Cannot get the string from char**

Question

I got a program here using double char pointer

#include <stdio.h>
#include <stdlib.h>

void loadarray(char ** message)
{
    int size = 10;
    *message = (char*)malloc(size * sizeof(char));
    int i = 0;
    char stringarr[10]={"hello"};
    char msg_byte;

    for (i = 0; i < size; i++)
    {
        //load stringarr into msg_byte
        msg_byte = stringarr[i];
        char* pmsg      = *message;
        *pmsg = (char)msg_byte;
        printf("data location %d is %X\n", i, *pmsg);
        pmsg++;

    }
}


void main()
{
    char* arr;
    loadarray(&arr);
    printf("array = %02X %02X %02X %02X %02X\n", arr[0], arr[1], arr[2], arr[3], arr[4]);
}

The output I have is

data location 0 is 68
data location 1 is 65
data location 2 is 6C
data location 3 is 6C
data location 4 is 6F
data location 5 is 0
data location 6 is 0
data location 7 is 0
data location 8 is 0
data location 9 is 0
array = 00 00 00 00 00

For some reason I can't just pass the string back to main. What am I doing wrong? Thanks for your help first.

@LinusKleen Daily reminder to look one of the [most famous SO questions](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Igor Pejic, Aug 28 '14 at 17:14
Your loop logic is wrong. You keep writing to the same location on every iteration. — nobody, Aug 28 '14 at 17:14

score 1 · Answer 1 · answered Aug 28 '14 at 17:14

1

char* pmsg = *message;

You are initializing pmsg on each iteration and therefore it always points to the first char, put it before the for loop.

answered Aug 28 '14 at 17:14

David Ranieri

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score 1 · Answer 2 · answered Aug 28 '14 at 17:17

That's because you have:

    char* pmsg      = *message;

in the loop. pmsg only ever points to the first object of *message.

Change the function loadarray so that pmsg is initialized before the for loop.

void loadarray(char ** message)
{
    int size = 10;
    *message = malloc(size * sizeof(char));
    int i = 0;
    char stringarr[10]={"hello"};
    char msg_byte;

    char* pmsg      = *message;
    for (i = 0; i < size; i++)
    {
        //load stringarr into msg_byte
        msg_byte = stringarr[i];
        *pmsg = (char)msg_byte;
        printf("data location %d is %X\n", i, *pmsg);
        pmsg++;

    }
}

Cannot get the string from char**

2 Answers2