c - Usage of F_GETFL and F_SETFL

Question

While trying to use fcntl() with command F_GETFL and F_SETFL, I got some questions:

1. Why the flag returned by fcntl(

Answer 1

1) The return of fcnl is a code that described if the function succceded and how:

RETURN VALUE

   For a successful call, the return value depends on the operation:

   F_DUPFD  The new 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 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 appropriately.

ERRORS

   EACCES or EAGAIN
          Operation is prohibited by locks held by other processes.

   EAGAIN The operation is prohibited because the file has been memory-
          mapped by another process.

   EBADF  fd is not an open file descriptor

   EBADF  cmd is F_SETLK or F_SETLKW and the file descriptor open mode
          doesn't match with the type of lock requested.

   EBUSY  cmd is F_SETPIPE_SZ and the new pipe capacity specified in arg
          is smaller than the amount of buffer space currently used to
          store data in the pipe.

   EBUSY  cmd is F_ADD_SEALS, arg includes F_SEAL_WRITE, and there
          exists a writable, shared mapping on the file referred to by
          fd.

   EDEADLK
          It was detected that the specified F_SETLKW command would
          cause a deadlock.

   EFAULT lock is outside your accessible address space.

   EINTR  cmd is F_SETLKW or F_OFD_SETLKW and the operation was
          interrupted by a signal; see signal(7).

   EINTR  cmd is F_GETLK, F_SETLK, F_OFD_GETLK, or F_OFD_SETLK, and the
          operation was interrupted by a signal before the lock was
          checked or acquired.  Most likely when locking a remote file
          (e.g., locking over NFS), but can sometimes happen locally.

   EINVAL The value specified in cmd is not recognized by this kernel.

   EINVAL cmd is F_ADD_SEALS and arg includes an unrecognized sealing
          bit.

   EINVAL cmd is F_ADD_SEALS or F_GET_SEALS and the filesystem
          containing the inode referred to by fd does not support
          sealing.

   EINVAL cmd is F_DUPFD and arg is negative or is greater than the
          maximum allowable value (see the discussion of RLIMIT_NOFILE
          in getrlimit(2)).

   EINVAL cmd is F_SETSIG and arg is not an allowable signal number.

   EINVAL cmd is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid
          was not specified as zero.

   EMFILE cmd is F_DUPFD and the process already has the maximum number
          of file descriptors open.

   ENOLCK Too many segment locks open, lock table is full, or a remote
          locking protocol failed (e.g., locking over NFS).

   ENOTDIR
          F_NOTIFY was specified in cmd, but fd does not refer to a
          directory.

   EPERM  Attempted to clear the O_APPEND flag on a file that has the
          append-only attribute set.

   EPERM  cmd was F_ADD_SEALS, but fd was not open for writing or the
          current set of seals on the file already includes F_SEAL_SEAL.

2) Flags to be set is your choice: :

F_SETFL (int)

   Set the file status flags to the value specified by arg.  File
   access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
   flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
   ignored.  On Linux this command can change only the O_APPEND,
   O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.  It is not
   possible to change the O_DSYNC and O_SYNC flags; see BUGS,
   below.

3) HERE you have a complete description.