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    SubjectRe: [PATCH v2] add man-page for pthread_mutexattr_setrobust_np()
    On Sun, Dec 21, 2008 at 21:59, Sebastian Andrzej Siewior
    <sebastian@breakpoint.cc> wrote:
    > * Michael Kerrisk | 2008-12-07 11:44:54 [-0500]:
    >
    >>Sebastian,
    > Michael,
    Sebastian, Michael,

    > +.SH EXAMPLE
    > +The code example shows how to share a lock between two applications without
    > +System V IPC.
    > +An advantage over System V semaphores is that the kernel is not invoked in
    > +case the lock is not hold.
    > +If one of the applications dies while holding the lock or the system reboots
    > +unexpectedly, the new owner of lock marks the lock state consistent.
    > +In this example the lock owner does not need to perform any validation of the
    > +resource protected by the lock.
    > +
    > +.nf
    > +#define _GNU_SOURCE
    > +
    > +#include <errno.h>
    > +#include <fcntl.h>
    > +#include <pthread.h>
    > +#include <stdio.h>
    > +#include <stdlib.h>
    > +#include <string.h>
    > +#include <unistd.h>
    > +
    > +#include <sys/mman.h>
    > +#include <sys/stat.h>
    > +#include <sys/types.h>
    > +
    > +static const char *lock_name = "/dev/shm/limi_lock";
    > +static pthread_mutex_t *limi_mutex;
    > +
    > +static int open_existing_lock(void)
    > +{
    > + int fd;
    > + int ret;
    > + struct stat buf;
    > + int retry = 5;
    > +
    > + fd = open(lock_name, O_RDWR);
    > + if (fd < 0)
    > + return fd;
    > + do {
    > + ret = fstat(fd, &buf);
    > + if (ret < 0)
    Isn't here a close(2) missing?

    > + return ret;
    > +
    > + if (buf.st_size == sizeof(*limi_mutex))
    > + return fd;
    > +
    > + close(fd);
    Isn't this close(2) wrong here?

    > + sleep(1);
    > + retry\-\-;
    > + } while (retry);
    > +
    > + close(fd);
    > + return \-1;
    > +}
    > +
    > +static int create_new_lock(void)
    > +{
    > + int fd;
    > + pthread_mutex_t cmutex = PTHREAD_MUTEX_INITIALIZER;
    > + pthread_mutexattr_t attr;
    > + int ret;
    > +
    > + pthread_mutexattr_init(&attr);
    > + pthread_mutexattr_setrobust_np(&attr, PTHREAD_MUTEX_ROBUST_NP);
    > + pthread_mutex_init(&cmutex, &attr);
    > +
    > + fd = open(lock_name, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR |
    > + S_IRGRP | S_IWGRP);
    > + if (fd < 0)
    > + return fd;
    > +
    > + ret = write(fd, &cmutex, sizeof(cmutex));
    I think its undefined behavior if you copy a struct pthread_mutex. You
    should use mmap here too.

    > + if (ret < 0) {
    > + fprintf(stderr, "Write to %s failed: %s\\n",
    > + lock_name, strerror(errno));
    > + exit(1);
    > + }
    > + return fd;
    > +}


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