如何在共享内存中的矩阵上写？

Question

I am trying to write a matrix of strings in a piece of shared memory. I'm constantly getting a segmentation fault error. Why do I keep getting a segmentation fault error?

/* Includes */
#include <unistd.h>     /* Symbolic Constants */
#include <sys/types.h>  /* Primitive System Data Types */ 
#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <stdlib.h>     /* General Utilities */
#include <pthread.h>    /* POSIX Threads */
#include <string.h>     /* String handling */
#include <semaphore.h>  /* Semaphore */

#include <wait.h>
#include <sys/stat.h>
#include <fcntl.h>      // Para as variáveis O_CREAT, O_EXCL etc
#include <sys/mman.h>
#include <time.h>

#define str_len 10

typedef struct{
   char **posix;
} line;

int main(void){

    int fd;
    line *m;
    int data_size = sizeof(line)*4;
    //char m[4][3][str_len];
    //int i,j;

    if ((fd = shm_open("/shm_ex13s", O_CREAT|O_RDWR,S_IRUSR|S_IWUSR)) < 0) {    // Abrir o objeto da memória partilhada
        fprintf(stderr, "ERROR: No shm_open()\n");
        exit(EXIT_FAILURE);
    }                                                                       
    ftruncate(fd, data_size);                                               // Ajustar o tamanho da memória partilhada                                                                  
    m = (line *) mmap(NULL, data_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);  // Mapear a memória partilhada


    m->posix[0] = "ASDAS";

    //printf("%s\n", m[0][0]);

    // Desfaz mapeamento
    munmap(m, data_size);
    // Fecha o descritor devolvido pelo shm_open
    close(fd);
    // O Leitor apaga a memória Partilhada do Sistema 
    shm_unlink("/shm_ex13s"); 
    exit(EXIT_SUCCESS);

    exit(EXIT_SUCCESS);
 }

Answer 1

You cannot share pointers between processes. Each process has its own virtual memory. Addresses cannot be shared.

Futhermore with instruction

mmap(NULL, data_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

you are creating a shared memory of 4 line variable, where pointer of pointers are uninitialized.

Accessing it and assigning it with

m->posix[0] = "ASDAS";

it is Undefined Behavior : you are assigning to posix[0] that is not inizialized.

Finally you cannot do what you think to do. What you can do is to create a shared memory and copy your literal strings into it, you cannot referring to them due to the fact that you cannot share pointers between processes.

For example you can:

char *string = "ASDAS";
char *m = (line *) mmap(NULL, strlen(string)+1, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

strcpy(m, string, strlen(string));