What does the `#include` preprocessor directive do in C?

The #include preprocessor directive in C is used to bring in the contents of a file, typically containing function definitions, declarations, or macros, into the program before compilation. When you use this directive, it effectively tells the compiler to include the specified header file, which often contains the declarations for functions and variables that you want to use from external libraries. This is essential for accessing predefined functions and libraries, such as those in the standard library, which facilitate various operations without needing to rewrite code.

Using #include ensures that your program has access to the necessary declarations and definitions, which helps in preventing issues like undefined references during the linking stage of compilation. This directive does not execute external programs, link object files, or define new data types; instead, it strictly includes the specified external code, making it crucial for effective programming in C.