close(2) - Linux man page
Name
close - close a file descriptor
Synopsis
#include <unistd.h>
int close(int fd);
Description
close() closes a file descriptor, so that it no longer refers to any file and may be reused. Any record locks (see fcntl(2) ) held on the file it was associated with, and owned by the process, are removed (regardless of the file descriptor that was used to obtain the lock).
NOTE: file descriptor的scope是process，而不是OS；
If fd is the last file descriptor referring to the underlying open file description (see open(2) ), the resources associated with the open file description are freed; if the descriptor was the last reference to a file which has been removed using unlink(2) the file is deleted.
NOTE: 这段话非常重要，它描述了OS何时release掉file table entry，显然OS管理file table entry的方式类似于引用计数的方式；
Return Value
close() returns zero on success. On error, -1 is returned, and errno is set appropriately.
Errors
1、EBADF
fd isn't a valid open file descriptor.
2、EINTR
The close() call was interrupted by a signal; see signal(7).
3、EIO
An I/O error occurred.