Thread: Initialize a 1D or 2D array in shared memory (POSIX)

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 S.O. gurus) which does not use shared memory and combines all 4 processes (separated by /**********/) into one. It's below.


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

Code:

#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_RDWR, 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)

Code:

#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;
}

Originally Posted by itCbitC

As far as shared memory goes, it is conceptually the same as any client-server program. First off you'd code a server program that uses shmget() to create a shared memory segment; and shmat() to attach it to its address space. Then you'd initialize it as you'd initialize any other variable of that type. Finally code a client program that connects to that shared memory ID and reads what the server program put in there.

Thanks.
The call that creates the shared object in our book is mmap(). Apparently this is the newer way of doing it, from what I've read. In an example void* ptr = mmap(.....).The pointer is then used to add the strings "Hello" and "World". The pointer is incremented the length of the string after adding each string to memory.

Code:

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

/* write to  the  shared  memory  object */
sprintf(ptr,"%s",message 0); 
ptr += strlen(message 0); 
sprintf(ptr,"%s",message 1);
ptr += strlen(message 1);

All examples use the void pointer, but its to add a single string or integer. How can I make this work using mmap() with an array?

Note: shared memory is outside scope of the C standard. This really belongs in a unix/posix programming forum.


For a 1D array it is easy. Declare ptr as the appropriate type of pointer (e.g. a pointer to char) and then use array syntax as normal until it comes time for your program to release the shared memory.

Generally, you would not share a multi-dimensional array directly. Instead, map as a 1D array, and transform indices as needed. For a 2D array (with dimensions row and columns) one way of mapping might be....

Code:

char some_2D_array[rows][columns];

/*   initialise some_2D_array somehow */

char *ptr = mmap(0, rows*columns,  PROT_WRITE,   MAP_SHARED,  shm_fd, 0);
for (i = 0; i < rows; ++i)
{
     for (j = 0; j < columns; ++j)
         ptr[i + rows*j] = some_2D_array[i][j];
}

And just to follow on from Grumpy's solution,

Code:

char (*some_2D_array)[columns];
 
some_2D_array = (char(*)[columns])mmap(0, rows*columns,  PROT_WRITE,   MAP_SHARED,  shm_fd, 0);

// Now you can do some_2D_array[row][col] directly in your shared memory

Originally Posted by grumpy

Note: shared memory is outside scope of the C standard. This really belongs in a unix/posix programming forum.


For a 1D array it is easy. Declare ptr as the appropriate type of pointer (e.g. a pointer to char) and then use array syntax as normal until it comes time for your program to release the shared memory.

Generally, you would not share a multi-dimensional array directly. Instead, map as a 1D array, and transform indices as needed. For a 2D array (with dimensions row and columns) one way of mapping might be....

Code:

char some_2D_array[rows][columns];

/*   initialise some_2D_array somehow */

char *ptr = mmap(0, rows*columns,  PROT_WRITE,   MAP_SHARED,  shm_fd, 0);
for (i = 0; i < rows; ++i)
{
     for (j = 0; j < columns; ++j)
         ptr[i + rows*j] = some_2D_array[i][j];
}

Originally Posted by Salem

And just to follow on from Grumpy's solution,

Code:

char (*some_2D_array)[columns];
 
some_2D_array = (char(*)[columns])mmap(0, rows*columns,  PROT_WRITE,   MAP_SHARED,  shm_fd, 0);

// Now you can do some_2D_array[row][col] directly in your shared memory

Okay, good stuff. This is all new to me though. How would I edit, read, or otherwise manipulate that data as a one dimensional array? Say I wanted to add a new entry in
some_2D_array [1][0] or list all the strings in the first column. Sorry if this sounds basic, but I've never heard of treating a multidimensional array as a single array...or vise versa.

SALEM: Are you saying all I have to do, with your addition to the solution, is treat it directly as a 2D array as I would in a normal program? Because that would be great, and I'll give it a try in the meantime.

Thanks so far guys.