Question

If I allocate memory for a char a[256], initialise each element to a different value and pass this to some function:

int subFunc(a);

how does the stack frame of subFunc look?

Does the stack frame contain all the 256 bytes in it or just a pointer to the address of a (4 bytes).

If a pointer, then how does it access the data inside the subFunc or where is this data available to subFunc?

Answer 1

subFunc will receive a pointer to char, not an array of char. The details of how that pointer is passed will vary with the implementation. The pointer value may be pushed onto the stack, or it may be passed in a register (gcc 2.96 on Red Hat does the former, while gcc 4.1.2 on SLES 10 does the latter), and that may depend on things like optimization settings, whether you’re generating a debug version, etc. The only way to know for sure how it works on your platform is to code it up, build it, and look at the generated machine code.

In most circumstances, an expression of type “N-element array of T” will be converted (“decay”) to an expression of type “pointer to T“, and the value of the expression will be the address of the first element of the array. The exceptions to this rule are when the array expression is an operand of the sizeof, _Alignof, or unary & operators, or is a string literal being used to initialize the contents of another array in a declaration.

Given the code

char a[256];
...
subFunc(a);

the expression a in the call to subFunc has type “256-element array of char“; by the rule above, it is replaced with an expression of type “pointer to char” (char *) and its value is the address of a[0].

Thus, subFunc receives a pointer value, not an array of char.

Note that, in the context of a function parameter declaration, T a[] and T a[N] are interpreted as T *a; if your prototype for subFunc is

int subFunc(char a[256])

it will be interpreted as

int subFunc(char *a)