c - Why am I able to use isspace() without including the required header? -

this question has answer here:

• why doesn’t putchar require header? 2 answers

why code work:

    #include <stdio.h>      int main()     {         int x = isspace(getchar());         printf("%d", x);         return 0;     } 

whenever enter whitespace isspace() returns 8 , when don't, gives 0.

shouldn't produce error @ compile time? didn't add #include <ctype.h> @ top. why allowed?

you're seeing because compiler (sadly, still) supports implicit declaration of function.

if enable strict checking, compiler should refuse compile code. on , above c99, implicit function declaration has been made non-standard. (to add, hopefully, future versions of compiler strictly disallow this, default.)