The ifdef directive checks for the existence of macro definitions.
If the identifier specified is defined as a macro, the tokens that immediately follow the condition are passed on to the compiler after a newline.
The ifdef directive has the form:
.----------------. V | >>-#--ifdef--identifier----token_sequence-+--------------------><
The following example defines MAX_LEN to be 75 if EXTENDED is defined for the preprocessor. Otherwise, MAX_LEN is defined to be 50.
#ifdef EXTENDED # define MAX_LEN 75 #else # define MAX_LEN 50 #endif
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