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Re: [compile error] execute.c b/c local variable 'stdin' in 'debug_print

From: Eric Blake
Subject: Re: [compile error] execute.c b/c local variable 'stdin' in 'debug_print_input '
Date: Thu, 1 Nov 2018 15:07:32 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 11/1/18 2:54 PM, Eric Blake wrote:

"Applications shall not declare or define identifiers with the same name as an identifier reserved in the same context."

Thus, I read it that if we included <stdio.h>, the name 'stdio' is

the name 'stdin'

reserved for use as the external identifier, which means the implementation is free to implement a macro by that name, and our attempt to shadow the name without using #undef is a bug in our part, and not in the headers for using a macro.

The cygwin case expands the 'stdin' macro and results in invalid C code.

Rather, the C code got lucky on glibc with it's incorrect use of a reserved name for a non-reserved purpose not actually colliding, but failed on Cygwin due to a clash in definitions where the fault lies in the application and not in the system headers.

Then again, this is just splitting hairs. Your suggestion is good and
I'll commit the patch soon.

Yes, the patch to use a non-reserved name is ideal in any case.

One more point - a local variable shadowing an identifier with external linkage does not necessarily conflict - so let's check one more piece of information in the standards:

"The <stdio.h> header shall define the following macros which shall expand to expressions of type "pointer to FILE" that point to the FILE objects associated, respectively, with the standard error, input, and output streams:

    Standard error output stream.
    Standard input stream.
    Standard output stream."

The fact that the standard requires 'stdin' to be a macro, and does not require the macro to expand to merely a single identifier, is good evidence that you should not attempt to shadow it.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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