Re: [PATCH v2] add man-page for pthread_mutexattr_setrobust_np()

["Bert Wesarg" <bert.wesarg@googlemail.com>]

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;
> +}