# Ifdef, # endif, # ifndef, # define, # Use and effect of endif

Generally, all the rows in the source program are compiled. However, sometimes you want to compile a part of the content only when certain conditions are met, that is, to specify the compilation conditions for a part of the content. This is "Conditional compilation ". Sometimes, you want to compile a group of statements when a condition is met, and compile another group of statements when the condition is not met.
The most common form of Conditional compilation commands is:
# Ifdef identifier
Procedure 1
# Else
Procedure 2
# Endif

It is used to compile program segment 1 when the identifier has been defined (generally defined using the # define command). Otherwise, compile program segment 2.
The # else part can also be absent, I .e:
# Ifdef
Procedure 1
# Denif

The "program segment" can be a statement group or a command line. This Conditional compilation can improve the universality of the C source program. If a C source program runs on different computer systems, there are some differences between different computers. For example, we have a data type. In Windows, we should use the long type, while in other platforms, we should use the float representation. In this case, we often need to make necessary modifications to the source program, this reduces the versatility of the program. You can compile with the following conditions:
# Ifdef WINDOWS
# Define MYTYPE long
# Else
# Define MYTYPE float
# Endif

If you compile a program on Windows, you can add
# Define WINDOWS

In this way, compile the following command line:
# Define MYTYPE long

If the following command line appears before this set of Conditional compilation commands:
# Define WINDOW

After pre-compilation, the MYTYPE in the program is replaced by float. In this way, the source program can be used in different types of computer systems without any modification. Of course, the above is just a simple case. You can design other conditions for compilation based on this idea.
For example, when debugging a program, you often want to output some required information, but do not output this information after the debugging is complete. You can insert the following Conditional compilation segments in the source program:
# Ifdef DEBUG
Print ("device_open (% p) \ n", file );
# Endif

If you have the following command line before it:
# Define DEBUG

The value of the file pointer is output when the program is running for debugging and analysis. After debugging, you only need to delete the define command line. Some people may think that Conditional compilation can achieve this goal, that is, adding a batch of printf statements during debugging, and deleting the printf statements one by one after debugging. Indeed, this is acceptable. However, when many printf statements are added during debugging, the modification workload is huge. If you use Conditional compilation, you do not need to delete the printf statement one by one. You only need to delete the previous "# define DEBUG" command, at this time, all the Conditional compilation segments using DEBUG as identifiers make the printf statement ineffective, that is, unified control, just like a "Switch.
The following format is also used:
# Ifndef identifier
Procedure 1
# Else
Procedure 2
# Endif

The first line is different from the first form: Change "ifdef" to "ifndef ". It is used to compile program segment 1 if the identifier is not defined; otherwise, compile program segment 2. This form is opposite to the first form.
The usage of the above two forms is similar. You can choose one as needed, depending on your convenience.
Another form is: # if is followed by an expression instead of a simple identifier:
# If expression
Procedure 1
# Else
Procedure 2
# Endif

It is used to compile program segment 1 when the specified expression value is true or non-zero. Otherwise, compile program segment 2. You can specify certain conditions in advance so that the program can execute different functions under different conditions.
For example, enter a line of letters, set the condition for compiling as needed, so that all letters can be changed to uppercase for output, or all letters can be changed to lowercase for output.
# Define LETTER 1
Main ()
{
Char str [20] = "C Language", c;
Int I = 0;
While (c = str [I])! = '\ 0 '){
I ++;
# If LETTER
If (c> = 'A' & c <= 'Z') c = c-32;
# Else
If (c> = 'A' & c <= 'Z') c = c + 32;
# Endif
Printf ("% c", c );
}
}

Running result: C LANGUAGE
Now let's first define LETTER as 1. In this way, the first if statement is compiled because LETTER is true and non-zero in the pre-processing condition compiling command. At runtime, lowercase letters are changed to uppercase letters. If you change the first line of the program:
# Define LETTER 0

During preprocessing, the second if statement is compiled to convert uppercase letters into lowercase letters, and the ASCII code of the corresponding lowercase letters is 32 ). The running status is as follows:
C language
Some people may ask: what are the advantages of using Conditional compilation commands to directly use if statements without Conditional compilation commands? Indeed, this problem can be completely solved without the need for Conditional compilation, but in this case, the target program is long because all statements are compiled). Using Conditional compilation can reduce compiled statements, this reduces the length of the target. When there are many Conditional compilation segments, the length of the target program can be greatly reduced.

Supplement:
# Ifndef in the header file

Never ignore # ifndef in the header, which is critical. For example, you have two C files, both of which include the same header file. During compilation, these two C files need to be compiled into a runable file together, so the problem arises and a large number of declarations conflict.

Put the header file content in # ifndef and # endif. Whether or not your header file will be referenced by multiple files, you must add this. The general format is as follows:

# Ifndef <ID>
# Define <ID>

......
......

# Endif

<Identifier> in theory, it can be freely named, but the "identifier" of each header file should be unique. The naming rule of the identifier is generally that the header file name is in uppercase, followed by an underscore, and the "." In the file name is also changed to an underscore, such as stdio. h.

# Ifndef _ STDIO_H _
# Define _ STDIO_H _

......

# Endif

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