Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.10 alarmandpauseFunctions 343

Here, we specifically want a slow system call to be interrupted. We’ll see a portable

way to do this in Section 10.14.

Example

Let’s redo the preceding example usinglongjmp.This way, we don’t need to worry

about whether a slow system call is interrupted.

#include "apue.h"

#include <setjmp.h>

static void sig_alrm(int);

static jmp_buf env_alrm;

int

main(void)

{

int n;

char line[MAXLINE];

if (signal(SIGALRM, sig_alrm) == SIG_ERR)

err_sys("signal(SIGALRM) error");

if (setjmp(env_alrm) != 0)

err_quit("read timeout");

alarm(10);

if ((n = read(STDIN_FILENO, line, MAXLINE)) < 0)

err_sys("read error");

alarm(0);

write(STDOUT_FILENO, line, n);

exit(0);

}

static void

sig_alrm(int signo)

{

longjmp(env_alrm, 1);

}

Figure 10.11 Callingreadwith a timeout, usinglongjmp

This version works as expected, regardless of whether the system restarts interrupted

system calls. Realize, however,that we still have the problem of interactions with other

signal handlers, as in Figure10.8.

If we want to set a time limit on an I/O operation, we need to uselongjmp,as

shown previously,while recognizing its possible interaction with other signal handlers.

Another option is to use theselectorpollfunctions, described in Sections 14.4.1 and

14.4.2.