What is if Def in C?

Description. In the C Programming Language, the #ifdef directive allows for conditional compilation. The preprocessor determines if the provided macro exists before including the subsequent code in the compilation process.

What is the meaning of if defined?

The #ifdef identifier statement is equivalent to #if 1 when identifier has been defined. It’s equivalent to #if 0 when identifier hasn’t been defined, or has been undefined by the #undef directive.

What does #ifndef mean in C?

Description. In the C Programming Language, the #ifndef directive allows for conditional compilation. The preprocessor determines if the provided macro does not exist before including the subsequent code in the compilation process.

What is preprocessing directive in C?

Preprocessor directives are lines included in a program that begin with the character #, which make them different from a typical source code text. They are invoked by the compiler to process some programs before compilation.

What is #ifndef #define and #endif used for?

#ifndef checks whether the given token has been #defined earlier in the file or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif statement.

What is #if #endif in C?

The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. If the expression you write (after the #if) has a nonzero value, the line group immediately following the #if directive is kept in the translation unit.

Why undef is used in C?

In the C Programming Language, the #undef directive tells the preprocessor to remove all definitions for the specified macro. A macro can be redefined after it has been removed by the #undef directive. Once a macro is undefined, an #ifdef directive on that macro will evaluate to false.

What is ## operator in C?

Token-pasting operator (##) Allows tokens used as actual arguments to be concatenated to form other tokens. It is often useful to merge two tokens into one while expanding macros. This is called token pasting or token concatenation. The ‘##’ pre-processing operator performs token pasting.