C segmentation fault in array of strings

Question

I expect the following code to work. It compiles (gcc), but the executions gives Segmentation fault: 11. How come? Thanks

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

#define MAX_INDEX_LENGTH 20

main()
{
    char **indici;
    int n = 4;

    indici = (char **)malloc(n * sizeof(char*));
    int i;
    for (i = 0; i < n; i++)
    {
        indici[i] = (char *)malloc(MAX_INDEX_LENGTH * sizeof(char*));
    }

    strncpy(indici[0], "aaa", MAX_INDEX_LENGTH);
    strncpy(indici[0], "bbbb", MAX_INDEX_LENGTH);
    strncpy(indici[0], "ccccc", MAX_INDEX_LENGTH);
    strncpy(indici[0], "ddddddd", MAX_INDEX_LENGTH);

    for (i = 0; i < n; i++)
    {
        printf("Index %d is %s\n", n, indici[n]);
    }
}

Helpful answers have been posted yet, but I want to point out that you are allocating MAX_INDEX_LENGTH * sizeof(char*) instead of MAX_INDEX_LENGTH * sizeof(char) which is what you actually want I guess. — Michael Schmeißer
– Michael Schmeißer, Commented Jan 28, 2012 at 13:22

cnicutar · Accepted Answer · 2012-01-28 13:27:06Z

2

You're not populating all the elements, only indici[0] so they remain uninitialized and likely point to garbage. So when the second for runs it will dereference uninitialized stuff.

Maybe you meant:

strncpy(indici[0], "aaa", MAX_INDEX_LENGTH);
strncpy(indici[1], "bbbb", MAX_INDEX_LENGTH);
strncpy(indici[2], "ccccc", MAX_INDEX_LENGTH);
strncpy(indici[3], "ddddddd", MAX_INDEX_LENGTH);

A second problem is that you're trying to print indici[n], which is outside the allocated memory. You probably meant

printf("Index %d is %s\n", i, indici[i]);

Another subtler problem is this:

(char *)malloc(MAX_INDEX_LENGTH * sizeof(char*));
                                         ^^^^^^

You're allocating more than you need. You need to allocated a number of chars, so it should be sizeof(char). But sizeof(char) is always 1 so it should be:

malloc(MAX_INDEX_LENGTH);

edited Jan 28, 2012 at 13:27

answered Jan 28, 2012 at 13:18

cnicutar

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1 Comment

taskinoor Over a year ago

Try to run this, it will crash too. OP has problem in last loop.

Joachim Isaksson · Accepted Answer · 2012-01-28 13:18:40Z

1

You're copying all of the strings into indici[0], so the other 3 are uninitialized.

When you print the uninitialized ones, kaboom.

answered Jan 28, 2012 at 13:18

Joachim Isaksson

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