不使用标准库函数进行字符串比较

Question

I am new to C programming. This is just a beginners question. I am trying to implement string compare without using standard function.Here i have used dynamic memory allocation and used fgets() . But the second string is not inputted. Can anyone help me point out the problem? The code is given below.

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

int my_strcmp(char*, char*);

int main()
{
    int a, n;
    printf("enter the length of strings\n");
    scanf("%d",&n);
    getchar();

    char *s1 = (char *)malloc(n * sizeof(char));
    printf("enter the string1\n");
    fgets(s1, n, stdin);
    getchar();

    char *s2 = (char *)malloc(n * sizeof(char));
    printf("enter the string2\n");
    fgets(s2, n , stdin);

    if (s1 == NULL)
    {
        printf("malloc error!!");
    }

    if (s2 == NULL)
    {
        printf("malloc error!!");
    }

    a = my_strcmp( s1, s2);

    if (a == 0)
    {
        printf("the strings are equal");
    }
    else
    {
        printf("the strings are not equal");
    }
    free (s1);
    free (s2);
    return 0;
}

int my_strcmp( char *s1, char*s2)
{
    while (*s1)
    {
        if (*s1 == *s2)
        {
            s1++;
            s2++;
        }
        else
            break;
    }

    if ( *s1 == *s2)
    {
        return 0;
    }
    else if (*s1 > *s2)
    {
        return 1;
    }
    else
    {
        return -1;
    }
} 

Answer 1

The n parameter of fgets is the size of the buffer including null-terminator . Thus, it reads up to at most n - 1 characters, and fills the last spot with a null-terminator. Your second call to getchar (after the first fgets ) then reads that last character, not the newline, so the second call to fgets stops early because it immediately hits a newline.

Instead, you'll want to allocate n + 1 characters for each string, and call fgets with n + 1 as well.

Also, you should check for malloc failure before trying to write to s1 or s2 .

Answer 2

The code can be changed like this.

getchar(); //Fix
fgets(s1, n, stdin);
getchar();
char *s2 = (char *)malloc(n * sizeof(char));

Set n=n+1 after getting from user. This is to deal '\\0' character