Why is #include not required to use printf()?

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You had originally tagged this C++, but it would appear to be a C program. C will automatically provide an implicit declaration for a function if there is no prototype in scope (such as due to the omission of #include <stdio.h>). The implicit declaration would be:

int printf();

Meaning that printf is a function that returns an int and can take any number of arguments. This prototype happened to work for your call. You should #include <stdio.h>

Finally, I should add that the current C standard (ISO/IEC 9899:1999 or colloquially “C99”) does not allow implicit declarations, and this program would not conform. Implicit declarations were removed. I believe your compiler does not support C99. C++ also requires correct prototypes and does not do implicit declarations.

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