C Beginner - Copying a char array to another char array

Question

I've been struggling with this for a stupidly long amount of time. Basically, I need to copy an array of char pointers to another array of char pointers.

Right now, I have the function:

void copyArray(char *source[], char *destination[]) {
    int i = 0;

    do {
        destination[i] = malloc(strlen(source[i]));
        memcpy(destination[i], source[i], strlen(source[i]));
    } while(source[i++] != NULL);
}

This results in a segmentation fault. Could someone please help?

Thanks!

EDIT: sample program

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

// Copy the contents of one array into another
void copyArray(char *source[], char *destination[]){
    // printf("In copy array");
    int i = 0;

    do {
        destination[i] = malloc(strlen(source[i]));
        memcpy(destination[i], source[i], strlen(source[i]));
    } while(source[i++] != NULL);
}

void addToHistory(char *history[][40], char *args[]){
    int i;
    for(i = 1; i < 10; i++){
        copyArray(history[i], history[i-1]);
    }
    i = 0;
    copyArray(args, history[0]);
}

int main(void){
    char *history[10][40];
    char *args[40];

    history[0][0] = NULL;

    args[0] = "ls";
    args[1] = NULL;

    addToHistory(history, args);
}

are you certain that the array source[] has a final NULL value? — BRPocock
– BRPocock, Commented Jan 22, 2014 at 17:52
Can you show a full (but small) example program which demonstrates this problem? — simonc
– simonc, Commented Jan 22, 2014 at 17:53
For my purposes, I do know that source will end with a final NULL value (it is the way the string is parsed in my program). I am working on a simple shell program and I uploaded a small test-code snippet.. — user3224639
– user3224639, Commented Jan 22, 2014 at 17:59
Please check the answer for your qn, here : stackoverflow.com/questions/36565328/… — user3857354
– user3857354, Commented Apr 12, 2016 at 15:54

barak manos · Accepted Answer · 2014-01-22 18:10:02Z

1

Make sure that the last element in the source array is NULL, before you pass it to copyArray.
In copyArray, put the while instead of the do, and increment i only at the end of the loop.

Instead all of the above, you can simply change i++ to ++i in function copyArray.

But it will crash if the first element in the source array passed to this function is NULL.

edited Jan 22, 2014 at 18:10

answered Jan 22, 2014 at 17:59

barak manos

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Comments

Menachem · Accepted Answer · 2014-01-22 19:08:53Z

0

I think you have an off-by-one error:

do {
    destination[i] = malloc(strlen(source[i]));
    memcpy(destination[i], source[i], strlen(source[i]));
} while(source[i++] != NULL);
               ^^^

You check if i was NULL after you already used it, and then end the loop. Try replacing it with

} while (source[++i] != NULL);           // or while (source[++i]), for short

You can try logging a short message after each iteration to see where the code faults.

Edit: Is there a reason you are using memcpy() (which will not copy the terminating '\0') rather than strcpy() (which will)?

(Note to @wildplasser: I believe strdup() may not be standard C).

edited Jan 22, 2014 at 19:08

answered Jan 22, 2014 at 18:02

Menachem

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Comments

wildplasser · Accepted Answer · 2014-01-22 19:14:46Z

0

void copyArray(char *source[], char *destination[]) {

    while ((*destiantion = *source)) {
        *destination++ = strdup( *source++ );
    }
}

BTW: it is common to make destination the first argument, just as in strcpy()

void copyArray(char *destination[], char *source[]) { ... }

edited Jan 22, 2014 at 19:14

answered Jan 22, 2014 at 18:16

wildplasser

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