Static char array initialization with simple function says length is 6, but shouldn't it be 1?

Question

I've discovered that this program continues returning 6 until I start returning 16 or greater instead of 1, at which point the program prints 0. Why? My intention was to use pass the result of a function directly into the static array initialization.

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

int ret_1() { return 1; }
int main( int argc, const char* argv[] ) 
{   
    char arr[ret_1()];  
    int l = strlen(arr); 

    printf("The size is: %d\n", l);     
    return 0; 
}

The size is: 6

I wonder if this is undefined behavior generating an arbitrary value, or if there's an underlying issue I'm missing.

Answer 1

Static char array initialization with simple function

This statement

char arr[ret_1()]; 

defines an array of as much elements as given by ret_1(), but leaves it uninitialised.

Then this call

int l = strlen(arr); 

reads the (uninitialised) array's content/elements and with this invokes undefined behaviour. Anything can happen from this moment on.

To fix this do:

char arr[ret_1()]; 
memset(arr, 0, sizeof arr); /* Initialise array by (all) zero(s). */

After applying the fix, strlen(arr) returns 0, meaning that 0 elements in arr are used to represent a "string". It stores the equivalent to "", the empty string.

Note:

This expression sizeof arr actually returns the number of bytes allocate for arr, which in your example would be 1.