The ifndef directive checks for the existence of macro definitions.
If the identifier specified is not defined as a macro, the tokens that immediately follow the condition are passed on to the compiler after a newline.
The ifndef directive has the form:
.----------------. V | >>-#--ifndef--identifier----token_sequence-+-------------------><
An identifier must follow the #ifndef keyword. The following example defines MAX_LEN to be 50 if EXTENDED is not defined for the preprocessor. Otherwise, MAX_LEN is defined to be 75.
#ifndef EXTENDED # define MAX_LEN 50 #else # define MAX_LEN 75 #endif
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