2010-07-25 21:29:06

by Parmenides

Subject: why are there some redundant variable definition

Hi,
I have read a macro like this
++++ include/linux/kernel.h
159: #define min_t(type,x,y) \
160: ({ type __x = (x); type __y = (y); __x < __y ? __x: __y; })
++++ net/ipv4/tcp_output.c
654: int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));

The definitions of __x and __y are somewhat redundant. I think a more

2010-07-25 21:44:56

by Randy Dunlap

Subject: Re: why are there some redundant variable definition

On Mon, 26 Jul 2010 05:29:04 +0800 Parmenides wrote:

> Hi,
> I have read a macro like this
> ++++ include/linux/kernel.h
> 159: #define min_t(type,x,y) \
> 160: ({ type __x = (x); type __y = (y); __x < __y ? __x: __y; })
> ++++ net/ipv4/tcp_output.c
> 654: int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));
>
> The definitions of __x and __y are somewhat redundant. I think a more
> --

The local variables __x and __y are used so that 'x' and 'y' are not
re-evaluated multiple times. That could be a problem if either one of
them is an expression.

Anything else?

---
~Randy
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