For a successful call, the return value depends on the operation:
F_DUPFD
The new file descriptor.
F_GETFD
Value of file descriptor flags.
F_GETFL
Value of file status flags.
F_GETLEASE
Type of lease held on file descriptor.
F_GETOWN
Value of file descriptor owner.
F_GETSIG
Value of signal sent when read or write becomes possible,
or zero for traditional SIGIO behavior.
F_GETPIPE_SZ, F_SETPIPE_SZ
The pipe capacity.
F_GET_SEALS
A bit mask identifying the seals that have been set for
the inode referred to by fd.
All other commands
Zero.
On error, -1 is returned, and errno is set to indicate the error.
