Incorrect String Length Using Pointer

Question

I'm trying to create a program that displays the string length of an entered string with a pointer to aforementioned string. When I input a string without spaces, the length is correct. When I enter a string with spaces, the length is incorrect. Please describe my mistake and recommend ways to fix it.

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

int main()
{
    int l,n;
    char s[n], *p=s;
    *p = malloc(sizeof(char)*5);
    n = sizeof(*p);
    if (p==NULL)
    printf("Unable to allocate memory");

    printf("Input a string: ");
    scanf("%s", p);
    l = strlen(p);
    printf("\nLength of given string: %d\n", l);
}

Input:

Input a string: June 27

Output:

Length of given string: 4

Answer 1

Aside from the obvious memory allocation shortcomings mentioned in the comments, "%s" used with scanf will read up to the first whitespace. If you want to read complete lines (up to newline), consider using fgets instead.

Answer 2

The program has the following defects:

1. The variable n is uninitialized before its usage in char s[n].

2. You're trying to read the whole line by scanf() which only reads to the next whitespace.

3. Allocated memory of 5 length, which is an overflow and tends to Undefined Behavior.

4. The (char *) is not explicitly defined during memory allocation.

---

Code redefined:

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

#define MAX 100

int main(void) {
    char *s;

    s = (char *)malloc(sizeof(char) * MAX); // allocating memory correctly

    if (s == NULL) {
        printf("Memory allocation failed.\n");
        return -1;
    }

    printf("Enter a string: ");
    fgets(s, MAX, stdin); // reading the entire line

    size_t len = 0;

    while (s[len++]); // alternative of strlen()

    printf("String length is: %d\n", len);

    return 0;
}

A sample output:

$ gcc -o prog prog.c; ./prog
Enter a string: Hello world, how are you today?
String length is: 33