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UNKNOWN! runtime error

Discussion in 'C' started by lionaneesh, Mar 23, 2010.

  1. lionaneesh

    lionaneesh Active Member

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    Code:
    #include<stdio.h>
    
    void reverse(char str[]);
    
    int main()
    {
        char str[100];
        int i;
    
        printf("Enter to reverse : ");
        scanf("%s",str);
        reverse(str);
        return(0);
    }
    /* Function */
    
    void reverse(char str[])
    {
    
        int i;
        for(i=99;i>=0;i--)
        {
            printf("%c",str[i]);
        }
        putchar('\n');
    }
    
    
    
    :embarasse:embarasse:embarasse:embarasse:embarasse:embarasse:embarasse:embarasse:embarasse:embarasse:embarasse:embarasse\


    Why there is a runtime error in the above programme..........

    help!!!!!!
    URJENT!!!!!!!!
     
  2. virxen

    virxen New Member

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    Code:
    #include<stdio.h>
    [COLOR=Red]#include <string.h>[/COLOR]
    
    void reverse(char str[]);
    
    int main()
    {
        char str[100];
        int i;
    
        printf("Enter to reverse : ");
        scanf("%s",str);
        reverse(str);
        return(0);
    }
    /* Function */
    
    void reverse(char str[])
    {
    
        int i;
        [COLOR=Red]int z=strlen(str);[/COLOR]
        for(i=[COLOR=Red]z-1[/COLOR];i>=0;i--)
        {
            printf("%c",str[i]);
        }
        putchar('\n');
    }
    
    
    the error is that you reserve 100 positions in memory for the string entered
    but it uses actually only those the user enter.

    so if i enter the word car-->'c' 'a' 'r' '\0' so it uses 4 positions instead of 100
    so instead of starting from the end of 100positions you must start from the end of 4positions down to 0
     
    shabbir likes this.
  3. xpi0t0s

    xpi0t0s Mentor

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    That doesn't explain why there would be a runtime error. It would explain why garbage is displayed and if I modify the program to add
    Code:
    	for (i=0; i<100; i++)
    		str[i]='~';
    
    to main just after the int i declaration, the output is:
    Enter to reverse : hello
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ olleh

    but even without this modification I don't get a runtime error (in Visual Studio 2008 building the program in Debug mode).

    When it prompted you to enter a string, did you enter more than 100 characters? That might cause a runtime error - this is the infamous "buffer overflow" bug.

    If not then I can only suggest this is a compiler bug. Maybe the RTL has a problem with %c for non-printable characters. You can test this hypothesis with a simple program:
    Code:
    printf("Starting test\n");
    fflush(stdout);
    for (i=0; i<256; i++)
    {
      printf("%d %c ",i,i);
      fflush(stdout);
    }
    
    The fflush ensures that the last character before the runtime error is displayed; normally output is buffered which can cause confusion when a program crashes - it looks like it crashed before it actually went tits up.
     

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