[Bug 3547] shell read is maybe too safe

Denys Vlasenko vda.linux at googlemail.com
Wed May 11 00:05:29 UTC 2011

On Tuesday 10 May 2011 01:27, Rich Felker wrote:
> On Tue, May 10, 2011 at 01:09:30AM +0200, Laurent Bercot wrote:
> > > There is no reason for a correct program to ever check for EINTR
> > > unless it specifically installs signal handlers and fails to specify
> > > SA_RESTART for them. Checking for ret==-1&&errno==EINTR in your read
> > > loop is purely bloat that does not belong in most programs.
> > 
> >  Well, I disagree, and I'll keep disagreeing until there's a release of
> > the Single Unix specification that says explicitly that the default
> > behaviour for every signal that does not terminate the process MUST be
> The default action for a signal is either ignoring it or stopping or
> terminating the process. Ignoring it obviously does not cause EINTR,
> and for terminating the process it's irrelevant since no further code
> is executed. This leaves only stopping. I'm uncertain if syscalls
> interrupted by SIGSTOP are required to be restarted, but I'd certainly
> consider restarting them a quality-of-implementation requirement for
> any implementation I wanted to use..

Try SIGTOPing and SIGCONTing this:

#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
static void sig(int n)
     char buf[128];
     int e = errno;
     sprintf(buf, "sig: %d %s\n", n, strsignal(n));
     write(1, buf, strlen(buf));
     errno = e;
int main()
     signal(SIGSTOP, sig);
     signal(SIGCONT, sig);
     signal(SIGWINCH, sig);
     signal(SIGABRT, sig);
     printf("PID: %d\n", getpid());
     errno = 0;
     printf("errno:%d %s\n", errno, strerror(errno));
     return 0;

I am getting on 2.6.35-rc4:

$ ./a.out
PID: 2821
sig: 18 Continued
errno:4 Interrupted system call

As you see, sleep() (nanosleep syscall)
is returning EINTR on SIGSTOP/SIGCONT...


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