cstringsizeofstrlennull-terminated

Why does sizeof("Hello") return 6 instead of 5 in C?

I was trying to calculate the length of a string using sizeof(), but I noticed something unexpected:

#include <stdio.h>

int main() {
    printf("%lu\n", sizeof("Hello"));  // Output: 6
    return 0;
}

I expected the output to be 5 (since "Hello" has 5 characters), but the result is 6.

Why does sizeof("Hello") return 6 instead of 5?

How is sizeof different from strlen() in this case?

Solution

  • 1. String Literals Include a Null Terminator

    When you write "Hello", the compiler automatically appends a null character (\0) at the end to indicate the end of the string.

    So, "Hello" in memory is actually:

    ['H', 'e', 'l', 'l', 'o', '\0']

    Thus, sizeof("Hello") counts all 6 bytes, including the \0

    2. Difference Between sizeof and strlen

    Also, %lu is the wrong conversion specifier when the argument is the return value from strlen or the value of the sizeof operator since the type is size_t, not unsigned long. %zu is the correct conversion specifier for size_t.

    Example:

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

int main() {
    printf("sizeof: %zu\n", sizeof("Hello"));  // Output: 6
    printf("strlen: %zu\n", strlen("Hello"));  // Output: 5
    return 0;
}

    How to Avoid This Confusion?