Thread: Returning a double pointer help.

Multi-dimensional arrays are an advanced feature of C, and they don't work wth the rest of the language as you think they should. Unfortunately most C primers introduce multi-dimensional arrays immediately after single dimensional arrays, ignorin gtheir complexities.

A double pointer is not really a pointer to a 2D array. It is a related idea, but not the same thing. A double pointer is a pointer to a pointer, or a pointer to a list of pointers. You often use a char ** for a list of strings. Each string should be allocated separately (e.g. by a call to malloc), then the list itself needs to be allocated.

> I'm trying to return a double pointer from a function. Here is my current code:
But you have a 2D array - which isn't the same thing.

char a[N] becomes a char *
but
char a[R][C] becomes char (*)[C], not char **. The array-becomes-pointer rule only applies to the left-most dimension, not all of them.

If you thought returning a ** was mind-bending enough, you're really not going to like returning a pointer to a 2D array.

Code:

// Declare includes.
#include <errno.h>
#include <libgen.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Declare defines.
#define FILENAME_SIZE                100
#define MAX_OLD_DOWNLOADS             10
#define PROGRAM_NAME_SIZE             50
#define YOUTUBE_DATA_DIRECTORY         "/dea/youtube_data/"

// Declare global variables.
char program_name[PROGRAM_NAME_SIZE + 1] = {0};

// Declare function prototypes.
char (*get_downloaded_list (char *account))[25];

int main (int argc, char *argv[])
{
// Decalre variables.
    char (*downloaded)[25] = {0};

// Get program name for error reporting.
    strcpy(program_name, basename(argv[0]));

// Check number of arguments.
    if(argc != 1)
    {
        fprintf(stderr, "Usage: %s\n", program_name);
        exit(EXIT_FAILURE);
    }

// Get downloaded videos list.
    downloaded = get_downloaded_list("AlfieAesthetics");

    if(downloaded)
    {;}

// Exit cleanly.
    exit(EXIT_SUCCESS);
}

char (*get_downloaded_list (char *account))[25]
{
    // Declare variables.
    int counter = 0;
    FILE *fp = NULL;
    char filename[FILENAME_SIZE + 1] = {0};
    static char downloaded[MAX_OLD_DOWNLOADS][25] = {0};

    // Build the filename
    strcpy(filename, YOUTUBE_DATA_DIRECTORY);
    strcat(filename, account);

    // Open the data file for reading.
    if((fp = fopen(filename, "r")) == NULL)
    {
        fprintf(stderr, "%s: get_downloaded_list error: fopen failed (%s) (%s)\n", program_name, strerror(errno), filename);
        exit(EXIT_FAILURE);
    }

    // Loop through all lines in the file.
    while(fgets(downloaded[counter], 25, fp) != NULL && counter < MAX_OLD_DOWNLOADS)
    {
        // Remove trailing newline.
        downloaded[counter][strlen(downloaded[counter]) - 1] = '\0';
        // Advance counter.
        counter++;
    }

    // Set end marker if list is not maxed out.
    if(counter != MAX_OLD_DOWNLOADS)
        downloaded[counter][0] = '\0';

    // Close the data file.
    fclose(fp);

    // Return the list.
    return(downloaded);
}

But if you really want the char** route (which is more flexible in the long run), then:

Code:

char **get_downloaded_list (char *account)
{
    // Declare variables.
    int counter = 0;
    FILE *fp = NULL;
    char filename[FILENAME_SIZE + 1] = {0};
    char **downloaded = malloc(MAX_OLD_DOWNLOADS * sizeof(*downloaded));
    char buff[25];

    // Build the filename
    strcpy(filename, YOUTUBE_DATA_DIRECTORY);
    strcat(filename, account);

    // Open the data file for reading.
    if((fp = fopen(filename, "r")) == NULL)
    {
        fprintf(stderr, "%s: get_downloaded_list error: fopen failed (%s) (%s)\n", program_name, strerror(errno), filename);
        exit(EXIT_FAILURE);
    }

    // Loop through all lines in the file.
    while(fgets(buff, sizeof(buff), fp) != NULL && counter < MAX_OLD_DOWNLOADS)
    {
        // Remove trailing newline.
        buff[strlen(buff)-1] = '\0';

        // save it
        downloaded[counter] = malloc( strlen(buff) + 1 );
        strcpy(downloaded[counter], buff);

        // Advance counter.
        counter++;
    }

    // Set end marker if list is not maxed out.
    if(counter != MAX_OLD_DOWNLOADS)
        downloaded[counter] = NULL;

    // Close the data file.
    fclose(fp);

    // Return the list.
    return(downloaded);
}