C Language Macro Definition Tips

Network Source: http://blog.chinaunix.net/uid-14022540-id-2849095.html

• usage of "#" and "# # " in macros

I. GENERAL usage

We use # to turn the macro parameter into a string , and use # # to fit the two macro parameters together .

 #include <stdio.h> #define  CONS (A, b) ((int) (a# #e # #b))  #define  STR (s) #s int   main () {printf (   %s\n   "    ,str (ABC)); printf (  %d\n   ", CONS (2 , 3  ));  return  0   /*  [email protected]:/work/dcc# GCC *.c;./a.out ABC2000  */ 

Second, when the macro parameter is another macro: you need to pay attention to the macro definition of Useful ' # ' or ' # # ' where the macro parameter is no longer expanded .

1, non- ' # ' and ' # # ' situations, naturally unfold:

#define TOW (2)

#define MUL (A, B) (a*b)

printf ("%d*%d=%d\n", TOW, TOW, MUL (Tow,tow));

This line of macros will be expanded to :

printf ("%d*%d=%d\n", (2), (2), ((2) * (2)));

the parameter TOW in the MUL will be expanded to (2).

2, when there is ' # ' or ' # # ' , use ' # ' or ' # # ' The local macro parameters are no longer expanded :

#define A (2)

#define CONS (A, b) ((int) (a# #e # #b))

printf ("%s\n", CONS (A, a)); Compile error

This line is :

printf ("%s\n", int (AeA));

But the solution to this problem is simple . add an extra layer of intermediate conversion macros : the macro that involves #, # #, uses another macro to wrap it.

#include <stdio.h>#define_get_file_name (f) (#f)#defineGet_file_name (f) _get_file_name (f)intMain () {//according to the definition of Get_file_name (f), it does not use the # directly, so the internal macro is fully expanded to get the correct result .     Static CharFilename[] = Get_file_name (__file__);//"TEST.c"printf"%s\n", FILENAME); //_get_file_name (f) When expanded, refers to the macro __file__//according to the definition of _get_file_name, the inside of the macro does not expand, but only treats it as a string, without getting the correct result.     Static CharFilename2[] = _get_file_name (__file__);//__file__printf"%s\n", FILENAME2); return 0;} /*[email protected]:/work/dcc# gcc *.c;./a.out "test.c" __file__*/

• The following is a list of some of the macro definitions commonly used in software :

1, prevents a header file from being repeated included

#ifndef Comdef_h

#define Comdef_h

Header File Contents

#endif

2 . Redefine some types to prevent differences in the number of types of bytes produced due to different platforms and compilers, which facilitates portability .

typedef unsigned char Boolean; /* Boolean value type. */

typedef unsigned long int uint32; /* Unsigned-bit value */

typedef unsigned short uint16; /* Unsigned-bit value */

typedef unsigned char uint8; /* Unsigned 8 bit value */

typedef signed Long int int32; /* Signed-bit value */

typedef signed short Int16; /* Signed-bit value */

typedef signed CHAR int8; /* Signed 8 bit value */

typedef unsigned char byte; /* Unsigned 8 bit value type. */

typedef unsigned short word; /* unsinged-bit value type. */

typedef unsigned long DWORD; /* Unsigned-bit value type. */

3 to get a byte or a word on the specified address

#define MEM_B (x) (* (BYTE *) (x))

#define MEM_W (x) (* ((Word *) (x)))

4, maximum and minimum values are calculated

#define MAX (x, Y) (((x) > (y))? (x): (y))

#define MIN (x, Y) (((x) < (y))? (x): (y))

7, convert two bytes into a Word in LSB format

#define FLIPW (Ray) (((Word) (Ray) [0]) + (Ray) [1])

8, convert a Word to two bytes in LSB format

#define FLOPW (Ray, Val) \

(Ray) [0] = ((val)/256); \

(Ray) [1] = ((val) & 0xFF)

convert one letter to uppercase

#define UPCASE (c) (((c) >= ' A ' && (c) = ' 0 ' && (c) = ' 0 ' && (c) = ' A ' && (c) = ' a ' && (c) (Val))? (val) +1: (val))

use some macro to track debugging

When the _DEBUG is defined, the output data information and the row where the file resides

#include <stdio.h>#include<time.h>#define_debug#ifdef _DEBUG#defineDebugmsg (msg, date) Do{printf ("msg:%s; file:%s, line:%d, date:%s\n", MSG, __file__, __line__, date); } while(0)    #else    #defineDebugmsg (Msg,date)#endif#if1intMain () {time_t now; Time (&Now ); Debugmsg ("Hello", CTime (&Now )); return 0;}#endif

macro definition To prevent use is wrong

Enclosed in parentheses .

with do{}while (0) statement contains multiple statements to prevent errors

Eg: #define ADD (A, b) do{a+b;\

a++;} while (0)

C Language Macro Definition Tips