C語言巨集macro 介紹與幾個sample寫法

例: 常見的 ## (連接), #@ (字元化), # (字串化)

1. x ## y :表示x連接y, 注意x與y需同一型態
2. #@ x :據說可以字元化, 但這是不常用的語法, 而且我編譯會出現error: ‘#’ is not followed by a macro parameter
3. # x :表示將x字串化

#define beConn(x, y)	x ## y
//#define ToChar(x)       #@ x   
#define beStr(x)		# x

int main(void) {
	int num = beConn(12, 34);
	printf("beConn(12, 34) : %d \n", num);
 
//	char c = ToChar(1);  
//	printf("beChar(1) : %c \n", c);
	
	printf("beStr(I am string) : %s \n", beStr(I am string));
	return 0;
}

#define beConn(x, y) x ## y

//#define ToChar(x) #@ x

#define beStr(x) # x

int main(void) {

int num = beConn(12, 34);

printf("beConn(12, 34) : %d \n", num);

// char c = ToChar(1);

// printf("beChar(1) : %c \n", c);

printf("beStr(I am string) : %s \n", beStr(I am string));

return 0;

}

輸出:
beConn(12, 34) : 1234
beStr(I am string) : I am string

例: 透過 #define 可以變成 function

定義更美觀易讀的語法, ” \” 表示視為同一行

#define ATK_CHK(list, cb, skb, dir)        \
{        \
int atk;        \
\
for (atk = 0 ; list[dir][atk] != EMPTY_VALUE ; atk++) {        \
if(cb[list[dir][atk]] && cb[list[dir][atk]](skb, (struct iphdr *)skb->nh.iph) == IS_ATTACK)        \
return AEGIS_BLOCK;            \
}        \
}

#define ATK_CHK(list, cb, skb, dir) \

{ \

int atk; \

for (atk = 0 ; list[dir][atk] != EMPTY_VALUE ; atk++) { \

if(cb[list[dir][atk]] && cb[list[dir][atk]](skb, (struct iphdr *)skb->nh.iph) == IS_ATTACK) \

return AEGIS_BLOCK; \

} \

}

例: 透過 #define 與內嵌X()變化, 做出enum與之字串化

延伸閱讀:
C語言巨集macro: 將enum的integer變數印出變成string

1. 假設這段程式碼可用於pwr_state.h檔案當作宣告 stringify_PWR_state_t()

#pragma once
#define PWR_STATES \
X(PWR_STATE_ENABLED, 0) \
X(PWR_STATE_DISABLED_REBOOT_REQUIRED, 1) \
X(PWR_STATE_DISABLED_MANUAL_ENABLE, 2) \
X(PWR_STATE_DISABLED, 3) \
X(PWR_STATE_PSW_NOT_AVAILABLE, 4) \
X(PWR_STATE_UNKNOWN, -1)

#define X(x_state, x_value) x_state = x_value,
typedef enum { PWR_STATES } pwr_state_t;
#undef X

const char* stringify_pwr_state_t(pwr_state_t state);

#pragma once

#define PWR_STATES \

X(PWR_STATE_ENABLED, 0) \

X(PWR_STATE_DISABLED_REBOOT_REQUIRED, 1) \

X(PWR_STATE_DISABLED_MANUAL_ENABLE, 2) \

X(PWR_STATE_DISABLED, 3) \

X(PWR_STATE_PSW_NOT_AVAILABLE, 4) \

X(PWR_STATE_UNKNOWN, -1)

#define X(x_state, x_value) x_state = x_value,

typedef enum { PWR_STATES } pwr_state_t;

#undef X

const char* stringify_pwr_state_t(pwr_state_t state);

2. 然後更改一下x()的定義, 假設在pwr_state.c檔案

#include <pwr_state.h>

#define X(x_state, x_value) case x_state: return # x_state;
const char* stringify_pwr_state_t(pwr_state_t state)
{
    switch (state) {
        PWR_STATES
    default:
        return "Unknown state.";
    };
}
#undef X

#include <pwr_state.h>

#define X(x_state, x_value) case x_state: return # x_state;

const char* stringify_pwr_state_t(pwr_state_t state)

{

switch (state) {

PWR_STATES

default:

return "Unknown state.";

};

}

#undef X

例: 常見的 ## (連接), #@ (字元化), # (字串化)

例: 透過 #define 可以變成 function

例: 透過 #define 與內嵌X()變化, 做出enum與之字串化

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