fgets 没有输入两个字符串

Question

I solved the question but still have a doubt Here's my code:

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

int main(int argv, char *argc[])
{

    if (argv != 2)
    {
        printf("Invalid Input, Please enter the length of the strings you wish to compare\n");
        return 1;
    }
    int n = atoi(argc[1]);
    char *a = malloc((sizeof(char) * n));
    printf("Enter the first string - ");
    fgets(a, n + 1, stdin);
    getc(stdin);
    char *b = malloc((sizeof(char) * n) + 1);
    printf("Enter second string  - ");
    fgets(b, n + 1, stdin);
    int d = 0;
    for (int i = 0; i < n; i++)
    {
        if (*(a + i) != *(b + i))
        {
            d++;
        }
    }
    if (d == 0)
    {
        printf("The two strings are identical\n");
    }
    else
    {
        printf("The two strings are not identical\n");
    }
    free(a);
    free(b);
}

My problem was solved after i added getc(stdin); But can someone please tell me what exactly it is doing? I don't know why it works!

Answer 1

fgets stops reading after input once it sees a newline (which isn't consumed if the buffer doesn't have enough space). So newline from first input is left in the input stream, which the second call to fgets sees and stops reading input.

When you add getc(stdin); , it consumes the leftover newline char, so it works.

As noted in comments, you're allocating only n bytes for a but attempting to read upto n + 1 bytes. This is undefined behaviour. So you'd need add " + 1" to the malloc call (like you do for the allocation of b ).

Another problem that you need to be aware of is, fgets will read the newline character too into a and b if the they have enough space (eg you enter just "hi" and n is 10).

And another problem is that if input given is less than n , you loop still compares n chars. This could be undefined as the rest of buffers are uninitialized.

You can also break out of the loop immediately when there's a mismatch. No need compare the rest of the chars.