Conditional compilation of C language pre-processing commands

In C,

The Conditional compilation command determines which code is compiled and which are not compiled. You can determine the compilation conditions based on the expression value or whether a specific macro is defined.

The following are some pre-processing commands:

Command usage
# Empty command, no effect
# Include contains a source code file
# Define definition macro
# Undef cancel a defined macro
# If the given condition is true, compile the following code
# Ifdef if the macro has been defined, compile the following code
# Ifndef if the macro is not defined, compile the following code
# Elif if the previous # if given condition is not true and the current condition is true, compile the following code:
# Endif end one. # if ...... # Else Conditional compilation Block
# Error stops compilation and displays error information

1. # if command

# The if command detects the constant expression following it. if the expression is true, compile the code after it until # else, # elif, or # endif appears. Otherwise, the Code is not compiled.

2. # endif command

# Endif is used to terminate # if preprocessing command.

# Include <stdio. h> DEBUG 0 DEBUG

If the # define statement is removed, the effect is the same.

3. # ifdef and # ifndef

# Include <stdio. h> DEBUG

4. # else command

# When the else command is used after a # if Command and the current # if command condition is not true, the code after # else is compiled. # The endif command terminates the preceding condition block.

# Include <stdio. h> DEBUG

5. # elif command

# Elif preprocessing commands combine the functions of # else and # if commands.

# Include <stdio. h> TWO defined TWO