Why is scanf not allowing any more input?

Question

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

int main() {
    char a[100], b[100];
    char *ret;
    scanf("%[^\n]s", a);
    scanf("%[^\n]s", b);
    ret = strstr(a, b);
    if (ret != NULL)
        printf("its a substring");
    else
        printf("not a substring");
    return 0;
}

My aim was to check whether a substring is present in the parent string in the string or not. I learned about the strstr() function from here.

I have previously used %[^\n]s in my codes before and they worked well. But, in this case as soon as I hit return/enter after typing one string, the output is not a substring.

What is it that I am doing wrong?

Answer 1

The first call to scanf() stops when it sees a newline ('\n') but it's still in the input stream. So, the second call fails immediately as it sees the (same) newline character.

You should always check the return value of scanf() calls for failure.

You can insert a getchar(); call in between the scanf() calls to consume the newline character. Or better use fgets() and handle the newline character.

---

This is how you could use fgets() in your code:

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

int main(void) {
     char a[100], b[100];
     char *ret;
     if (fgets(a, sizeof a, stdin) == NULL) {
        /* error handling */
     }

     a[strcspn(a, "\n")] = '\0';
     if (fgets(b, sizeof b, stdin) == NULL) {
       /* error handling */
     }

     b[strcspn(b, "\n")] = '\0';
     ret=strstr(a, b);
     if(ret!=NULL)
         printf("its a substring");
     else
         printf("not a substring");
     return 0; 
}

Answer 2

The scanf() format you use %[^\n] does not have a trailing s and you should provide the maximum number of characters to store into the destination array. Since the array has a size of 100, you should specify %99[^\n].

Furthermore, you must skip the newline between the 2 %99[^\n] otherwise the second conversion will fail since there would no characters different from \n to match.

Here is a simple correction using scanf(). usr has a good alternative using fgets() which is a simpler way to parse input:

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

int main(void) {
    char a[100], b[100], c;

    if (scanf("%99[^\n]%c%99[^\n]", a, &c, b) == 3 && c == '\n') {
        if (strstr(a, b))
            printf("its a substring\n");
        else
            printf("not a substring\n");
    }
    return 0;
}