Error message from Helgrind:
$ valgrind --tool=helgrind ./prog
...
==6587== Thread #1: Bug in libpthread: sem_wait succeeded on semaphore without
prior sem_post
==6587== at 0x4850069: sem_wait_WRK (hg_intercepts.c:3155)
==6587== by 0x4851180: sem_wait@* (hg_intercepts.c:3174)
==6587== by 0x109271: main (in /media/sf_antiX/erros/duvida/prog)
…
Error message from Drd:
$ valgrind --tool=drd --trace-semaphore=yes ./prog
...
==6828== [1] sem_wait 0x487f000 value 0 -> 4294967295
==6828== Invalid semaphore: semaphore 0x487f000
==6828== at 0x4866C1A: sem_wait_intercept (drd_pthread_intercepts.c:1570)
==6828== by 0x4866C1A: sem_wait@* (drd_pthread_intercepts.c:1578)
==6828== by 0x109271: main (in /media/sf_antiX/erros/duvida/prog)
==6828== semaphore 0x487f000 was first observed at:
==6828== at 0x4865A16: sem_open_intercept (drd_pthread_intercepts.c:1527)
==6828== by 0x4865A16: sem_open@* (drd_pthread_intercepts.c:1538)
==6828== by 0x1091D3: main (in /media/sf_antiX/erros/duvida/prog)
…
So the issue seems to be that according to Drd a semaphore that has the value
of zero, with a sem_wait() gets decremented to 4294967295. (????????)
I have the sample code on the end of this message, and I have tried to run the
code on several recent versions of Valgrind (32.3 and others), and on x86-64
using antiX Linux, MX Linux, Ubuntu Linux and FreeBSD. I have also tried ARM 64
bits under Ubuntu, and the behaviour is the same. The sample program seems to
work ok.
My thanks to all the Valgrind developers for the wonderful software they
created, and I sincerely hope the issue is on my side (between chair and
keyboard).
Paulo Ferreira
======================
#include <stdio.h>
#include <fcntl.h>
#include <semaphore.h>
#include <unistd.h>
#include <sys/wait.h>
#define NUM 10
int main(void)
{
int i;
sem_t *sem1; // First semaphore
sem_t *sem2; // Second semaphore
// create, initialize semaphores
sem1 = sem_open("/semaphore1", O_CREAT, 0600, 0);
sem2 = sem_open("/semaphore2", O_CREAT, 0600, 1);
pid_t pid = fork();
if (pid>0) // Parent process
{
for (i = 0; i < NUM; i++)
{
sem_wait(sem2); // Lock the semaphore
write(1, "a\n", 2);
sem_post(sem1); // Release the semaphore lock
}
wait(NULL);
}
else // Child process
{
for (i = 0; i < NUM; i++)
{
sem_wait(sem1); // Lock the semaphore
write(1, "b\n", 2);
sem_post(sem2); // Release the semaphore lock
}
}
// Close the Semaphores
sem_close(sem1);
sem_unlink("/semaphore1");
sem_close(sem2);
sem_unlink("/semaphore2");
return 0;
}
==========================
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