Fix deadlock, busy waiting and comments in futex example from http://man7.org/linux/man-pages/man2/futex.2.html

futex_demo.c

/* 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 <stdatomic.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 0, and then set the value to 1. */

static void
fwait(int *futexp)
{
    int s;

    /* atomic_compare_exchange_strong atomically performs
       the equivalent of:

       bool cmpexch(int *val, int *exp, int newval)
       {
           if (*val == *exp) {
               *val = newval;
               return true;
           } else {
               *exp = *val;
               return false;
           }
       }

       */

    while (1) {

        /* Is the futex available? */
        int expected = 0;
        if (atomic_compare_exchange_strong(futexp, &expected, 1))
            break;

        /* Futex is not available; wait */

        s = futex(futexp, FUTEX_WAIT, 1, 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 1, set its value to 0 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;

    /* atomic_compare_exchange_strong() was described in comments above */

    int expected = 1;
    if (atomic_compare_exchange_strong(futexp, &expected, 0)) {
        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 = 1;        /* State: unavailable */
    *futex2 = 0;        /* 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);
}

See also the upstream git repository for the futex(2) man page:
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/man2/futex.2

Mailinglist thread:
https://lore.kernel.org/linux-man/20191025173526.GA32433@dell12.lru.li/T/#t