What are the uses of #ifndef in C language?

The #ifndef in the C language is a conditional compilation directive used to control the compilation of code. Its purpose is to skip the following code if a specified identifier has already been defined, or to compile the following code if not.

The main purposes of #ifndef include the following:

1. To prevent header file duplication: using #ifndef and #endif in header files can prevent them from being included multiple times, thus avoiding redefinition errors.
2. Conditional compilation can be achieved by using #ifndef and #endif in the code to determine whether a certain section of code should be compiled. Different code can be compiled based on different conditions, allowing for compilation in different platforms or environments.
3. Definition of an identifier: Using #define after #ifndef allows for the definition of an identifier, which can then be used for conditional checks in subsequent code.

In summary, #ifndef is mainly used to prevent header file duplication and conditional compilation, which can make the program more flexible and portable.