# Define usage

1.SimpleDefineDefinition

# Define maxtime1000

2. DefineOf"Function Definition"

Define can accept some parameters as the function does, as follows:

# Define max (x, y) (x)> (y )? (X) :( y );

Because this "function" does not have a type check, it is like a function template. It is safe without a template.

However, there are hidden risks. The example is as follows:
# Define add (A, B) A + B; if it encounters C * Add (a, B) * D, the problem may occur.

Another example is as follows:
# Define pin (int *);
Pin A, B;
The intention is that A and B are both int-type pointers, but they actually become int * a, B;
A is an int pointer while B is an int variable.
In this case, typedef should be used instead of define, so that both A and B are int pointers.

When defining it, we develop a good habit. We recommend that you add brackets to all layers.

3.Single Row definition of macros (rarely seen)

# Define a (x) T _ # x
# Define B (X) # @ x
# Define C (x) # x

Suppose: x = 1, there are:

A (1) ------> T_1
B (1) ------> '1'
C (1) ------> "1"

It can be seen that # Is a simple connector,

# @ Is used to add single quotes to parameters and convert them into single characters (the last character)

# It is used to convert parameters into strings by adding double quotation marks

4. DefineMulti-line definition

Define can replace multiple lines of code, such as the macro definition in MFC (very classic, although disgusting)

# Define macro (arg1, arg2) do {\
/* Declarations */\
Stmt1 ;\
Stmt2 ;\
/*...*/\
} While (0)/* (no trailing ;)*/
The key is to add "\" to each line feed "\"

5.In large-scale development, especially cross-platform and system software,DefineThe most important feature is Conditional compilation.

That is:
# Ifdef windows
......
......
# Endif
# Ifdef Linux
......
......
# Endif

You can use # define to set the compiling environment during compilation.

6.How to define and cancel macros

# Define [macroname] [macrovalue] // fixed macro
# UNDEF [macroname] // cancel the macro
# Define Pi (3.1415926) // normal macro
# Define max (A, B) (a)> (B )? (A), (B) // macro with Parameters
7.Conditional compilation
# Ifdef XXX... (# Else )... # Endif
For example
# Ifdef dv22_aux_input
# Define aux_mode 3
# Else
# Define auy_mode 3
# Endif
# Ifndef XXX... (# Else )... # Endif

8. Prevents Repeated inclusion of a header file
Because header files can be nested, the C file may contain the same header file multiple times, and the definition may be repeated.
Use the Conditional compilation switch to avoid repeated inclusion (repeated definition)

# Ifndef comdef_h

# Define comdef_h

// Header file content

# Endif

Because header files can be nested, the C file may contain the same header file multiple times, and the definition may be repeated. Use the Conditional compilation switch to avoid repeated inclusion (repeated definition ). Because # define comdef_h is not always defined when it compiles the first header file, it will compile all the content in the header file, including the definition # define comdef_h. In this way, if the header file to be compiled is encountered during the next compilation, the statement # ifndef comdef_h will not repeatedly compile this header file because of the existence of the statement # ifndef comdef_h.

1.Obtains a byte or word on the specified address.

# Define mem_ B (x) (* (byte *) (x )))

# Define mem_w (x) (* (word *) (X )))

2.Calculate the maximum and minimum values

# Define max (x, y) (x)> (y ))? (X): (y ))

# Define min (x, y) (x) <(y ))? (X): (y ))

3.GetFieldIn struct(Struct)Offset in

# Define FPOs (type, field) & (type *) 0)-> field)

4.Get a structFieldNumber of bytes occupied

# Define fsiz (type, field) sizeof (type *) 0)-> field)

5.FollowLSBFormat to convert two bytes into oneWord

# Define flipw (Ray) (Word) (Ray) [0]) * 256) + (Ray) [1])

6.FollowLSBFormatWordConvert to two bytes

# Define flopw (Ray, Val )\

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

(Ray) [1] = (VAL) & 0xff)

7.Get the address of a variable (WordWidth)

# Define B _ptr (VAR) (byte *) (void *) & (VAR ))

# Define w_ptr (VAR) (word *) (void *) & (VAR ))

8.Obtain the high and low bytes of a word.

# Define word_lo (XXX) (byte) (Word) (XXX) & 255 ))

# Define word_hi (XXX) (byte) (Word) (XXX)> 8 ))

9.Returns a ratio.XThe largest closest8Multiples

# Define rnd8 (x) + 7)/8) * 8)

10.Converts a letter to uppercase.

# Define upcase (C) (c)> = 'A' & (c) <= 'Z ')? (C)-0x20): (c ))

11.Judge whether the character is10Number of the input value

# Define decchk (C) (c)> = '0' & (c) <= '9 ')

12.Judge whether the character is16Number of the input value

# Define hexchk (C) (c)> = '0' & (c) <= '9') | \

(C)> = 'A' & (c) <= 'F') | \

(C)> = 'A' & (c) <= 'F '))

13.A method to prevent overflow

# Define inc_sat (VAL) (val = (VAL) + 1> (VAL ))? (VAL) + 1: (VAL ))

14.Returns the number of array elements.

# Define arr_size (A) (sizeof (a)/sizeof (A [0]) <! -- [If! Supportannotations] --> [lst1] <! -- [Endif] -->

15.Returns an unsigned number.NEnd valueMod_by_power_of_two (x, n) = x % (2 ^ N)

# Define mod_by_power_of_two (Val, mod_by )\

(DWORD) (VAL) & (DWORD) (mod_by)-1 ))

16.ForIoSpace ing in the bucket structure, Input/Output Processing

# Define Indium (port) (* (volatile byte *) (port )))

# Define inpw (port) (* (volatile word *) (port )))

# Define outp (port, Val) (* (volatile byte *) (port) = (byte) (VAL )))

# Define outpw (port, Val) (* (volatile word *) (port) = (Word) (VAL )))

17.Debug with some macro tracking

The ANSI standard specifies five predefined macro names. They are:

_ Line __

_ File __

_ Date __

_ Time __

_ Stdc __

C ++ also defines _ cplusplus

If the compiler is not standard, only a few of the above macro names are supported or not supported at all. Remember that the Compilation Program may also provide other predefined macro names.

_ Line _ and _ file _ macro instructions. # The line command can change its value. In short, during compilation, it contains the current number of lines and file names of the program.

The _ date _ macro command contains a string in the form of month, day, or year, which indicates the date when the source file was translated to the code.

The _ time _ macro command contains the program Compilation Time. Time is represented by a string in the form of minute: Second

The significance of the _ stdc _ macro command is defined during compilation. Generally, if _ stdc _ has been defined, the compiler will only accept standard C/C ++ code that does not contain any non-standard extensions. If the implementation is standard, macro _ stdc _ contains the decimal constant 1. If it contains any other number, the implementation is non-standard.

The consistent compiler of _ cplusplus and Standard C ++ defines it as a value containing at least six characters. A compiler that is inconsistent with the standard C ++ will use a value of 5 or less.

You can define macros, for example:

When _ debug is defined, the output data and the row of the file

# Ifdef _ debug

# Define debugmsg (MSG, date) printf (MSG); printf ("% d", date, _ line _, _ file _)

# Else

# Define debugmsg (MSG, date)

# Endif

 18.Macro definition to prevent the inclusion of braces by mistake.

For example:

Definition in question: # define dump_write (ADDR, NR) {memcpy (bufp, ADDR, NR); bufp + = nR ;}

Definition: # defne do (a, B) do {A + B; A ++;} while (0)

For example:

If (ADDR)

Dump_write (ADDR, NR );

Else

Do_somethong_else ();

This will happen after the macro scale:

If (ADDR)

{Memcpy (bufp, ADDR, NR); bufp + = nR ;};

Else

Do_something_else ();

When GCC encounters the ";" in front of Else, it considers that the if statement has ended, so the subsequent else is not in the IF statement. With the do {} while (0) definition, there is no problem under any circumstances. The definition of # define do (a, B) do {A + B; A ++;} while (0) will not go wrong under any circumstances.

19. DefineSpecial identifier in

# Define conn (x, y) x # Y
# Define tochar (x) # @ x
# Define tostring (x) # x

Int A = conn (12, 34 );
Char B = tochar ();
Char C [] = tostring ();
The result is a = 1234, B = 'A', c = "";

It can be seen that # Is a simple connector,

# @ Is used to add single quotes to parameters and convert them into single characters (the last character)

# It is used to convert parameters into strings by adding double quotation marks