Signals I

Signals and system calls

The following program, from APUE3 section 1.9, adds signal handling to the simple shell program we studied before:

#include "apue.h"
#include <sys/wait.h>

static void	sig_int(int);		/* our signal-catching function */

int
main(void)
{
        char	buf[MAXLINE];	/* from apue.h */
        pid_t	pid;
        int		status;

        if (signal(SIGINT, sig_int) == SIG_ERR)
                err_sys("signal error");

        printf("%% ");	/* print prompt (printf requires %% to print %) */
        while (fgets(buf, MAXLINE, stdin) != NULL) {
                if (buf[strlen(buf) - 1] == '\n')
                        buf[strlen(buf) - 1] = 0; /* replace newline with null */

                if ((pid = fork()) < 0) {
                        err_sys("fork error");
                } else if (pid == 0) {		/* child */
                        execlp(buf, buf, (char *)0);
                        err_ret("couldn't execute: %s", buf);
                        exit(127);
                }

                /* parent */
                if ((pid = waitpid(pid, &status, 0)) < 0)
                        err_sys("waitpid error");
                printf("%% ");
        }
        exit(0);
}

void
sig_int(int signo)
{
        printf("interrupt\n%% ");
}

Three problems:

1. “Slow” system calls may get interrupted on signals

   • errno set to EINTR

2. Signals get lost

   • signal handling resets after each signal

3. Signal handler calls non-reentrant functions

   • malloc(), free(), standard I/O functions

Solutions:

• Don’t use signal(); use sigaction() instead

• From a signal handler, call only async-signal-safe functions (see man 7 signal)

Last updated: 2019–01–28