Ignoring the stdio.h file in a C file

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http://www.unix.com – I am facing a problem in the below given code. Code: int main() { printf("\nHello Geeks\n\n") ; return 0 ; } In the above mentioned code i left including "#include ". And if i compile and execute this piece of code, the output is printed as expected. But "#include " being the most important thing in a C program, i have ignored it and still the compilation is done without any errors but with warning. Why is this happening? (HowTos)