What is the purpose of global variables in the C programming language?

When a global variable is defined in a C program, it will exist throughout the entire lifespan of the program and can be accessed by all functions. Global variables are typically declared outside of functions and can be read from and modified within any function.

The main functions of global variables include:

1. Data sharing: When multiple functions need to share the same data, global variables can be used to store this data instead of passing parameters in each function.
2. Cross-function calls: Global variables can pass information between different functions, facilitating data sharing and interaction.
3. Configuration parameters: Global variables can be used to store the program’s configuration parameters or global state, facilitating the management and control of the program.
4. Reduce parameter passing: Avoid frequently passing large amounts of data as function parameters to improve code readability and simplicity.

However, there are also some disadvantages and considerations associated with global variables.

1. Scope expansion: The scope of global variables being too large may lead to unintended data modifications, increasing program complexity and debugging difficulty.
2. Naming conflicts: Global variables can easily lead to naming conflicts, affecting the maintainability of the code, so it is best to avoid using overly generic variable names.
3. Data protection: Global variables may be modified by any function, potentially leading to unexpected results. Careful design and management of global variable usage is necessary.

Therefore, when using global variables, one must carefully consider their impact on the program’s structure and logic, avoid overusing global variables, in order to ensure the maintainability and reliability of the program.