Question

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

#define n ((sizeof(char)) * 100 )

int stringlength(char * str)
{
    int count=0;
    while(*str)
    {  
        if(*str == '\n')
        {
            *str=0;
        }
        else
            count++, str++;
    }
    return count;
}


int palin1(char *str, int k)
{
    char * pend = str + k - 1;
    if(*pend != *str)
        return 0;
    else
        palin1(str+1, k-1);
       return 1;
}    

int palin(char *str)
{
    int length = stringlength(str), f=0;
    char *pend = str + length - 1;
    while(str <= pend)
    {
        if(*str == *pend) f=1;
        else
            return (f = 0);
        str++, pend--;
    }
    return 1;
}

main()
{
    char * ps = (char *)malloc(n);
    int flag;
    if(ps == NULL) printf("Malloc Fail\n");
    else
    {
        printf("Malloc Succeeded, you have memory of %d bytes\n", n);
        printf("This program checks if String is Palindrome or not\n\
        \nEnter your String: ");
        fgets(ps, 100, stdin);
        printf("You entered: %s of length %d", ps, stringlength(ps));
        int i = 0;
        printf("\n\nEnter:\n1.Using iteration\n2.Using Recursion ");
        scanf("%d", &i);
        switch(i)
        {
            case 1:
                flag=palin(ps);
                break;
            case 2:
                flag=palin1(ps,stringlength(ps));
                break;
            default:
                printf("Invalid input");
        }

        if(flag) printf("\nYou entered a Palindrome");
        else  printf("\nNot a Palindrome");
    }
    free (ps);
    return 0;
}

Why does the above program http://www.ideone.com/qpGxi does not give any output on putting the input:

mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

I know fgets(ps,100,stdin) will take only 100 characters and not more than that, but why does the program halt execution?

Answer 1

You should check for fgets failure, as recommended by the fgets spec.

if ( fgets(ps,100,stdin) == NULL ) {
    printf("Input failed.");
    //check for 'feof' or 'ferror' here
    return -1;
}
printf("You entered: %s of length %d",ps,stringlength(ps));

I don’t see why fgets would be failing, but you would get an uninitialized character buffer back, which would crash printf.

EDIT: You should really pay attention to your compiler warnings, too.

prog.c:49: warning: return type defaults to ‘int’
prog.c: In function ‘main’:
prog.c:59: warning: ignoring return value of ‘fgets’, declared with attribute warn_unused_result
prog.c:63: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result
prog.c: In function ‘palin’:
prog.c:46: warning: control reaches end of non-void function
prog.c: In function ‘main’:
prog.c:52: warning: ‘flag’ may be used uninitialized in this function

You can see that even your compiler recommends checking fgets for null. Also, flag should be set to 0 in the default case, otherwise you will get undefined behavior if the user enters something other than 1 or 2.

EDIT 2: Oh for Christ’s sake! your program works fine! You forgot to check “run program” in Ideone!!!

http://www.ideone.com/7ecZd