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

[Alexey Tourbin <at@altlinux.ru>]

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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?

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#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;
}

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