Sizeof vs Strlen

c sizeof strlen
beparas picture beparas · Mar 30, 2012 · Viewed 54.6k times · Source 
#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\n\n", 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

Sean picture Sean · Mar 30, 2012

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.

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