[英]A macros in the C language (#define)
I am reading source code of hoard memory allocator, and in the file of gnuwrapper.cpp, there is the following code 我正在阅读囤积内存分配器的源代码,并在gnuwrapper.cpp的文件中,有以下代码
#define CUSTOM_MALLOC(x) CUSTOM_PREFIX(malloc)(x)
What's the meaning of CUSTOM_PREFIX(malloc)(x)
? CUSTOM_PREFIX(malloc)(x)
的含义是什么? is CUSTOM_PREFIX
a function? CUSTOM_PREFIX
是一个函数吗? But as a function it didn't defined anywhere. 但作为一个功能,它没有在任何地方定义。 If it's variable, then how can we use variable like var(malloc)(x)
? 如果它是变量,那么我们如何使用变量如var(malloc)(x)
?
More code: 更多代码:
#ifndef __GNUC__
#error "This file requires the GNU compiler."
#endif
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <malloc.h>
#ifndef CUSTOM_PREFIX ==> here looks like it's a variable, so if it doesn't define, then define here.
#define CUSTOM_PREFIX
#endif
#define CUSTOM_MALLOC(x) CUSTOM_PREFIX(malloc)(x) ===> what's the meaning of this?
#define CUSTOM_FREE(x) CUSTOM_PREFIX(free)(x)
#define CUSTOM_REALLOC(x,y) CUSTOM_PREFIX(realloc)(x,y)
#define CUSTOM_MEMALIGN(x,y) CUSTOM_PREFIX(memalign)(x,y)
In your code, since CUSTOM_PREFIX is defined to be nothing, the string CUSTOM_PREFIX(malloc)(x)
will expand to 在您的代码中,由于CUSTOM_PREFIX被定义为空,字符串CUSTOM_PREFIX(malloc)(x)
将扩展为
(malloc)(x)
which is equivalent to the usual 这相当于平常
malloc(x)
However, the CUSTOM_PREFIX allows the developer to choose a different memory management function. 但是,CUSTOM_PREFIX允许开发人员选择不同的内存管理功能。 For example, if we define 例如,如果我们定义
#define CUSTOM_PREFIX(f) my_##f
then CUSTOM_PREFIX(malloc)(x)
will be expanded to 那么CUSTOM_PREFIX(malloc)(x)
将扩展为
my_malloc(x)
CUSTOM_PREFIX
is defined as nothing, so it will just disappear, leaving behind (malloc)(x)
, which is the same as malloc(x)
. CUSTOM_PREFIX
被定义为无,因此它将消失,留下(malloc)(x)
,这与malloc(x)
相同。 Why? 为什么? I don't know. 我不知道。 Perhaps other places in the code set CUSTOM_PREFIX
to something else. 也许代码中的其他地方将CUSTOM_PREFIX
设置为其他内容。
At a guess, its a macro which changes calls to malloc(x) etc. into something like: 猜测,它是一个宏,它将对malloc(x)等的调用更改为:
DEBUG_malloc( x );
You can choose to supply the macro yourself, to provide a customised prefix for the functions, or not in which case the names won't be changed. 您可以选择自己提供宏,为函数提供自定义前缀,或者不在何种情况下不更改名称。
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