Why call macro in this way?

Question

I read larbin source code recently. But I have a doubt. I the global.h file define the global class, at the end of this file have a macro like this:

#define setPoll(fds, event) \ global::pollfds[global::posPoll].fd = fds; \ global::pollfds[global::posPoll].events = event; \ global::posPoll++

But in the fetch/fetchPipe.cc file, call this macro like this:

global::setPoll(n, POLLOUT);

The question is why use global:: to call this macro? I think use

setPoll(n, POLLOUT);

is ok. Any body can tell me why?

global::setPoll looks like a bug to me. It would expand to global::global::pollfds[... — ugoren
– ugoren, Commented Jul 30, 2012 at 12:48
@eharvest: Technically C++, since there's apparently a class, but spiritually C since it uses macros instead of inline methods. — MSalters
– MSalters, Commented Jul 30, 2012 at 12:56

Community · Accepted Answer · 2017-05-23 12:03:16Z

The source code is a complete mess, it won't even compile. It seems that global.h has been changed between version 2.2.2 and the current version 2.6.3 without addressing those changes in fetch/fetchPipe.h. Also have a look at those include statements in global.cc:

#include <iostream.h> // iostream.h? #include <unistd.h> // twice, see below #include <errno.h> #include <string.h> // mixing C++ and C libraries #include <sys/types.h> #include <unistd.h> ...

This code is outdated and not standard C++. There are several other things wrong. But back to your question: yes, setPoll(n, POLLOUT); should be sufficient. Using global::setPoll will not result in a bug, since this will expand to

global::global::pollfds[global::posPoll].fd = fds; global::pollfds[global::posPoll].events = event; global::posPoll++;

and global is a struct (see Mike Seymour's comment).

You are right, it's not standard c++. I surprise is that it can compile ok with global::setPoll(n, POLLOUT) at gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)
global::global::pollfds is not an error if global is a class; a class-name is injected into the class's scope, so global, global::global and global::global::global::global::global are all equivalent.
@MikeSeymour: Accidentally misread the declaration of global and thought it was a namespace. My fault.

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