = p;
		else {
			argv[argc] = 0;
			break;
		}

		/* scan over arg */
		while(*p != '\0' && !isspace(*p))
			p++;
		/* terminate arg: */
		if(*p != '\0' && i < argvsize-1)
			*p++ = '\0';
	}

	return argc;
}

Calling makeargv is straightforward:

	char *av[10];
	int i, ac = makeargv(string, av, 10);
	for(i = 0; i < ac; i++)
		printf("\"%s\"\n", av[i]);

If you want each separator character to be significant, for instance if you want two tabs in a row to indicate an omitted field, it's probably more straightforward to use strchr:

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

char string[] = "this\thas\t\tmissing\tfield";
char *p = string;

while(1) {		/* break in middle */
	char *p2 = strchr(p, '\t');
	if(p2 != NULL)
		*p2 = '\0';
	printf("\"%s\"\n", p);
	if(p2 == NULL)
		break;
	p = p2 + 1;
}

All the code fragments presented here modify the input string, by inserting \0's to terminate each field (meaning that the string must be writable; see question 1.32). If you'll need the original string later, make a copy before breaking it up.

References: K&R2 Sec. B3 p. 250
ISO Sec. 7.11.5.8
H&S Sec. 13.7 pp. 333-4
PCS p. 178

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