Thread: Why does this program crash when I do this?

Hi I'm a newbie in C programming attending a night course. Right now I'm trying to learn about: arrays, calloc and pointers.
None of these things has truly sunken in yet, I'm still not comfortable using them.

Using:
* Windows Vista 32-bit
* Bloodshed Dev-C++ v.4.9.9.2
* 4 GB RAM


Example code 1: /* Complex_Numbers_01_INT.c */ - [ semi-OK ]
I have 2 example codes. The first works fine except it can't handle floating-point numbers properly so you can only test it using Integers.


Example code 2: /* Complex_Numbers_02_LF.c */ - [ BAD ]
The second example code I basically replaced all INT data types with DOUBLE data types and replaced most %d with %lf when using:
scanf() and printf()


The Crash / The Problem
Only when I run the second example code with the DOUBLE and %lf in the source code does the program crash. You have to run it around 6-10 times before it crashes.
I'm suspecting it's some kind of memory leak problem but I am too newbie to see it and handle it.
What am I doing wrong? What have I misunderstood?


Example code 1: - [ semi-OK ]

Code:

/* Complex_Numbers_01_INT.c */

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


int main(int argc, char *argv[])
{
  int *tempPointer_a1;
  int *tempPointer_b1;
  int n,i,j;


  printf("How many complex numbers do you want to input? --> "); scanf("%d",&n);
/* calloc = Memory allocation for array */
  tempPointer_a1 = (int *) calloc(n,sizeof(int));
  tempPointer_b1 = (int *) calloc(n,sizeof(int));

    for(i=0 ; i<n ; i++){
        printf("\n\nInput a complex number Re(a%d)+Im(b%d), for element number <%d>.", i, i, i);
        printf("\ni.e. input 2 floating-point numbers: --> ");   
        scanf("%d %d", tempPointer_a1+i, tempPointer_b1+i );
    }

  for(i=0 ; i<n ; i++)
     printf("\n< Element number %d > , Re(a%d)+Im(b%d) = %d + %d (j)", i, i, i, tempPointer_a1[i], tempPointer_b1[i] );

  free(tempPointer_a1);
  free(tempPointer_b1);
  printf("\n\n");

  system("PAUSE");	
  return 0;
}

Example code 2: - [ BAD ]

Code:

/* Complex_Numbers_02_LF.c */

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


int main(int argc, char *argv[])
{
  double *tempPointer_a1;
  double *tempPointer_b1;
  int n,i,j;


  printf("How many complex numbers do you want to input? --> "); scanf("%d",&n);
/* calloc = Memory allocation for array */
  tempPointer_a1 = (double *) calloc(n,sizeof(int));
  tempPointer_b1 = (double *) calloc(n,sizeof(int));

    for(i=0 ; i<n ; i++){
        printf("\n\nInput a complex number Re(a%d)+Im(b%d), for element number <%d>.", i, i, i);
        printf("\ni.e. input 2 floating-point numbers: --> ");   
        scanf("%lf %lf", tempPointer_a1+i, tempPointer_b1+i );
    }

  for(i=0 ; i<n ; i++)
     printf("\n< Element number %d > , Re(a%d)+Im(b%d) = %lf + %lf (j)", i, i, i, tempPointer_a1[i], tempPointer_b1[i] );

  free(tempPointer_a1);
  free(tempPointer_b1);
  printf("\n\n");

  system("PAUSE");	
  return 0;
}

Double's are usually larger than integers... Therefore your calloc() in the second program is wrong. You simply haven't allocated enough space to store n * sizeof(double).

To avoid that mistake, I would suggest:

Code:

double * x = NULL;
/* ... */
x = calloc(n, sizeof *x);

I would also suggest:
* Checking the return value of calloc().
* Don't cast malloc(), realloc(), calloc().
* Error checking for user input. Passing n straight from scanf() to calloc() is silly.

Yeah, you don't need to cast void pointers in C (such as returned by calloc/malloc) -- the type conversion is automatic.

You do need to do it in C++, which is why code sometimes appears this way.