Why does this program crash when I do this?

This is a discussion on Why does this program crash when I do this? within the C Programming forums, part of the General Programming Boards category; Hi I'm a newbie in C programming attending a night course. Right now I'm trying to learn about : arrays, ...

  1. #1
    Registered User Witch's Avatar
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    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;
    }
    Last edited by Witch; 03-18-2010 at 07:03 PM.

  2. #2
    Woof, woof! zacs7's Avatar
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    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.

  3. #3
    Registered User claudiu's Avatar
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    Well, the main issue is that in the second example you are doing:

    Code:
    tempPointer_a1 = (double *)(calloc(n,sizeof(int));
    You are allocating space for an int, to a variable that is going to hold a double. It should be sizeof(double).

    Also, stop casting malloc() and calloc(). Search the FAQ or the threads of this forum for the explanation.

  4. #4
    spurious conceit MK27's Avatar
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    segmentation fault
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    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.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  5. #5
    Registered User Witch's Avatar
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    Thumbs up

    Roger on the casting tips! I have an old skool teacher.

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