Hi,
Given the following C program:
--8<---------------cut here---------------start------------->8---
#include <stdio.h>
#include <signal.h>
int main(void)
{
struct sigaction act;
sigaction(SIGINT, NULL, &act);
printf("%p %p %p\n", act.sa_handler, SIG_IGN, SIG_DFL);
}
--8<---------------cut here---------------end--------------->8---
and its ouput from various executions:
1) $ ./a.out => (nil) 0x1 (nil)
2) $ guile -c '(system "./a.out")' => (nil) 0x1 (nil)
3) $ guile -c '(system* "./a.out")' => 0x1 0x1 (nil)
We can see that 3) does not honor restoration of `SIGINT' to `SIG_DFL'
like `system(3)' does. This seems to be because of the following
sigaction before the creation of the process:
--8<---------------cut here---------------start------------->8---
1697 scm_dynwind_sigaction (SIGINT,
1698 scm_from_uintptr_t ((uintptr_t) SIG_IGN),
1699 SCM_UNDEFINED);
1700 #ifdef SIGQUIT
1701 scm_dynwind_sigaction (SIGQUIT,
1702 scm_from_uintptr_t ((uintptr_t) SIG_IGN),
1703 SCM_UNDEFINED);
1704 #endif
1705
1706 err = piped_process (&pid, prog, args,
1707 SCM_UNDEFINED, SCM_UNDEFINED);
--8<---------------cut here---------------end--------------->8---
>From execve(2):
POSIX.1 specifies that the dispositions of any signals that are
ignored or set to the default are left unchanged. POSIX.1 specifies
one exception: if SIGCHLD is being ignored, then an implementation
may leave the disposition unchanged or reset it to the default; Linux
does the former.
Therefore, setting `SIG_IGN' for `SIGINT' before the fork/execve results
in ignoring the signals in the child, which is unexpected as a user of
`system(3)'. The solution would be to restore `SIG_DFL' before
`execve(2)' if before the call to `system*' the action was not
`SIG_IGN'.
In the mean time, I have a solution that works for single-threaded
application:
--8<---------------cut here---------------start------------->8---
(define (system* . args)
(let ((handler+flags (sigaction SIGINT)))
(dynamic-wind
(lambda ()
(sigaction SIGINT SIG_IGN))
(lambda ()
(let ((cpid (primitive-fork)))
(if (zero? cpid)
(catch #t
(lambda ()
(sigaction SIGINT SIG_DFL)
(apply execlp (car args) args))
(lambda _
(primitive-exit EXIT_FAILURE)))
(waitpid cpid))))
(lambda ()
(sigaction SIGINT
(car handler+flags)
(cdr handler+flags))))))
--8<---------------cut here---------------end--------------->8---
Thanks,
Olivier
--
Olivier Dion
oldiob.ca
