Deep understanding of the C preprocessor _c language

The C preprocessor is not part of the compiler and is a separate step in the compilation process. The C preprocessor is just a text replacement tool that instructs the compiler to complete the required preprocessing before actual compilation.

All preprocessor commands begin with a pound sign (#). It must be the first non-null character, and in order to enhance readability, preprocessor directives should start with the first column.

The following table contains all the important preprocessor directives:

Instructions	Describe
#define	Defining macros
#include	Contains a source code file
#undef	Cancel a defined macro
#ifdef	If the macro is already defined, return the true
#ifndef	If the macro is not defined, return the true
#if	If the given condition is true, compile the following code
#else	Alternatives to #if
#elif	If the previous #if the given condition is not true and the current condition is true, compile the following code
#endif	End a #if ... #else conditional compilation block
#error	Output error message when standard error is encountered
#pragma	Use the standardized method to publish special commands to the compiler
#line	Provides the line number for compiler information

Predefined macros

ANSI c defines a number of macros. You can use these macros in programming, but you cannot directly modify these predefined macros.

Macro	Describe
__date__	The current date, the amount of the word constants in the "MMM DD YYYY" format.
__time__	The amount of Word constants in the current time, in "HH:MM:SS" format.
__file__	Contains the current file name, string constant.
__line__	Contains the current line number, the decimal constant.
__stdc__	When the compiler compiles in ANSI standard, it is defined as 1.
__stdc__version__	If the compiler follows C99, the value of the macro is 199901L, and in other cases, the macro is undefined.
__stdc__hosted__	The current host system, which has a value of 1, is currently a stand-alone system with a macro value of 0.
__stdc__iec__559__	If the floating-point implementation follows the IEC 60599 standard, the macro value is 1, otherwise there is no definition.
__stdc__iec__559__complex__	If the complex operation is implemented in accordance with the IEC 60559 standard, the macro value is 1, otherwise undefined.
__stdc__iso10646__	is defined as a long integer constant.

Preprocessor operators

The C preprocessor provides the following operators to help create macros:

1. Macro continuation operator (\)

A macro is usually written on a single line. However, if the macro is too long for a single row to fit, the macro continuation operator (\) is used.

2, string constant quantization operator (#)

In a macro definition, when you need to convert a macro parameter to a string constant, the string literal operator (#) is used. The operator used in a macro has a specific parameter or parameter list.

3, Tag paste operator (# #)

The tag paste operator (# #) within a macro definition merges two parameters. It allows two separate tags in a macro definition to be merged into one tag.

4, defined () operator

The preprocessor defined operator is used in a constant expression to determine whether an identifier is already in use #define定义过. If the specified identifier is defined, the value is true (not 0). If the specified identifier is undefined, the value is False (0).

Parameterized macros

CPP A powerful feature is that you can use parameterized macros to simulate functions.

Before you use a macro with parameters, you must use the #define Directive definition. The argument list is enclosed in parentheses and must be immediately after the macro name. Spaces are not allowed between the macro name and the left parenthesis.

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