Argument-Passing Mechanism

This section describes how arguments are passed in ANSI/ISO C.

All arguments to C functions are passed by value.

Actual arguments are passed in the reverse order from which they are declared in a function declaration.

Actual arguments which are expressions are evaluated before the function reference. The result of the expression is then placed in a register or pushed onto the stack.

(SPARC)

Functions return integer results in register %o0, float results in register %f0, and double results in registers %f0 and %f1.

long long [Not available in -Xc mode.] integers are passed in registers with the higher word order in %oN, and the lower order word in %o(N+1). In-register results are returned in %i0 and %i1, with similar ordering.

All arguments, except doubles and long doubles, are passed as four-byte values. A double is passed as an eight-byte value. The first six four-byte values (double counts as 8) are passed in registers %o0 through %o5. The rest are passed onto the stack. Structures are passed by making a copy of the structure and passing a pointer to the copy. A long double is passed in the same manner as a structure.

Upon return from a function, it is the responsibility of the caller to pop arguments from the stack. Registers described are as seen by the caller.

(Intel)

Functions return integer results in register %eax.

long long results are returned in registers %edx and %eax. Functions return float, double, and long double results in register %st(0).

All arguments, except structs, unions, long longs, doubles and long doubles, are passed as four-byte values; a long long is passed as an eight-byte value, a double is passed as an eight-byte value, and a long double is passed as a 12-byte value.

structs and unions are copied onto the stack. The size is rounded up to a multiple of four bytes. Functions returning structs and unions are passed a hidden first argument, pointing to the location into which the returned struct or union is stored.

Upon return from a function, it is the responsibility of the caller to pop arguments from the stack, except for the extra argument for struct and union returns that is popped by the called function.