Semaphore deadlock when using unnamed semaphores in POSIX with shared memory

Question

I have 2 semaphores. I have a shared memory segment. I am trying to synchronize processes so that they wait for each other until a certain task is finished but when sem_post_util( sem_sync ) is used in the other processes just stop and the last processes continues and exits. Why doesn't it increment the unnamed semaphore in the shared memory?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "../../utils.h"
#include <signal.h>

#define SHARED_MEM "/shmm115"
#define SEM "nammed_sem115"
#define SEM_SYNC "unnamed_sem115"
#define SIZE 256
#define n_proc 3

sem_t *sem, sem_sync;
void* addr;

void cleanup(){
    sem_close( sem );
    sem_close( &sem_sync );
    munmap_util( addr, SIZE );
    shm_unlink( SHARED_MEM );
}

int main(){
    sem = sem_open( SEM, O_CREAT, 0666, 1 );/*opent the named semaphore we got*/

    /*Open shared memory segment and put a shared variable inside of it*/
    struct sigaction handler;
    handler.sa_handler = cleanup;

    sigaction( SIGINT, &handler, NULL);

    int fd;
    short count = 0;
    fd  = shm_open_util(SHARED_MEM, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH | S_IRWXU);

    struct stat stat_buf;
    fstat( fd, &stat_buf );
    if( stat_buf.st_size == 0){/*if it is the first one to write to it*/
        ftruncate_util(fd, SIZE);
        addr = mmap_util(0, SIZE, PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);
        memcpy( addr, &count, sizeof(short) );
        memcpy( addr+sizeof(short), &sem_sync, sizeof(sem_t) );
    }
    else
        addr = mmap_util(0, SIZE, PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);

    close(fd);
    
    sem_init_util( &sem_sync, 1, 0 );

    sem_wait_util( sem );
    printf("Reading count\n");
    memcpy(&count, addr, sizeof(short));
    printf("Incrementing and writing the count: %d\n", count);
    count++;
    memcpy(addr, &count, sizeof(short));
    sem_post_util( sem );

    if( count == n_proc ){
        printf("%d unblocks all the processes\n", getpid());
        for( int i = 0 ; i<n_proc-1; ++i )
            sem_post_util( addr + sizeof(short) );
    }
    else{
        printf("%d pauses\n", getpid());
        sem_wait_util( &sem_sync );
    }

    printf("%d continues\n", getpid());
    sem_close( sem );
    sem_close( &sem_sync );
    munmap_util( addr, SIZE );
    shm_unlink( SHARED_MEM );
}

I am running it with ./a.out & ./a.out & ./a.out

This is the output

[70] 19882
[71] 19883
Reading count
Incrementing and writing the count: 0
19882 pauses
Reading count
Incrementing and writing the count: 1
19883 pauses
Reading count
Incrementing and writing the count: 2
19884 unblocks all the processes
19884 continues

As you can see only the last process continued and processes 19882 and 19883 didn't continue and hanged there waiting. What am I doing wrong?

I want the semaphore to be in a shared memory along with some other data inside there.

mevets · Accepted Answer · 2021-04-24 13:07:15Z

0

You didn't put up the source to util.h, but it seems you are acquiring the semaphore via sem_wait_util( &sem_sync );, but releasing it via sem_post_util( addr + sizeof(short) );.

I would have expected it to be released via sem_post_util(&sem_sync);

answered Apr 24, 2021 at 13:07

mevets

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pilcrow · Accepted Answer · 2021-04-24 13:16:45Z

0

Operations on copies of POSIX semaphores are undefined. You must post and wait on the very memory that was initialized by sem_init or sem_open.

So, the memcpy of your sem_sync, initialized in ordinary process memory, to shared memory won't make that memory a viable semaphore.

Instead, try something like this:

struct shared_items {
  sem_t sem;
  short count;
} *shmem;

....

*shmem = mmap(0, sizeof(*shared_bundle), ..., shm_opened_fd, 0);

if (i_am_the_first_to_open_this_shared_memory) {
  sem_init(&shmem->sem, 1, 0);
}

....

sem_post(&shmem->sem);

answered Apr 24, 2021 at 13:16

pilcrow

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