Initialize a 1D or 2D array in shared memory

Question

I am trying to initialize a 2D char array of strings into POSIX shared memory to be shared between 3 other processes. There are plenty of tutorials on how to use a pointer to share a single string or an integer between processes, but I could find no examples on how to initialize 1D or 2D arrays using mmap() . I have posted what I have so far below. It is the first program, which creates the shared memory object and initialize the array char files[20][2][100] with the value files[0][0][0] = '\\0' .

What is the proper method to initialize and share an array in C?

For context, I coded a simple version of this project (at the suggestion of helpful SO gurus) which does not use shared memory and combines all 4 processes (separated by /**********/ ) into one. It's below.

I've asked a similar question involving structs in a previous post, but I was required to use a multidimensional array instead for my project. Any insights on that answer would be helpful HERE .

Thanks.

CODE SO FAR: (program initializes shared memory object and array)

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>

int main (int argc, char *argv[])
{
    char files [20][2][100]; 


    /* the size of shared memory object */
    int size = sizeof(files);

    /* name of the shared memory object */
    const char *name = "/PROJ4_SHARED_MEM";

    /* shared memory file descriptor */
    int shm_fd;

    /* pointer to shared memory obect */
    void *ptr;

    /* create the shared memory object */
    shm_fd = shm_open(name, O_CREAT | O_RDRW, 0666);

    /* configure the size of the shared memory object */
    ftruncate(shm_fd, size);

    /* memory map the shared memory object */
    ptr = mmap(0, size, PROT_WRITE, MAP_SHARED, shm_fd, 0);

    /* save array to the shared memory object. */
    /*****WHERE I LOSE IT*****/

    return 0;
}

CONTEXT PROGRAM: (not POSIX)

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>

int main (int argc, char *argv[])
{
    char files [20][2][100]; 
    files [0][0][0] = '\0';
    char file_name[100];

    char file_contents[100];
    char search[100];

    char answer;
    int again = 0;
    int counter;
    int i = 0;

    /**************************************************/

    while (again == 0)
    {
        printf("Enter a filename:  ");
        scanf("%s", &file_name);
        getchar();
        printf("Enter file contents (string):  ");
        fgets (file_contents, 100, stdin);

        for (i = 0; i < 20; i++)
        {
            if (files[i][0][0] == '\0')
            {
                strcpy(files[i][0],file_name);
                strcpy(files[i][1],file_contents);
                files [i+1][0][0] = '\0';       
                break;
            }
            else if (i == 19)
            {
                printf("ERROR: The directory is full.\n\n");
            }
        }

        counter++; 

        printf("Save another file y/n ?:  ");
        scanf(" %c", &answer);

        if (answer == 'n')
        {
            again = 1;
        }

    }
    printf("\n\n");
    again = 0;

    /**************************************************/

    printf("You have saved the following files:\n");
    for (i = 0; i < 20; i++)
    {
        if (files[i][0][0] == '\0')
        {
            break;
        }

        printf("%s \n", files[i][0]);
    }
    printf("\n\n");

    /**************************************************/

    printf("Would you like to open a file? (y/n) ?:  ");
    scanf(" %c", &answer);
    if (answer == 'n')
        again = 1;

    while (again == 0)
    {
        printf("Enter a filename:  ");
        scanf(" %s", &file_name);

        for (i = 0; i < 20; i++)
        {
            if (strcmp(files[i][0], file_name) == 0)
            {
                printf("%s \n", files[i][1]);       
                break;
            }
            else if (i == 19)
            {
                printf("ERROR: The file was not found.\n\n");
            }
        }

        printf("Search for another file (y/n) ?:  ");
        scanf(" %c", &answer);

        if (answer == 'n')
        {
            again = 1;
        }
        printf("\n\n");
    }


    getchar();

    return 0;
}

Answer 1

The code has a pointer to the shared memory therefore, like with any pointer, the code can access any specific item in the shared memory by indexing off that pointer.

#define INDEX_1_SIZE (20)
#define INDEX_2_SIZE (2)
#define INDEX_3_SIZE (100)
#define SIZE         (INDEX_1_SIZE * INDEX_2_SIZE * INDEX_3_SIZE)
ptr = mmap(0, SIZE, PROT_WRITE, MAP_SHARED, shm_fd, 0);

then:

int i, j, k; // loop counters

// initialize the shared memory to '\0'
for(i=0;i<INDEX_1_SIZE; i++_
{
    for(j=0; j<INDEX_2_SIZE; j++)
    {
        for(k=0;k<INDEX_3_SIZE; k++)
        {
            ptr[i][j][k] = '\0';
        }
    }
}

there are faster ways to perform the initialization to '\\0' for instance:

memset( ptr, '\0', SIZE );

There is no need for the local variable: 'files'

However, since a number of different processes can access the shared memory at the same time,
the code needs a named semaphore that all the processes use so only one process is accessing the shared memory at any one time, to avoid 'race' conditions. here is a good discussion, with example usage, for semaphores http://www.cs.cf.ac.uk/Dave/C/node26.html the code should be checking for errors, something like this: ( of course, using your own variable names )

des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode);

if (des_mutex < 0)
{
    perror("failure on shm_open on des_mutex");
    exit(1);
}

if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1)
{
    perror("Error on ftruncate to sizeof pthread_cond_t\n");
    exit(-1);
}