preprocessing command
1. Macro definition
The C language standard allows the use of an identifier to represent a string in a program, called a macro. The identifier is the macro name. When compiling and preprocessing, all macro names in the program are replaced with corresponding strings. This process is called macro substitution. There are two types of macros: macros without parameters and macros with parameters.
1. Macro without parameters
The general form of a parameterless macro definition is: #define identifier string
"#" means that this line is a compilation preprocessing command. define is the keyword of the macro definition, and the identifier is the macro name. The string is the content replaced by the macro name, which can be a constant, an expression, etc.
E.g:
2. Macro definition with parameters
A macro definition with parameters is similar to a function with parameters, and the general form of its definition is:
#define Identifier (parameter list) string.
E.g:
2. The file contains
File inclusion refers to including the entire contents of the specified source file into the current source program file.
The general form of a file containment command is:
#include "filename" or #include <filename>
3. Conditional compilation
Conditional compilation means that under certain conditions, the conditions are met and those that are not met are processed separately - some statements are compiled when the conditions are met, and other statements are compiled when the conditions are not met.
Conditional compilation commands have the following modes.
Mode one:
#ifndef Identifier
Block 1:
#endif
The meaning is: if no identifier is defined, then compile segment 1.
The program segment 1 here can be either a statement group or an execution command.
E.g:
#ifndef getkey; #define getkey; #include<sys\types.h> #endif;
The meaning of the code is: if the symbolic constant getkey is not defined, the output is defined and the header file sys/types.h is included.
Mode two:
#ifndef Identifier
block 1
#else
block 2
#endif
Its meaning is: if no identifier is defined, compile program segment 1, otherwise, compile program segment 2.
Mode three:
#ifdef identifier
block 1
#endif
The meaning is: if the identifier is defined, program segment 1 will be compiled, otherwise, the program segment will not be compiled.
E.g:
#ifdef DEBUG
printf("a=%d,b=%d",a,b);
#healthiness
When calling a program, you can add the following statement at the beginning of the program:
#define DEBUG
Mode four:
#ifdef identifier
block
#else
block 2
#endif
Its meaning is: if the identifier is defined, compile program segment 1, otherwise compile program segment 2.
Mode five:
#if expressions
block 1
#endif
Its meaning is: if the expression is established, then compile the program segment 1, otherwise, do not compile the program segment.
Mode six:
#if expressions
block 1
#else
block 2
#endif
Its meaning is: if the expression is true, compile the program segment 1, otherwise, compile the program segment 2.