Question

Can you please tell me what I did wrong?

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


void read(int *p,int n)
{
    int *q,i,j;
    q=p;
    for(i=0;i<n;i++)
        for(j=0;j<n;j++)
        {
            printf("matrix[%d][%d]=",i,j);
            scanf("%d",q);
            q=q+1;
        }
    printf("\n");
}

void alocate(int *p,int n)
{
    p=(int*)malloc(n*n*sizeof(int));
    if(p==NULL)
    {
        printf("Allocation error\n");
        exit(1);
    }
}

void realocate(int *p,int n)
{
    p=(int*)realloc(p,n*n*sizeof(int));
    if(p==NULL)
    {
        printf("Reallocation error\n");
        exit(1);
    }
}

void show(int *p,int n)
{
    int *q,i,j;
    q=p;
    for(i=0;i<n;i++)
    {
        for(j=0;j<n;j++)
        {
            printf("%d\t",*q);
            q=q+1;
        }
        printf("\n");
    }
}

void cleaner(int *p)
{
    free(p);
}
int main() {
    int *p,n;
    p=NULL;
    printf("n=");
    scanf("%d",&n);
    alocate(p,n);
    read(p,n);
    show(p,n);
    realocate(p,2);
    read(p,2);
    show(p,2);
    cleaner(p);
    return 0;
    system("pause");
}

NetBeans (MinGW):

RUN FAILED (exit value 5)

Signal received: SIGSEGV (?) with sigcode ? (?)
From process: ?
For program cppapplication_1, pid -1

Visual Studio:

Unhandled exception at 0x5c81e42e (msvcr100d.dll) in Capp.exe: 0xC0000005: Access violation writing location 0x00000000.

---

And if I delete p=NULL; from main function, it says:

Run-Time Check Failure #3 – The variable ‘p’ is being used without being initialized.
Unhandled exception at 0x5b4ee42e (msvcr100d.dll) in Capp.exe: 0xC0000005: Access violation writing location 0xcccccccc.

Answer 1

Your alocate function correctly allocates memory, but does not return a pointer to the allocated memory. You can fix it like this

int *alocate(int n)
{
    int *p=(int*)malloc(n*n*sizeof(int));
    if(p==NULL)
    {
        printf("Allocation error\n");
        exit(1);
    }

    return p;
}

In your main function, you would use alocate like this:

p = alocate(n);

You need to make a similar change to your realocate function.