Sun Studio 12: C User's Guide

F.3 Argument-Passing Mechanism

This section describes how arguments are passed in ISO C.

F.3.1 32-Bit SPARC

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

long long 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 %o0 and %o1, 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.

Registers described are as seen by the caller.

F.3.1.1 64-Bit SPARC

All integral arguments are passed as eight-byte values.

Floating-point arguments are passed in floating-point registers when possible.


Functions return results in the following registers:

Table F–21 Registers Used by x86 Functions to Return Types


Type Returned  



long long

%edx and %eax

float, double, and long double


float _Complex

%eax for the real part and %edx for the imaginary part

double _Complex and long double _Complex

The same as a struct that contains two elements of the corresponding floating point type. 

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.