매크로가 아직 정의되지 않은 경우에만 정의하는 이유는 무엇입니까?

매크로가 아직 정의되지 않은 경우에만 정의하는 이유는 무엇입니까?

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C 코드베이스 전체에서 모든 매크로가 다음과 같이 정의 된 것을 볼 수 있습니다.

#ifndef BEEPTRIM_PITCH_RATE_DEGPS
#define BEEPTRIM_PITCH_RATE_DEGPS                   0.2f
#endif

#ifndef BEEPTRIM_ROLL_RATE_DEGPS
#define BEEPTRIM_ROLL_RATE_DEGPS                    0.2f
#endif

#ifndef FORCETRIMRELEASE_HOLD_TIME_MS
#define FORCETRIMRELEASE_HOLD_TIME_MS               1000.0f
#endif

#ifndef TRIMSYSTEM_SHEARPIN_BREAKINGFORCE_LBS
#define TRIMSYSTEM_SHEARPIN_BREAKINGFORCE_LBS       50.0f
#endif

매크로를 정의하는 대신 이러한 정의 검사를 수행하는 이유는 무엇입니까?

#define BEEPTRIM_PITCH_RATE_DEGPS                   0.2f
#define BEEPTRIM_ROLL_RATE_DEGPS                    0.2f
#define FORCETRIMRELEASE_HOLD_TIME_MS               1000.0f
#define TRIMSYSTEM_SHEARPIN_BREAKINGFORCE_LBS       50.0f

웹에서이 관행이 설명 된 것을 찾을 수 없습니다.

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이렇게하면 컴파일 할 때 매크로를 재정의 할 수 있습니다.

gcc -DMACRONAME=value

헤더 파일의 정의가 기본값으로 사용됩니다.

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댓글에서 말했듯이 다음 상황을 상상해보십시오.

foo.h

#define FOO  4

defs.h

#ifndef FOO
#define FOO 6
#endif

#ifndef BAR
#define BAR 4
#endif

bar.c

#include "foo.h"
#include "defs.h"

#include <stdio.h>

int main(void)
{
    printf("%d%d", FOO, BAR);
    return 0;
}

인쇄 44됩니다.

그러나 조건부 ifndef가없는 경우 결과는 MACRO 재정의에 대한 컴파일 경고가되고 64.

$ gcc -o bar bar.c
In file included from bar.c:2:0:
defs.h:1:0: warning: "FOO" redefined [enabled by default]
 #define FOO 6
 ^
In file included from bar.c:1:0:
foo.h:1:0: note: this is the location of the previous definition
 #define FOO 4
 ^

---

I do not know the context but this can be used to give the user the availability to override the values set by those macro definitions. If the user explicitly defines a different value for any of those macros it will be used instead of the values used here.

For instance in g++ you can use the -D flag during compilation to pass a value to a macro.

---

This is done so that the user of the header file can override the definitions from his/her code or from compiler's -D flag.

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Any C project resides on multiple source files. When working on a single source file the checks seem to (and actually) have no point, but when working on a large C project, it's a good practice to check for existing defines before defining a constant. The idea is simple: you need the constant in that specific source file, but it may have been already defined in another.

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You could think about a framework/library that gives to the user a default preset that allow the user to compile and work on it. Those defines are spreaded in different files and the final user is advised to include it's config.h file where he can config its values. If the user forgot some define the system can continue to work because of the preset.

---

Using

#ifndef BEEPTRIM_PITCH_RATE_DEGPS
#define BEEPTRIM_PITCH_RATE_DEGPS                   0.2f
#endif

allows the user to define the value of the macro using the command line argument (in gcc/clang/VS) -DBEEPTRIM_PITCH_RATE_DEGPS=0.3f.

There is another important reason. It is an error to re-define a preprocessor macro differently. See this answer to another SO question. Without the #ifndef check, the compiler should produce an error if -DBEEPTRIM_PITCH_RATE_DEGPS=0.3f is used as a command line argument in the compiler invocation.

참고URL : https://stackoverflow.com/questions/32398612/why-only-define-a-macro-if-its-not-already-defined