C Language Basics Tutorial (iv) Pointers, structures, unions, and enumerations (19)

preprocessing directives
As stipulated by ANSI standards, preprocessing directives mainly include:
#define
#error
#if
#else
#elif
#endif
#ifdef
#ifndef
#undef
#line
#pragma
As can be seen from the above instructions, each preprocessing instruction has a symbol "#". Here are just a few common instructions.
1. #define指令
#define指令是一个宏定义指令, the general form of the definition is:
#define A macro substitution name string (or numeric value)
Once defined by the #define directive, it is replaced with a defined string (or numeric value) each time the macro substitution name is encountered in the program.
For example, use the following statement to define true to indicate a value of 1, false to 0.
#define TRUE 1
#define FALSE 0
Once you have used true and false in the source program, the compilation is automatically replaced with 1 and 0.
Attention:
1. There is no ";" After the name of the macro definition.
2. In the Turbo C program, it is customary to replace names with uppercase characters as macros and often at the beginning of a program.
3. A macro definition also has the feature that a macro substitution name can have formal parameters that, when used in a program, replace these formal parameters.
For example:
#define MAX (x, y) (x>y)? x:y
Main ()
{
int i=10, j=15;
printf ("The Maxmum is%d", MAX (i, j);
}
The last example of a macro definition statement is to replace the name Max (x,y) with a macro instead of the larger of the x,y, and also to define:
#define MIN (X,y) (x<y) x:y
Represents the substitution of a macro for min (x,y) to replace the lesser of the x,y.
2. #error指令
This instruction is used for program debugging, and it stops compiling when the #error instruction is encountered in the compilation. Its general form is:
#error error message
The error message is not quoted, and when the compiler encounters this instruction, it displays the following information and stops compiling.
Fatal:filename linename error directive
3. #include directives
The purpose of the #include directive is to instruct the compiler to embed another source file that is indicated by the directive in the program in which the #include directive resides, and the file should be enclosed in double quotes or angle brackets. The header file of the Turbo C library function is generally indicated by the #include instruction in the program switch.
For example:
#include <stdio.h>
Programs also allow embedding of other files, such as:
Main ()
{
#include }
Where Help.c is another file, the content can be
printf ("Glad to meet for you here!");
The example above compiles to find the embedded file by the included file path specified in the Options/directories/include directories of the integrated development environment.

4. #if, #else, #endif指令
#if, #els和 #endif instruction is conditional optional instruction, its general form is:
#if constant expression
Statement section;
#else
Statement section;
#endif
The meaning of the above structure is: if the constant expression after the #if instruction is true, compile the program segment between #if and #else, or compile the program segment between #else and #endif.
For example:
#define MAX 200
Main ()
{
#if max>999
printf ("Compiled for bigger\n");
#else
printf ("Compiled for Small\n");
#endif
}

5. #undef指令
#undef指令用来删除事先定义的宏定义, its general form is:
#undef a macro substitution name
For example:
#define TRUE 1
...
#undef ture
#undef主要用来使宏替换名只限定在需要使用它们的程序段中.