question regarding SYSCALL macros

Sudheer Divakaran inbox1.sudheer at
Thu Apr 28 06:16:14 EDT 2011

 I was going through the file kernel/time.c and found the definition
of 'gettimeofday' system call as follows.

SYSCALL_DEFINE2(gettimeofday, struct timeval __user *, tv,
		struct timezone __user *, tz)   <--- NOTE THE COMMA between variable
types and names.

In syscalls.h I could find the needed macros for expanding the
definition to its final form i.e.,

asmlinkage long sys_gettimeofday(struct timeval __user * tv, struct
timezone __user *tz)

[syscalls.h macros]

#define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)

#define SYSCALL_DEFINEx(x, sname, ...)	\
	__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

#define __SYSCALL_DEFINEx(x, name, ...)	\
	asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__))

#define __SC_DECL1(t1, a1)	t1 a1
#define __SC_DECL2(t2, a2, ...) t2 a2, __SC_DECL1(__VA_ARGS__)

Note the comma between the variable types and names in the original
definition  (e.g., "struct timeval __user *, tv" instead of "struct
timeval __user *tv").

What is is the reason behind separating variable types and variable
names using 'comma' and latter removing the comma character using
_SC_DECLx macros?


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