Thread: Mysterious Crash

  1. #1
    Registered User
    Join Date
    May 2013

    Mysterious Crash

    So I am doing a problem from C How To Program 7th Edition by Deitel. The problem is 8.10 for those who might have access to the book.

    In a nutshell, I am comparing strings up to n characters. The program funs fine however at the end it crashes. I cannot figure out why

    #include <string.h>
    #include <stdio.h>
    int main(void){
        char *string1, *string2;
        int n, compare;
        puts("Enter a string");
        fgets(string1,20, stdin);
        puts("Enter a string");
        fgets(string2,20, stdin);
        puts("Enter the amount of characters to compare");
        compare = strncmp(string1, string2, n);
        if(compare == 0)
            printf("Strings compares are equal\n");
            printf("string 1 is %s String 2\n", (compare > 0) ? "greater than" : "less than");
        printf("Ending ....\n");
        return 0;

  2. #2
    Registered User
    Join Date
    May 2009
    You NEED to allocate space for the C-string!!
    Your pointers are pointer to random locations; of course it crashes sometimes.

    This code creates to pointers that point to a random location; likely to a location that you do not own or a location you do own that gets over written.
    char *string1, *string2;
    Simple fix is this line instead
    char string1[20+1], string2[20+1];
    Tim S.
    Last edited by stahta01; 05-26-2013 at 05:02 PM.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  3. #3
    Registered User
    Join Date
    May 2013
    Thank You. You are right, it does crash SOMETIMES.

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