Re: macros with global variables?

[Glynn Clements <glynn@gclements.plus.com>]

Robert P. J. Day wrote:

>   not sure if this is outside the scope of this mailing list but i
> just downloaded the source for a simple SSL-aware ftp client
> (netkit-ftp-ssl), did the configure and the make and got:
> 
> ftp.c: In function ârecvrequestâ:
> ftp.c:1127: error: âpdataâundeclared (first use in this function)
> ftp.c:1127: error: (Each undeclared identifier is reported only once
> ftp.c:1127: error: for each function it appears in.)
> 
>   ok, fair enough, check the offending header file to find the
> following macro definition:
> 
> #define is_ssl_fd(X,Y)    ( (SSL_get_fd((X))==0) || \
>                             (SSL_get_fd((X))==1) || \
>                             (SSL_get_fd((X))==pdata) || \
>                             (SSL_get_fd((X))==(Y)) \
> 
>   ?????.  so i have a macro which accepts two arguments, but expands
> to include an explicit reference to something called "pdata", which
> does not exist, hence the error message.
> 
>   i tend to avoid defining macros that refer to anything but their own
> arguments.  what might the above mean?  is there some well-known idiom
> for C programmers that makes the above, in some way, acceptable?
> (i've perused the code and the object "pdata", whatever that is, is
> simply not defined anywhere.  so is there a context in which the above
> makes sense in some way?)

At the risk of stating the obvious, the only context where it makes
sense is one where "pdata" is defined, as either a global variable,
local variable or macro.

Macros with external references are generally best avoided, although
sometimes it can simplify the code signficantly.

-- 
Glynn Clements <glynn@gclements.plus.com>
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