файловый контроль (file control)
Возвращаемое значение (Return value)
Upon successful completion, the value returned shall depend on
cmd as follows:
F_DUPFD A new file descriptor.
F_DUPFD_CLOEXEC
A new file descriptor.
F_GETFD Value of flags defined in <fcntl.h>. The return
value shall not be negative.
F_SETFD Value other than -1.
F_GETFL Value of file status flags and access modes. The
return value is not negative.
F_SETFL Value other than -1.
F_GETLK Value other than -1.
F_SETLK Value other than -1.
F_SETLKW Value other than -1.
F_GETOWN Value of the socket owner process or process group;
this will not be -1.
F_SETOWN Value other than -1.
Otherwise, -1 shall be returned and errno set to indicate the
error.
