An application that had used the stdio routine fopen() to open a file should use the corresponding fclose() routine rather than close(). Once a file is closed, the file descriptor no longer exists, since the integer corresponding to it no longer refers to a file.

If the file descriptor is associated with a standard I/O stream (for example, if the file descriptor was obtained by calling fileno() on a stream, or if an associated stream was created using fdopen()), the file descriptor should be closed using fclose() on the associated stream (or pclose() if the stream was opened using popen()) instead of calling close() or posix_close() on the file descriptor. Once a file descriptor is closed, the integer corresponding to it no longer refers to a file. However, the same integer can be allocated on a subsequent file open and would then become associated with the stream if the stream has not been closed.

If the file descriptor is associated with a directory stream (for example, if the file descriptor was obtained by calling dirfd() on a directory stream, or if a directory stream was created using fdopendir()), the file descriptor should be closed using closedir() on the associated directory stream instead of calling close() or posix_close() on the file descriptor.