cmd2.c

#include "apue.h"
#include <setjmp.h>

#define	TOK_ADD	   5

jmp_buf	jmpbuffer;

int
main(void)
{
	char	line[MAXLINE];

	if (setjmp(jmpbuffer) != 0)
		printf("error");
	while (fgets(line, MAXLINE, stdin) != NULL)
		do_line(line);
	exit(0);
}

 . . .

void
cmd_add(void)
{
	int		token;

	token = get_token();
	if (token < 0)		/* an error has occurred */
		longjmp(jmpbuffer, 1);
	/* rest of processing for this command */
}