Question

I’m working on manually constructing an IDT table in x86 assembly. I have the following macros defined using the C preprocessor in my .S file:

// sets up an entry in the idt for a trap type
#define SETUP_IDT_ENTRY( name, num, istrap, segment, dpl ) \
    lea name, %edx; \
    movl $(KCODE << 16), %eax; \
    movw $0x8e00, %dx; \
    lea (idt + (8 * num)), %edi; \
    movl %eax, (%edi); \
    movl %edx, 4(%edi);

// sample set of args to a call to setup_dt_entry
#define M_DIVIDE _t_divide_ep, T_DIVIDE, 0, KCODE, 0

// the call
SETUP_IDT_ENTRY( M_DIVIDE )

However, gcc complains: error: macro "SETUP_IDT_ENTRY" requires 5 arguments, but only 1 given

I thought that #define’d arguments to #define’d functions were expanded before the function call was evaluated, in which case M_DIVIDE would expand to the five arguments required and SETUP_IDT_ENTRY would be happy. I’ve tried various combinations of parentheses and nothing seems to be working; is there a way to make this work?

Note: I know there are alternate approaches for building IDT’s in x86 assembly, but that’s not the question I’m trying to answer here; I’m just trying to figure out if macros can be expanded as macro arguments.

Answer 1

The arguments themselves are expanded, but the number of arguments must match the macro definition. You’ll need an extra macro to make it work:

#define IDT1(x) SETUP_IDT_ENTRY(x)

IDT1(M_DIVIDE)

More info here and here.