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Date: Sun, 2 Jul 2023 22:30:08 -0400
From: Rich Felker <dalias@...c.org>
To: 程智星 <zhixing@...iscas.ac.cn>
Cc: musl@...ts.openwall.com
Subject: Re: Re: Re: On the redefinition of stdin, stdout, etc. in
stdio. h
On Mon, Jul 03, 2023 at 08:37:17AM +0800, 程智星 wrote:
> hi,friend:
>
> I have a question to ask:
>
> 1. The stdin and stdout are redefined in the stdio. h file in the Musl source code, as follows:
>
> extern FILE *const stdin;
> extern FILE *const stdout;
> extern FILE *const stderr;
>
> #define stdin (stdin)
> #define stdout (stdout)
> #define stderr (stderr)
> but,In glibc, they are only redefined as themselves, as follows:
>
> /* Standard streams. */
> extern FILE *stdin; /* Standard input stream. */
> extern FILE *stdout; /* Standard output stream. */
> extern FILE *stderr; /* Standard error output stream. */
> /* C89/C99 say they're macros. Make them happy. */
> #define stdin stdin
> #define stdout stdout
> #define stderr stderr
> Based on what considerations we will make this change?
>
> 2. Based on the above modifications, I will report the following errors when compiling packages such as lcr and lxc using Musl as the basic library for yocto embedded compilation:
> -o json/.libs/liblxc_la-json_common.o
> | In file included from ../../../lxc-4.0.3/src/lxc/json/read-file.h:5,
> | from ../../../lxc-4.0.3/src/lxc/json/defs.c:6:
> | ../../../lxc-4.0.3/src/lxc/json/defs.c: In function 'make_defs_hook':
> | ../../../lxc-4.0.3/src/lxc/json/defs.c:86:26: error: expected identifier before '(' token
> | 86 | if (ctx->stderr > 0)
> | | ^~~~~~
> | ../../../lxc-4.0.3/src/lxc/json/defs.c:87:34: error: expected identifier before '(' token
> | 87 | fprintf(ctx->stderr, "WARNING: unknown key found: %s\n", tree->u.object.keys[i]);
> | | ^~~~~~
> | ../../../lxc-4.0.3/src/lxc/json/defs.c:87:21: error: too few arguments to function 'fprintf'
> | 87 | fprintf(ctx->stderr, "WARNING: unknown key found: %s\n", tree->u.object.keys[i]);
> | | ^~~~~~~
> | In file included from ../../../lxc-4.0.3/src/lxc/json/read-file.h:5,
> | from ../../../lxc-4.0.3/src/lxc/json/defs.c:6:
>
>
> Is there a solution to this error report; I tried to synchronize
> their definitions in the Musl source code with those in Glibc, and
> there was no problem compiling images. Can it be modified to be
> consistent with glibc?
No, this is a bug in the application. Per 7.21.1 ¶3, stdin, stdout,
and stderr are macros which expand to expressions. And per 7.1.3
"Reserved Identifiers", ¶2, "If the program declares or defines an
identifier in a context in which it is reserved (other than as allowed
by 7.1.4), or defines a reserved identifier as a macro name, the
behavior is undefined."
An application cannot use the names stdin, stdout, or stderr in the
manner above if it is including stdio.h. The names of those struct
members need to be changed to something that is not in the reserved
namespace.
If anything, at some point we may be making changes further in the
opposite direction, so that the expressions are not lvalues. This has
value to catch/preclude other types of application errors. musl would
not give up the ability to do things like this for the sake of
compatibility with erroneous application sources that could easily be
fixed.
Rich
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