Realloc an int array

Question

I'm trying to create an array to hold an int, then when another int is to be added increase it in size to hold another int.. and so on..

I know it's not an efficient use of realloc, but it's proof on concept more than anything else. Just to get it working would allow me to optimise it and be able to apply it to something useful. A working example. The problem comes when i call the print function and it just segfaults. Any help would be appreciated.

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

typedef char String[100];

void begin(int *);
void add(int *, int);
void print(int *);

int tempcount=0;

int main(void)
{
    int *n=NULL;
    String menu;

    begin(n);

    while(true)
    {
        scanf("%9s", menu);

        if(!strcmp("a", menu)) //add
        {
            int i=0;
            scanf("%d", &i);
            add(n, i);
        }
        else if(!strcmp("p", menu)) //print
        {
            print(n);
        }
        else if(!strcmp("q", menu)) //quit
        {
            free(n);
            break;
        }

    }

    return 0;
}

void begin(int *n)
{
    n=malloc(sizeof(int));

    if(n==NULL)
    {
        printf("Error in malloc!");
        return;
    }

    n[0]=0;

    printf("Added %d \n", n[0]);
}

void add(int *n, int numToAdd)
{
    static int sizeCount=0;
    sizeCount++;
    tempcount=sizeCount;

    int *temp;

    temp=realloc(n, (sizeCount+1) * sizeof(int));

    if(temp==NULL)
    {
        printf("Error in realloc!");
        return;
    }

    n=temp;

    n[sizeCount]=numToAdd;

    printf("Added %d \n", n[sizeCount]);

}

void print(int *n)
{
    int i;
    for(i=0; i<tempcount; i++)
    {
        printf("%d ", n[i]);
    }
}

This is related and thus of interest: stackoverflow.com/q/13098037/856199 — Nikos C.
– Nikos C., Commented Nov 2, 2012 at 19:28
@n.m.: realloc returns a pointer because it can't guarantee the "resized" block of memory will be in the same spot as the old. If it has to move the block, it needs some way to tell you where the new block is. — cHao
– cHao, Commented Nov 2, 2012 at 19:32
@n.m. Next time ask "What do you think is the reason..." to prevent misunderstandings. — Daniel Fischer
– Daniel Fischer, Commented Nov 2, 2012 at 19:46

hexist · Accepted Answer · 2012-11-02 19:32:41Z

8

You need to pass a pointer to your pointers in add/begin so they can modify your pointer in main

begin(&n);
...
add(&n, i);

and your definition

void begin(int **n)
{
    *n=malloc(sizeof(int));

    if(*n==NULL)
    {
        printf("Error in malloc!");
        return;
    }

    (*n)[0]=0;

    printf("Added %d \n", (*n)[0]);
}

and

void add(int **n, int numToAdd)
{
    static int sizeCount=0;
    sizeCount++;
    tempcount=sizeCount;

    int *temp;

    temp=realloc(*n, (sizeCount+1) * sizeof(int));

    if(temp==NULL)
    {
        printf("Error in realloc!");
        return;
    }

    *n=temp;

    (*n)[sizeCount]=numToAdd;

    printf("Added %d \n", (*n)[sizeCount]);

}

Right now what you're doing is modifying local copies of your pointer in begin/add, so when you change it in those functions it's not modifying your pointer n in main

Also, fun fact, if you pass NULL as the first parameter to realloc it acts like a malloc, so if you initialize n to NULL, you can simply call add without first doing a begin.

edited Nov 2, 2012 at 19:32

answered Nov 2, 2012 at 19:25

hexist

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2 Comments

Some programmer dude Over a year ago

The same, passing the array by reference, has to be done for the add function as well.

Draconian Times Over a year ago

@hexist: Hey, thanks a lot for your answer, it really helped and i have the program working! Thanks!

Nik · Accepted Answer · 2012-11-02 19:30:49Z

0

Check your function add - are you sure you update the pointer value? Try with ** as a parameter - I think it will help.

answered Nov 2, 2012 at 19:30

Nik

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