[devel] system(3) blocks alarm(2) under strace(1)

[devel] system(3) blocks alarm(2) under strace(1)

[Alexey Tourbin]

[-- Attachment #1.1: Type: text/plain, Size: 226 bytes --]

Столкнулся в перле, но воспроизводится без всякого перла.

$ gcc alarm.c
$ ./a.out
diff=3 seconds
$ strace -o /dev/null ./a.out
diff=3 seconds
$ strace -f -o /dev/null ./a.out
diff=6 seconds
$

В чем здесь секрет?

[-- Attachment #1.2: alarm.c --]
[-- Type: text/plain, Size: 362 bytes --]

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <time.h>

time_t start_time, alarm_time;

void alarm_handler(int signo)
{
	alarm_time = time(NULL);
}

int main()
{
	start_time = time(NULL);
	signal(SIGALRM, alarm_handler);
	alarm(3);
	system("sleep 6");
	printf("diff=%d seconds\n", alarm_time - start_time);
	return 0;
}

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

Re: [devel] system(3) blocks alarm(2) under strace(1)

[Dmitry V. Levin]

[-- Attachment #1: Type: text/plain, Size: 1047 bytes --]

On Wed, Jun 23, 2004 at 12:39:37PM +0400, Alexey Tourbin wrote:
> Столкнулся в перле, но воспроизводится без всякого перла.
> 
> $ gcc alarm.c
> $ ./a.out
> diff=3 seconds
> $ strace -o /dev/null ./a.out
> diff=3 seconds
> $ strace -f -o /dev/null ./a.out
> diff=6 seconds
> $
> 
> В чем здесь секрет?

В strace(1) про -f написано:

Trace child processes as they are created by currently traced processes
as a result of the fork(2) system call.  The new process is attached
to as soon as its pid is known (through the return value of fork(2) in
the parent process). This means that such children may run uncontrolled
for a while (especially in the case of a vfork(2)), until the parent is
scheduled again to complete its (v)fork(2) call.

If the parent process decides to wait(2) for a child that is currently
being traced, it is suspended until an appropriate child process either
terminates or incurs a signal that would cause it to terminate (as
determined from the child's current signal disposition).


-- 
ldv

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]