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Note: would be nice to explain syscall macros

Forum: How System Calls Work on Linux/i86
Keywords: sycall
Date: Fri, 13 Sep 1996 01:37:11 GMT
From: Tim Bird <>

The syscall macros are a little dense to decipher. It took me a while to determine how the macro syscall1(int,setuid,uid_t,uid) expanded into the assembly code shown.

It might be nice to show the macro, and explain a little about how it gets expanded.

Here is the source for the _syscall1 macro

	#define _syscall1(type,name,type1,arg1) \
	type name(type1 arg1) \
	{ \
	long __res; \
	__asm__ volatile ("int $0x80" \
        	: "=a" (__res) \
        	: "0" (__NR_##name),"b" ((long)(arg1))); \
	if (__res >= 0) \
        	return (type) __res; \
	errno = -__res; \
	return -1; \

When expanded, this become the code
	int setuid(uid_t uid)
		long __res;
		__asm__ volatile ("int $0x80" \
			: "=a" (__res) \
			: "0" (__NR_setuid), "b" ((long)(uid)));
		if (__res >= 0 )
			return (int) __res;
		errno = -__res;
		return -1;

It's pretty easy to see how the cleanup code converts
into assembly, but the setup code eluded me until
I figured out the following:
	"=a" (__res) means the result comes back in %eax
	"0" (__NR_setuid) means put the system call number
		into %eax on entry
	"b" ((long)(uid) means put the first argument
		into %ebx on entry

syscallX macros that use additional parameters use %ecx, %edx, %esi, and %edi to hold additional values passed through the call.