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/* Use a semaphore to implement mutual exclusion. */
#include <assert.h>
#include <fcntl.h> /* O_CREAT */
#include <pthread.h>
#include <semaphore.h>
#include <stdio.h> /* printf() */
#include <stdlib.h> /* exit() */
#include <unistd.h> /* sleep() */
/* Local functions declarations. */
static void* thread_func(void*);
/* Local variables. */
/* s_sem protects s_d3. */
static sem_t* s_sem;
static double s_d1; /* accessed before thread creation and in the created */
/* thread (not a race). */
static double s_d2; /* accessed in the created thread and after the join */
/* (not a race). */
static double s_d3; /* accessed simultaneously from both threads (race). */
static int s_debug = 0;
static int s_do_printf = 0;
static int s_do_mutual_exclusion = 0;
/* Function definitions. */
int main(int argc, char** argv)
{
int optchar;
pthread_t threadid;
char semaphore_name[32];
while ((optchar = getopt(argc, argv, "dmp")) != EOF)
{
switch (optchar)
{
case 'd':
s_debug = 1;
break;
case 'm':
s_do_mutual_exclusion = 1;
break;
case 'p':
s_do_printf = 1;
break;
default:
assert(0);
}
}
/*
* Use the ipcs and ipcrm commands to clean up named semaphores left by
* aborted instances of this process.
*/
snprintf(semaphore_name, sizeof(semaphore_name), "/drd-sem-open-test-%d",
getpid());
s_sem = sem_open(semaphore_name, O_CREAT | O_EXCL, 0600, 1);
if (s_sem == SEM_FAILED)
{
fprintf(stderr, "Failed to create a semaphore with name %s\n",
semaphore_name);
exit(1);
}
/*
* Switch to line-buffered mode, such that timing information can be
* obtained for each printf() call with strace.
*/
setlinebuf(stdout);
if (s_debug)
{
printf("&s_d1 = %p; &s_d2 = %p; &s_d3 = %p\n", &s_d1, &s_d2, &s_d3);
}
s_d1 = 1;
s_d3 = 3;
pthread_create(&threadid, 0, thread_func, 0);
sleep(1); /* Wait until thread_func() finished. */
{
if (s_do_mutual_exclusion) sem_wait(s_sem);
s_d3++;
if (s_do_mutual_exclusion) sem_post(s_sem);
}
/* Wait until the thread finished. */
pthread_join(threadid, 0);
if (s_do_printf) printf("s_d2 = %g (should be 2)\n", s_d2);
if (s_do_printf) printf("s_d3 = %g (should be 5)\n", s_d3);
sem_close(s_sem);
sem_unlink(semaphore_name);
return 0;
}
static void* thread_func(void* thread_arg)
{
if (s_do_printf)
{
printf("s_d1 = %g (should be 1)\n", s_d1);
}
s_d2 = 2;
{
if (s_do_mutual_exclusion) sem_wait(s_sem);
s_d3++;
if (s_do_mutual_exclusion) sem_post(s_sem);
}
return 0;
}
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