static const char * - 已定义但未使用

Question

We need to define a const static char pointer in each and every header (.h) and source (.cpp) file to comply with company coding standards.

static const char * one_time_param = "ABCDEFG";

When compiled, the compiler is generating lot of "defined but not used" warnings. Does someone have a solution to this issue, please?

-Wno-unused-parameter

Using the above compiler flag, we can suppress these warnings. But, this also suppresses some other unused parameters which might need attention. We tried these solutions which only work for function parameters.

Q_UNUSED

in Qt, and

#define UNUSED(x) ((void)(x))

Previous question of similar kind:

How can I hide "defined but not used" warnings in GCC?

Answer 1

在这种情况下它通常也是const指针，所以尝试使用：

static const char * const one_time_param = "ABCDEFG";

Answer 2

First - the company coding standards are arguably wasting space. If you're going to do that, then use an array instead of a char * so you store just the data and not a pointer and the data:

static const char one_time_param[] = "ABCDEFG";

Next, presumably this is for file identification - that, at least, is what I use it for. There are several things to be aware of, learned from experience over a number of years. (I still like to embed version numbers in the source files - I haven't whole-heartedly moved to DVCS because of this.)

1. To avoid the warnings, you have to make the symbols visible outside the file.
2. That, in turn, means you have to make the variable names unique.

3. I'm currently using names based on file name: jlss_id_filename_c[] etc.

    #ifndef lint /* Prevent over-aggressive optimizers from eliminating ID string */ const char jlss_id_errno_c[] = "@(#)$Id: errno.c,v 3.3 2011/09/07 22:33:45 jleffler Exp $"; #endif /* lint */ 

4. The AT&T SVR4 C compiler and support software supported a #ident directive:

    #ident "@(#)$Id: errno.c,v 3.3 2011/09/07 22:33:45 jleffler Exp $" 

   The compiler included the strings in a 'comments' section in the object file, and a tool ( mcs ) to manipulate the comments section (options -d to delete it and -c to compress it, IIRC). This section was part of the binary, but not loaded into memory at runtime.

5. At one point in GCC's evolution, in conjunction with the command line options I was using, I got warnings unless I declared as well as defined the variable, so my 'template' for new source file generates:

    #ifndef lint /* Prevent over-aggressive optimizers from eliminating ID string */ extern const char jlss_id_filename_c[]; const char jlss_id_filename_c[] = "@(#)$Id$"; #endif /* lint */ 

   However, I normally remove the declaration these days, and don't get compiler warnings.

6. As an alternative to using the file name as the basis of variable name, you could generate a UUID or GUID name in hex and use that as the variable name, with a prefix to ensure the first character is alphabetic.

7. In headers, you don't want that material defined in every source file that includes the header because (a) it becomes a noticable (but not necessarily significant) overhead on program size, and (b) you can't multiply define global variables (you can multiply declare them; that's not a problem). So, my headers have a stanza like:

    #ifdef MAIN_PROGRAM #ifndef lint /* Prevent over-aggressive optimizers from eliminating ID string */ const char jlss_id_stderr_h[] = "@(#)$Id: stderr.h,v 10.3 2011/11/28 04:49:24 jleffler Exp $"; #endif /* lint */ #endif 

   Then, when I want the headers to define the values, I have #define MAIN_PROGRAM at the top of the corresponding source file. For example, from running what errno on a program of that name, I get the output:

    errno: $Id: errno.c,v 3.3 2011/09/07 22:33:45 jleffler Exp $ $Id: range.h,v 1.8 2008/02/11 07:39:36 jleffler Exp $ $Id: stderr.h,v 10.3 2011/11/28 04:49:24 jleffler Exp $ $Id: errhelp.c,v 8.5 2009/03/02 19:13:51 jleffler Exp $ $Id: range2.c,v 1.8 2008/02/11 08:44:50 jleffler Exp $ $Id: stderr.c,v 10.7 2011/11/28 04:49:24 jleffler Exp $ stderr.c configured with USE_STDERR_FILEDESC stderr.c configured with USE_STDERR_SYSLOG 

Old-style

This is a complete (and very useful) program illustrating the old-style of doing business.

/*
@(#)File:            $RCSfile: al.c,v $
@(#)Version:         $Revision: 1.4 $
@(#)Last changed:    $Date: 1996/08/13 11:14:15 $
@(#)Purpose:         List arguments one per line
@(#)Author:          J Leffler
@(#)Copyright:       (C) JLSS 1992,1996
@(#)Product:         :PRODUCT:
*/

/*TABSTOP=4*/

#include <stdio.h>
#include <stdlib.h>

#ifndef lint
static const char sccs[] = "@(#)$Id: al.c,v 1.4 1996/08/13 11:14:15 johnl Exp $";
#endif

int main(int argc, char **argv)  
{ 
    while (*++argv) 
        puts(*argv);
    return(EXIT_SUCCESS); 
}

NB: When that is compiled, the version string is not included in the binary (or the object file). This does not currently give me any warning when compiled with GCC 4.6.1 compiled on MacOS X 10.7.2:

gcc -m64 -g -O -std=c99 -pedantic -Wall -Wshadow -Wpointer-arith -Wcast-qual \
    -Wstrict-prototypes -Wmissing-prototypes -o al al.c

When I run what al , I get no identification output.

Answer 3

看一下__attribute__((used)) 。

Answer 4

Will the variable ever be referenced by some external utility examining the executable, or is it just something you must have in the source?

If you just must have it in the source, and it doesn't have to be in the compiled executable, why not #if it out:

#if 0
static const char * one_time_param = "ABCDEFG";
#endif

The added benefit of this method is that you no longer have to worry about name clashes in the header files.

Answer 5

You could always hack around it. eg if (one_time_param[0] == one_time_param[0]); Minimal computational effort and it should remove the warning. It's possible that that line would be optimised to a no-op because it's inherently useless to the program.

It depends on how elegant you want the solution to be. Perhaps somebody can recommend a compiler flag that'll get rid of the warning.

Answer 6

在单个标头中定义它，在标头内定义一个内联函数以“获取”指针的值，然后在需要定义的任何位置包含此标头。

Answer 7

In a global header for your project declare a macro like:

#define DECLARE_ONETIME_CONST(name,value) \
    static const char* name = (value); \
    static const char nowarning_##name = name[0];

Then in your files say:

DECLARE_ONETIME_CONST(one_time_param, "ABCDEFG");

Seems to work. Whatever one might think about macros and token pasting, at least with a macro you'll be able to find these things and get rid of them when people realize they're silly. <shrugs>

Answer 8

one_time_param.h

#ifndef ONE_TIME_PARAM_H
#define ONE_TIME_PARAM_H

extern const char* one_time_param;

#endif

one_time_param.cpp

#include "one_time_param.h"

const char* one_time_param = "ABCDEFG";

Then include one_time_param.h in each & every header and source file.

< scrathes head > this of course won't work if you need it to be static .