c - Sizeof vs Strlen

Question

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

int main(int argc, char *argv[]) {
    char string[] = "october"; // 7 letters

    strcpy(string, "september"); // 9 letters

    printf("the size of %s is %d and the length is %d

", string,
        sizeof(string), strlen(string));

    return 0;
}

Output:

$ ./a.out
the size of september is 8 and the length is 9

Is there something wrong with my syntax or what?

Answer 1

sizeof and strlen() do different things. In this case, your declaration

char string[] = "october";

is the same as

char string[8] = "october";

so the compiler can tell that the size of string is 8. It does this at compilation time.

However, strlen() counts the number of characters in the string at run time. So, after you call strcpy(), string now contains "september". strlen() counts the characters and finds 9 of them. Note that you have not allocated enough space for string to hold "september". This is undefined behaviour.