msemaphore on linux?

Question

AIX (and HPUX if anyone cares) have a nice little feature called msemaphores that make it easy to synchronize granular pieces (e.g. records) of memory-mapped files shared by multiple processes. Is anyone aware of something comparable in linux?

To be clear, the msemaphore functions are described by following the related links here.

Answer 1

Under Linux, you may be able to achieve what you want with SysV shared memory; quick googling turned up this (rather old) guide that may be of help.

Answer 2

POSIX semaphores can be placed in memory shared between processes, if the second argument to sem_init(3), "pshared", is true. This seems to be the same as what msem does.

#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <time.h>
#include <unistd.h>
int main() {
    void *shared;
    sem_t *sem;
    int counter, *data;
    pid_t pid;
    srand(time(NULL));
    shared = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE,
            MAP_ANONYMOUS | MAP_SHARED, -1, 0);
    sem_init(sem = shared, 1, 1);
    data = shared + sizeof(sem_t);
    counter = *data = 0;
    pid = fork();
    while (1) {
        sem_wait(sem);
        if (pid)
            printf("ping>%d %d\n", data[0] = rand(), data[1] = rand());
        else if (counter != data[0]) {
            printf("pong<%d", counter = data[0]);
            sleep(2);
            printf(" %d\n", data[1]);
        }
        sem_post(sem);
        if (pid) sleep(1);
    }
}

This is a pretty dumb test, but it works:

$ cc -o test -lrt test.c
$ ./test
ping>2098529942 315244699
pong<2098529942 315244699
pong<1195826161 424832009
ping>1195826161 424832009
pong<1858302907 1740879454
ping>1858302907 1740879454
ping>568318608 566229809
pong<568318608 566229809
ping>1469118213 999421338
pong<1469118213 999421338
ping>1247594672 1837310825
pong<1247594672 1837310825
ping>478016018 1861977274
pong<478016018 1861977274
ping>1022490459 935101133
pong<1022490459 935101133
...

Because the semaphore is shared between the two processes, the pongs don't get interleaved data from the pings despite the sleeps.

Answer 3

This can be done using POSIX shared-memory mutexes:

pthread_mutexattr_t attr;
int pshared = PTHREAD_PROCESS_SHARED;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, &pshared);

pthread_mutex_init(&some_shared_mmap_structure.mutex, &attr);
pthread_mutexattr_destroy(&attr);

Now you can unlock and lock &some_shared_mmap_structure.mutex using ordinary pthread_mutex_lock() etc calls, from multiple processes that have it mapped.

Indeed, you can even implement the msem API in terms of this: (untested)

struct msemaphore {
    pthread_mutex_t mut;
};

#define MSEM_LOCKED 1
#define MSEM_UNLOCKED 0
#define MSEM_IF_NOWAIT 1

msemaphore *msem_init(msemaphore *msem_p, int initialvalue) {
    pthread_mutex_attr_t attr;
    int pshared = PTHREAD_PROCESS_SHARED;

    assert((unsigned long)msem_p & 7 == 0); // check alignment

    pthread_mutexattr_init(&attr);
    pthread_mutexattr_setpshared(&attr, &pshared); // might fail, you should probably check
    pthread_mutex_init(&msem_p->mut, &attr); // never fails
    pthread_mutexattr_destroy(&attr);

    if (initialvalue)
     pthread_mutex_lock(&attr);

    return msem_p;
}

int msem_remove(msemaphore *msem) {
    return pthread_mutex_destroy(&msem->mut) ? -1 : 0;
}

int msem_lock(msemaphore *msem, int cond) {
    int ret;
    if (cond == MSEM_IF_NOWAIT)
     ret = pthread_mutex_trylock(&msem->mut);
    else
     ret = pthread_mutex_lock(&msem->mut);

    return ret ? -1 : 0;
}

int msem_unlock(msemaphore *msem, int cond) {
    // pthreads does not allow us to directly ascertain whether there are
    // waiters. However, a unlock/trylock with no contention is -very- fast
    // using linux's pthreads implementation, so just do that instead if
    // you care.
    //
    // nb, only fails if the mutex is not initialized
    return pthread_mutex_unlock(&msem->mut) ? -1 : 0;
}