c文件中的#ifndef？ [英] #ifndef in c file?

问题描述

是否可以将 #ifndef 放在c文件的顶部？基本上我需要检查某个预处理器常量是否在运行程序时声明，并且我的程序会相应地改变。

我需要检查 - D DESCENDING_ORDER = 1 被添加为参数（无关紧要）。

我的代码位于我的顶部主要的c文件：

  #ifndef DESCENDING_ORDER 
 int ascending = 1; 
 #else 
 int ascending = 0; 
 #endif 
  

编译时自动运行，但在尝试编译时遇到错误一个Makefile，一些符合预期标识符之前的'int'for int ascending = 1 。

谢谢
$ b 编辑 - 添加了Makefile代码

  CC = gcc 
 CFLAGS = -g -Wall 
 INC = -include 
 RES_OBS = res.o 
 LIBS = 
 all：res 
 
 res：$（ RES_OBS）
 
 $（CC）$（CFLAGS）-o res $（RES_OBS）$（LIBS）$（INC）res.h -D DESCENDING_ORDER = 1 
 
 clean ：
 rm -f * .o 
 
 clobber：
 make clean 
 rm -f res 
  

在命令结尾处猜测并添加 $（INC）.... DESCENDING_ORDER = 1 所以这可能是为什么它不起作用。
命令我没有使用makefile：

  gcc res -include res。 h -D DESCENDING_ORDER = 1 
  

编辑2 - 有一点玩不同的参数，发现如果我在命令中删除 -include res.h ，我会得到同样的错误。仍然不确定如何正确引用makefile中的头文件？我在我的res.c文件中添加了 #includeres.h，但仍然出现错误。

$（CLAGS）之后， Makefile 中有一个错字。应该是 $（CFLAGS）。
通过运行 make -p ，了解更多关于 make 的知识，在 make 中使用它们（例如考虑使用 $（COMPILE.c）和 $（$ LINK.c） etc。）

不要忘记添加 -Wall 添加到您的 CFLAGS 中，因为您需要编译器的所有警告。您可能也需要调试信息，所以也要添加 g 。

在Linux上，我建议使用重拍用于调试 Makefile -s通过运行<$ c $标准做法是：

>


• 避免将 -include 传递给 gcc ，而是添加一个<$在相关的 *。c 源文件

的开始处附近的c＃c> #includeres.h


• 将 -D 粘贴到已定义的符号上，例如 -DDESCENDING_ORDER = 1




• 添加到 Makefile 将相关目标文件依赖于新的 #include -d文件 res.h ;注意这些依赖可以自动生成（通过将 -MD 传递给 gcc 等等） / p>




• 传递 -DDESCENDING_ORDER = 1 通过 CFLAGS 或更好 CPPFLAGS

不要忘记 $ h $>
$ b

您可能需要使用 code> gcc -C -E ，你可以有一个像



  res的规则。 i：res.c res.h 
 $（CC）-C -E $（CFLAGS）$（CPPFLAGS）$ ^ -o $ @ 
   make res.i 并用某个编辑器或寻呼机（可能> less  code>）预处理器输出 res.i ;或者，在命令行上执行此操作
  gcc -C -E -I。 -DDESCENDING_ORDER = 1 res.c |减去
  
您可以移除生成的行信息并执行操作 
 
 
  gcc -C -E -I。 -DDESCENDING_ORDER = 1 res.c | grep -v'^＃'> res_.i 
 gcc -Wall -c res_.i 
  
关键是预处理在 C 中是一个文本操作，并且您的预处理过的表单是错误的。
 
 
 
最近的Clang / LLVM（版本3.2）或GCC发布版本4.8）编译器为您提供更好的预处理消息。
 
Is it possible to put #ifndef at the top of a c file? Basically I need to check whether a certain preprocessor constant was declared when running the program and my program will change accordingly.

I need to check if -D DESCENDING_ORDER=1 is added as an argument (doesn't matter what value given).

I have this code at the top of my main c file:
#ifndef DESCENDING_ORDER
int ascending = 1;
#else
int ascending = 0;
#endif
Works when compiling by itself, but I get errors when I try compiling with a Makefile, something along the lines of "expected identifier before 'int' for int ascending = 1.

Thanks.

EDIT - Added Makefile code
CC=gcc
CFLAGS=-g -Wall
INC=-include
RES_OBS=res.o
LIBS=
all: res

res:    $(RES_OBS)

    $(CC) $(CFLAGS) -o res $(RES_OBS) $(LIBS) $(INC) res.h -D DESCENDING_ORDER=1

clean:
        rm -f *.o

clobber:
        make clean
        rm -f res
Kind of guessed and added $(INC)....DESCENDING_ORDER=1 to the end of the command, so that's probably why it's not working. 
Command I'm using without makefile:
gcc res -include res.h -D DESCENDING_ORDER=1
EDIT 2 - Had a little play with different arguments and found that I get the same error if I remove -include res.h in the command. Still not sure how to correctly reference the header file in the makefile? I've added the #include "res.h" in my res.c file but still get the error.
解决方案
There is a typo in your Makefile since $(CLAGS) should be $(CFLAGS).
 Learn a lot more about make, notably by running make -p which shows you the many built-in rules to make and use them (e.g. consider using $(COMPILE.c) and $(LINK.c) etc..)

Don't forget to add -Wall to your CFLAGS, because you want all the warnings from the compiler. You probably want debugging information too, so add g also.

On Linux, I do recommend using remake for debugging Makefile-s by running remake -x which helps a lot.

Standard practices are:


avoid passing -include to gcc, instead, add a#include "res.h" near the beginning of relevant *.c source files
glue the -D to the defined symbol, e.g. -DDESCENDING_ORDER=1
add in your Makefile the dependencies on relevant object files to the newly #include-d file res.h; notice that these dependencies could be automatically generated (by passing e.g. -MD to gcc, etc etc...)
pass the -DDESCENDING_ORDER=1  thru CFLAGS or better CPPFLAGS


Don't forget that the order of program arguments to gcc matters a lot.

addenda

You may want to generate the preprocessed form res.i of your source code res.c using gcc -C -E and you could have a rule like
  res.i: res.c res.h
           $(CC) -C -E $(CFLAGS) $(CPPFLAGS) $^ -o $@
then do make res.i and examine with some editor or pager (perhaps less) the preprocessor output res.i ; alternatively, do that on the command line
  gcc -C -E -I. -DDESCENDING_ORDER=1  res.c | less
you could remove the generated line information and do 
  gcc -C -E -I. -DDESCENDING_ORDER=1  res.c | grep -v '^#' > res_.i
  gcc -Wall -c res_.i
The point is that the preprocessing in C is a textual operation, and your preprocessed form is wrong.

BTW very recent Clang/LLVM (version 3.2) or GCC (just released version 4.8) compilers give you much better messages regarding preprocessing.

                        
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