Dynamically allocating char pointer pointer

Question

I have created the following test program:

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

char **messages = NULL;
int messageCount = 0;

void addMessage(char *message) {
    messages = realloc(messages, messageCount * sizeof(char *));
    if(messages == NULL) {
        printf("Unable to realloc");
        return;
    }
    messages[messageCount] = malloc(255 * sizeof(char));
    if(messages[messageCount] == NULL) {
        printf("Unable to alloc");
        return;
    }
    memset(messages[messageCount], '\0', 255);
    strncpy(messages[messageCount], message, 255);
    messageCount++;
}

void listMessages(void) {
    if(messageCount == 0) return;
    int i = 0;
    for(i = 0; i < messageCount; i++) {
        printf("%s\n", messages[i]);
    }
    printf("\n");
}

int main(void) {
    addMessage("This");
    listMessages();
    addMessage("is");
    listMessages();
    addMessage("a");
    listMessages();
    addMessage("test");
    listMessages();
    addMessage("for");
    listMessages();
    addMessage("dynamic");
    listMessages();
    addMessage("memory");
    listMessages();
    addMessage("allocation");
    listMessages();
}

It is pretty self explanatory as to what I am trying to do.

I get the following output before the program crashes:

This

This
is

This
is
a

This
is

Note that I am not receiving an "Unable to realloc" or an "Unable to alloc" message.

So why does it crash?

So I know how many messages I have, and thus know how much memory to allocate? — Jimmay
– Jimmay, Commented Aug 25, 2013 at 22:21
I meant that you're incrementing it after trying to write to memory allocated based on its value, rather than before....but I see that you've already accepted the answer pointing that out. — verbose
– verbose, Commented Aug 25, 2013 at 22:45

dcaswell · Accepted Answer · 2013-08-25 22:17:51Z

1

Here's what your code is doing:

Calling addMessage with "This"
Calling this function with messageCount = 0: messages = realloc(messages, messageCount * sizeof(char *));

And then continually writing into unallocated memory.

So, do a ++messageCount; at the top of addMessage and you can remove it from the bottom of the function.

edited Aug 25, 2013 at 22:17

answered Aug 25, 2013 at 22:12

dcaswell

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1 Comment

Jimmay Over a year ago

Nice catch! This has been driving me mad; I kept the messageCount++ at the bottome, just did a realloc(messages, (messageCount + 1) * sizeof(char *)) instead. Works a treat.

Lidong Guo · Accepted Answer · 2013-08-25 22:29:49Z

1

messageCount ++ before you alloc the memory.

messages = realloc(messages, messageCount * sizeof(char *));

if(messages == NULL) {

    printf("Unable to realloc");
    return;
}
messages[messageCount] = malloc(255 * sizeof(char));//here is wrong `

You alloc a memory of messageCount, the max index you can use is messageCount-1!

You need always use:

messages[messageCount-1] = malloc(...)

Also other place with message[messageCount] should replaced by messages[messageCount-1].

Or, you have a array overflow.

edited Aug 25, 2013 at 22:29

answered Aug 25, 2013 at 22:18

Lidong Guo

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Comments

zentrunix · Accepted Answer · 2013-08-25 22:20:47Z

0

The problem: in the the first realloc, messageCount is zero. messageCount should be incremented before the call to realloc.

I had written before that messages being NULL the first time was the culprit, but it seems that's not really a problem (by @user814064).

edited Aug 25, 2013 at 22:20

answered Aug 25, 2013 at 22:15

zentrunix

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1 Comment

dcaswell Over a year ago

Realloc() operates like this normally when passed a null pointer: pubs.opengroup.org/onlinepubs/7908799/xsh/realloc.html

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