C pre-processor, processor

C pre-processor, processor

The C Preprocessor is not a component of the compiler, but a separate step in the compilation process. The C Preprocessor is just a text replacement tool that instructs the compiler to complete the necessary preprocessing before the actual compilation.

All Preprocessor commands start with. It must be the first non-null character. To enhance readability, Preprocessor commands should start from the first column.

 

The following table contains all important Preprocessor commands:

Command	Description
# Define	Definition macro
# Include	Contains a source code file
# Undef	Cancel a defined macro
# Ifdef	If the macro has been defined, true is returned.
# Ifndef	Returns true if the macro is not defined.
# If	If the given condition is true, compile the following code:
# Else	# If alternative
# Elif	If the previous # if condition is not true and the current condition is true, compile the following code:
# Endif	End one # if ...... # Else Conditional compilation Block
# Error	When a standard error occurs, an error message is output.
# Pragma	Use the standardization method to publish special commands to the Compiler
# Line	Provide the row number used for Compiler Information

 

Predefined macros

Ansi c defines many macros. These macros can be used in programming, but they cannot be directly modified.

Macro	Description
_ DATE __	The current date, which is a character constant in the format of "mmm dd yyyy.
_ TIME __	The current time, which is a character constant in the format of "HH: MM: SS.
_ FILE __	Contains the current file name, A String constant.
_ LINE __	Contains the current row number, a decimal constant.
_ STDC __	When the compiler is compiled in ANSI standard, it is defined as 1.
_ Stdc1_version __	If the compiler complies with C99, the macro value is 199901L, which is not defined in other cases.
_ Stdc1_hosted __	The current macro is a host system. The macro value is 1 and the current macro value is 0.
_ Stdc1_iec0000559 __	If the floating point implementation follows the IEC 60599 standard, the macro value is 1, otherwise it is not defined.
_ Stdc1_iec1_5591_complex __	If the plural operation follows the IEC 60559 standard, the macro value is 1. Otherwise, it is not defined.
_ Stdc1_iso000046 __	It is defined as a long integer constant.

 

Preprocessor Operator

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

1. Macro continuation operator (\)

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

2. String constant operator (#)

In macro definition, when the macro parameter needs to be converted to a String constant, the string constant operator (#) is used (#). The operator used in a macro has a specific parameter or parameter list.

3. Mark and paste operator (##)

The mark-and-paste operator (#) in the macro definition merges two parameters. It allows two independent tags in the macro definition to be merged into one tag.

4. defined () Operator

The Preprocessor defined operator is used in constant expressions to determine whether an identifier has been defined using # define. If the specified identifier has been defined, the value is true (non-zero ). If the specified identifier is not defined, the value is false (zero ).

 

Parameterized macros

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

Before using a macro with parameters, you must use the # define command to define the macro. The parameter list is enclosed in parentheses and must be followed by the macro name. Spaces are not allowed between macro names and left parentheses.