How to know the size (in bytes) of a char array passed to a function?

Question

I am testing the sizeof operator. In two cases in my code, I get the size of the pointer (I think). In the other cases I get

Answer 1

When an array is passed to a function, what's actually happening is that a pointer to the first element in the array is being passed. Put another way, the array decays into a pointer to the first element.

In these two declarations:

void test (char arrayT[]);
void test2 (char *arrayU);

arrayT and arrayU are of exactly the same type due to this decay, and sizeof will return the same value for both, i.e. the size of a char *.

Contrast the above with this:

char array1[] = "a string";
char *array2;

Where array1 is actually an array of size 9, while array2 is a pointer whose size (on your system) is 4.

Because of this, there is no way to know the length of an array passed to a function. You need to pass in the size as a separate parameter:

void test (char arrayT[], size_t len);