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/*
* THIS PROGRAM IS ADAPTED FROM THE futex MAN PAGE.
*
* futex_demo.c
*
* Usage: futex_demo [nloops] (Default: 5)
*
* Demonstrate the use of futexes in a program where parent and child
* use a pair of futexes located inside a shared anonymous mapping to
* synchronize access to a shared resource: the terminal. The two
* processes each write 'num-loops' messages to the terminal and employ
* a synchronization protocol that ensures that they alternate in
* writing messages.
*/
/* #define _GNU_SOURCE */
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/time.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static int *futex1, *futex2, *iaddr;
static int
futex(int *uaddr, int futex_op, int val,
const struct timespec *timeout, int *uaddr2, int val3)
{
return syscall(SYS_futex, uaddr, futex_op, val, timeout, uaddr, val3);
}
/*
* Acquire the futex pointed to by 'futexp': wait for its value to
* become 1, and then set the value to 0.
*/
static void
fwait(int *futexp)
{
int s;
/*
* __sync_bool_compare_and_swap(ptr, oldval, newval) is a gcc
* built-in function. It atomically performs the equivalent of:
*
* if (*ptr == oldval)
* *ptr = newval;
*
* It returns true if the test yielded true and *ptr was updated.
* The alternative here would be to employ the equivalent atomic
* machine-language instructions. For further information, see
* the GCC Manual.
*/
while (1) {
/* Is the futex available? */
if (__sync_bool_compare_and_swap(futexp, 1, 0))
break; /* Yes */
/* Futex is not available; wait */
s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
if (s == -1 && errno != EAGAIN)
errExit("futex-FUTEX_WAIT");
}
}
/*
* Release the futex pointed to by 'futexp': if the futex currently
* has the value 0, set its value to 1 and the wake any futex waiters,
* so that if the peer is blocked in fpost(), it can proceed.
**/
static void
fpost(int *futexp)
{
int s;
/* __sync_bool_compare_and_swap() was described in comments above */
if (__sync_bool_compare_and_swap(futexp, 0, 1)) {
s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
if (s == -1)
errExit("futex-FUTEX_WAKE");
}
}
int
main(int argc, char *argv[])
{
pid_t childPid;
int j, nloops;
setbuf(stdout, NULL);
nloops = (argc > 1) ? atoi(argv[1]) : 5;
/*
* Create a shared anonymous mapping that will hold the futexes.
* Since the futexes are being shared between processes, we
* subsequently use the "shared" futex operations (i.e., not the
* ones suffixed "_PRIVATE").
*/
iaddr = mmap(NULL, sizeof(int) * 2, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_SHARED, -1, 0);
if (iaddr == MAP_FAILED)
errExit("mmap");
futex1 = &iaddr[0];
futex2 = &iaddr[1];
*futex1 = 0; /* State: unavailable */
*futex2 = 1; /* State: available */
/* Create a child process that inherits the shared anonymous mapping. */
childPid = fork();
if (childPid == -1)
errExit("fork");
if (childPid == 0) { /* Child */
for (j = 0; j < nloops; j++) {
fwait(futex1);
printf("Child (%ld) %d\n", (long) getpid(), j);
fpost(futex2);
}
exit(EXIT_SUCCESS);
}
/* Parent falls through to here */
for (j = 0; j < nloops; j++) {
fwait(futex2);
printf("Parent (%ld) %d\n", (long) getpid(), j);
fpost(futex1);
}
wait(NULL);
exit(EXIT_SUCCESS);
}
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