为什么某些代码文件使用close（）而不包含unistd.h？

Question

Take this example: http://codebase.eu/tutorial/linux-socket-programming-c and http://codebase.eu/tutorial/linux-socket-programming-c/code/tcpclient.cpp

Basically the first example app uses close() but does not include unistd.h. Some people mention this in the comments. It works for some without it and others have to include it.

My question is why is this: Why is it sometimes not necessary? Is it better to include it always? What does it mean when it is not necessary to include it?

Answer 1

The referenced code uses <iostream> . It is likely, that unistd.h gets included because of it. But you should not count on it and include unistd.h yourself if you want to use close() .

Answer 2

It is always better to #include <unistd.h> when using int close(int); because this is the only way to guarantee that a declaration of int close(int); will be available for your source code when compiling on POSIX platforms. In some cases other included files (or your buildchain) might include unistd.h or include a forward declaration for it.

When the C++ preprocessor processes your source code, it basically replaces your #include <something.h> line with the preprocessed contents of something.h . Hence if something.h (or recursively one of its included headers) has a #include <unistd.h> line then your source code will see the declaration of int close(int); even thou you didn't directly include the unistd.h header.

However, to ensure portability, always include unistd.h if you use the int close(int); function ( man 3p close )!

See this question if you want to find out where a header file might be included from. Similarly you can use the -E flag for g++ / clang++ to output a fully preprocessed version of your source code to find out where the declaration of int close(int); comes from.