Why use ifndef in c

The interface usually includes function prototypes, definitions for publicly accessible data structures, and some other miscellaneous stuff. Note that header files may be included in the source file multiple times, leading to compiler errors. Usually, this is prevented with the ifndef preprocessor directive, which is called wrapper ifndef.

In case the user includes the same header for the second time, the ifndef directive will evaluate false and ignore the code before the endif directive. Preprocessor directives. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info.

This is a guide to ifndef in C. Here we discuss the Introduction of ifndef in C and how it works along with different Examples and its Code Implementation. You can also go through our other suggested articles to learn more —. Submit Next Question. By signing up, you agree to our Terms of Use and Privacy Policy. Forgot Password? This website or its third-party tools use cookies, which are necessary to its functioning and required to achieve the purposes illustrated in the cookie policy.

By closing this banner, scrolling this page, clicking a link or continuing to browse otherwise, you agree to our Privacy Policy. Popular Course in this category. I wanted to manipulate a form which was opened by the form to manipulate.

It gaveme lots of errors and I didn't know what to do. CIsForCookies Punch "one definition rule" into your favorite search engine. QuantumFool 3 3 gold badges 9 9 silver badges 29 29 bronze badges. Roy Roy 1, 1 1 gold badge 13 13 silver badges 11 11 bronze badges. Identifiers starting with an underscore are reserved; you shouldn't define them yourself. I know this is an old comment, but actually the underscore restriction only applies to "external identifiers" - identifiers that could end up in the compiled object's symbol table, i.

It does not apply to macro names. Is Stu's comment true? I just read stackoverflow. In practice such symbols may be used to define macros in the standard libraries, which could collide with user-defined symbols of the same form I look forward to a response in ; greetings, from the age of the rat plague! Exception: user-defined literals. Add a comment. This prevent from the multiple inclusion of same header file multiple time.

They are called ifdef or include guards. Mohit Jain Mohit Jain 6 6 bronze badges.