Sleep | warning implicit declaration of function `sleep'?

The issue is in the libraries (header files):

on Windows:

#include <windows.h> and Sleep(1000); => 1000 milliseconds

on Linux:

#include <unistd.h> and sleep(1); => 1 second

The function sleep is not part of C programming language. So, C compiler needs a declaration/prototype of it so that it can get to know about about number of arguments and their data types and return data type of the function. When it doesn't find it, it creates an Implicit Declaration of that function.

In Linux, sleep has a prototype in <unistd.h> and in windows, there is another function Sleep which has a prototype in <windows.h> or <synchapi.h>.You can always get away with including header, if you explicitly supply the prototype of the function before using it. It is useful when you need only few functions from a header file.

The prototype of Sleep function in C on windows is:

VOID WINAPI Sleep(_In_ DWORD dwMilliseconds);

Remember, it is always a good practice to supply the prototype of the function being used either by including the appropriate header file or by explicitly writing it. Even, if you don't supply it, compiler will just throw a warning most of the time and it will make an assumption which in most cases will be something that you don't want. It is better to include the header file as API might change in future versions of the Library.

Windows doesn't have the sleep function. Instead, it has Sleep, which takes the number of milliseconds to sleep:

VOID WINAPI Sleep(  _In_ DWORD dwMilliseconds);

You'll need to either #include <windows.h> or #include <synchapi.h>, depending on the version of Windows you're running. See MSDN for more details.