This section describes how arguments are passed in 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 that are expressions are evaluated before the function reference. The result of the expression is then placed in a register or pushed onto the stack.
Functions return integer results in register %o0, float results in register %f0, and double results in registers %f0 and %f1.
All arguments, except double and long double, are passed as 4-byte values. A double is passed as an 8-byte value. The first six 4-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.
Registers described are as seen by the caller.
All integral arguments are passed as 8-byte values.
Floating-point arguments are passed in floating-point registers when possible.
Intel 386 psABI and AMD64 psABI are observed.
Functions return results in the following registers:
All arguments except structs, unions, long longs, doubles and long doubles are passed as four-byte values; a long long is passed as an 8-byte value, a double is passed as an 8-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, the caller is responsible for popping arguments from the stack except for the extra argument for struct and union returns that is popped by the called function.