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Date: Fri, 2 Feb 2018 14:54:27 +0100
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: CodingMarkus <CodingMarkus@...auska.name>
Cc: musl@...ts.openwall.com
Subject: Re: Why are stdin/stdout/stderr `FILE *const` in musl?
Hello CodingMarkus,
On Fri, 2 Feb 2018 14:24:28 +0100 CodingMarkus
<CodingMarkus@...auska.name> wrote:
> Hello everyone!
>
> Just a quick question:
> Why does musl define stdin, stdout, and stderr to be of type `FILE
> *const`? Neither the C standard, nor the POSIX standard require,
> recommend or even imply that it would be allowed that this is a
> `const` pointer. That’s why other C libraries define it as `FILE *`
> only because that matches the examples given by POSIX and that
> matches the description found in any ISO-C standard.
I don't think so. The C standard says
> stderr
> stdin
> stdout
> which are expressions of type "pointer to FILE" that point to the FILE objects associated, respectively,
which clearly states that these are "expressions" not lvalues, and so
your code
> void * getOutputPtr ( void ) {
> if (/* whatever */) {
> return &stdout;
> }
> return &stderr;
> }
is not conforming. In is undefined (by omission of defining it) to
take the address of such an expression.
(This is different from errno, e.g, which is defined as a modifiable
lvalue.)
> and it is correct by POSIX
> standard but it won’t compile with strict compiler settings when
> using musl as the compiler complains that the `const` is lost and
> there is no explicitly cast for that.
> error: cannot initialize return object of type 'void *' with
> an rvalue of type 'FILE *const *'
and this is a feature, because it points to where your program has
undefined behavior.
The C standard has freopen for the cases that you want to change the
association or the access mode of one of the standard streams.
Jens
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