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What is difference between preprocessor directive #if and normal if in C? I'm new to C.
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There are 4 Main Types of Preprocessor Directives: Macros. File Inclusion. Conditional Compilation. Other directives.
Preprocessor directives appear in source code. There are many different directives. One of them is #include , which is used to include a header file. Header files contain a collection of declarations, often for functions and types (and sometimes variables) found in a library.
What Does Preprocessor Directive Mean? 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.
Preprocessing directives are lines in your program that start with # . The # is followed by an identifier that is the directive name. For example, #define is the directive that defines a macro. Whitespace is also allowed before and after the # .
Statements with # in front of them are called preprocessor directives. They are processed by a parser before the code is actually compiled. From the first search hit using Google (http://www.cplusplus.com/doc/tutorial/preprocessor/):
Preprocessor directives are lines included in the code of our programs that are not program statements but directives for the preprocessor. These lines are always preceded by a hash sign (#). The preprocessor is executed before the actual compilation of code begins, therefore the preprocessor digests all these directives before any code is generated by the statements.
So a #if will be decided at compile time, a "normal" if will be decided at run time. In other words,
#define TEST 1 #if TEST printf("%d", TEST); #endif Will compile as
printf("%d", 1); If instead you wrote
#define TEST 1 if(TEST) printf("%d", TEST); The program would actually compile as
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