What is ifndef nedir?

The term "ifndef" is short for "if not defined" and is used in programming languages like C and C++ to check whether a particular macro or symbol has already been defined. When the preprocessor encounters an "ifndef" directive followed by a macro name, it checks whether that macro is already defined. If it's not defined, the preprocessor moves to the next block of code that follows the "ifndef" block. If it is defined, the preprocessor skips over the code block that follows the "ifndef" block and moves on to the next block of code.

The "ifndef" directive is often used in header files to prevent multiple includes of the same header file, which can lead to errors and conflicts in the code. By putting "ifndef" at the top of a header file and following it with a unique identifier, programmers can ensure that the header file is only included once, even if it's referenced multiple times in the code.