[英]C Macros - avoiding macro expansion
I have the following macros 我有以下宏
#define REG_PWR_CTRL 0x2D
#define REG_FIFO_CTL 0x38
#define VERBOSE(...) \
if(verbose) \
printf(__VA_ARGS__);
#define READ_REGISTER(i2c_dev_file, REGISTER, variable) \
{ \
variable = i2c_smbus_read_byte_data(i2c_dev_file, REGISTER); \
}
#define WRITE_REGISTER_VERBOSE(i2c_dev_file, REGISTER, value) \
{ \
short int var = 0; \
i2c_smbus_write_byte_data(i2c_dev_file, REGISTER, value); \
usleep(100); \
READ_REGISTER(i2c_dev_file, REGISTER, var); \
VERBOSE(#REGISTER " :0x%02X\n", var); \
}
I would like the REGISTER
field to not be expanded in the following line 我希望
REGISTER
字段不会在以下行中展开
VERBOSE(#REGISTER " :0x%02X\\n", var); \\
For example, When I write 例如,我写的时候
WRITE_REGISTER_VERBOSE(i2c_dev_fd, REG_PWR_CTRL, 0x1A);
WRITE_REGISTER_VERBOSE(i2c_dev_fd, REG_FIFO_CTL, 0xC6);
I get the output 我得到了输出
0x2D :0x1A
0x38 :0xC6
I would like to obtain 我想获得
REG_PWR_CTRL :0x1A
REG_FIFO_CTL :0xC6
I came across a lot of posts that spoke about adding an extra level of indirection. 我遇到了许多关于添加额外级别间接的帖子。
I tried the answer described here https://stackoverflow.com/a/2653351/1761555 ..although I believe that that answer is for a different problem altogether.. 我尝试了这里描述的答案https://stackoverflow.com/a/2653351/1761555 ..尽管我相信这个答案完全是针对不同的问题..
What I did was 我做的是
#define STRINGIFY(label) (#label)
#define WRITE_REGISTER_VERBOSE(i2c_dev_file, REGISTER, value) \
{ \
short int var = 0; \
i2c_smbus_write_byte_data(i2c_dev_file, REGISTER, value); \
usleep(100); \
READ_REGISTER(i2c_dev_file, REGISTER, var); \
VERBOSE("%s :0x%02X\n", STRINGIFY(REGISTER), var); \
}
But this still gives me the same output as before 但这仍然给我与以前相同的输出
Is there any way to achieve this? 有没有办法实现这个目标?
You could make REG_PWR_CTRL
and REG_FIFO_CTL
a value of some enum, like 您可以使
REG_PWR_CTRL
和REG_FIFO_CTL
成为某些枚举的值,例如
enum registers_en {
REG__NONE,
REG_PWR_CTRL = 0x2d,
REG_FIFO_CTL = 0x38,
};
Then REG_PWR_CTRL
becomes a real identifier of some enum value, and is not macro-expanded in something else (because an enum
definition is not a macro definition, and is not handled by the cpp
preprocessor ). 然后
REG_PWR_CTRL
成为某个枚举值的实际标识符,并且不会在其他内容中进行宏扩展(因为enum
定义不是宏定义,并且不由cpp
预处理器处理 )。
So define such an enum, and preprocess your source code (eg with gcc -C -E yoursource.c > yoursource.i
) then look (eg with less yoursource.i
) inside the preprocessed file. 因此,定义这样的枚举,并预处理您的源代码(例如,使用
gcc -C -E yoursource.c > yoursource.i
)然后在预处理文件中查看(例如,使用less yoursource.i
)。 All occurrences of REG_PWR_CTRL
will still be there. 所有出现的
REG_PWR_CTRL
仍然存在。
Be aware the the preprocessor is conceptually the first phase of a compiler: even in compilers like current GCC 4.8 where the preprocessor is not an external program but implemented thru a libcpp internal library, the compiler works by first preprocessing the source code and obtaining a stream of lexemes , and then occurrences of REG_PWR_CTRL
stay as lexemes (not as literal constants 0x2d
as when you #define REG_PWR_CTRL 0x2d
...). 请注意,预处理器在概念上是编译器的第一阶段:即使在当前GCC 4.8等编译器中,预处理器不是外部程序,而是通过libcpp内部库实现,编译器首先预处理源代码并获取流的词位 ,然后的出现
REG_PWR_CTRL
留为词位(而不是字面常量0x2d
当你为#define REG_PWR_CTRL 0x2d
...)。
You need to read more about the preprocessor cpp , and take the habit to look into the preprocessed form. 您需要阅读有关预处理器cpp的更多信息,并养成查看预处理表单的习惯。
Another advantage of enum
-s is that if you compile with debugging info (eg gcc -g
) the debugging info hence the debugger gdb
knows about enum
. enum
-s的另一个优点是,如果使用调试信息(例如gcc -g
)编译调试信息,则调试器gdb
知道enum
。
I modified your code for simplicity: 为简单起见,我修改了代码:
#include <stdio.h>
#define REG_PWR_CTRL (0x2D)
#define GET_VAR_NAME(var) (#var)
#define VERBOSE(...) (printf(__VA_ARGS__))
#define ANOTHER_LAYER(arg) ( \
VERBOSE("%s = %#X; %s = %#X\n", \
GET_VAR_NAME(REG_PWR_CTRL), REG_PWR_CTRL, \
GET_VAR_NAME(arg), arg) \
) \
int main(void)
{
int num = 5;
VERBOSE("%s = %#X\n", GET_VAR_NAME(REG_PWR_CTRL), REG_PWR_CTRL);
ANOTHER_LAYER(num);
return 0;
}
Output: 输出:
REG_PWR_CTRL = 0X2D
REG_PWR_CTRL = 0X2D; num = 0X5
Use macros for simple things. 使用宏来处理简单的事情。
This is because: 这是因为:
So just use functions for complex stuff. 所以只需将函数用于复杂的东西。 Use macros for simple stuff
使用宏来表示简单的东西
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