Thread: interpreting fcnrl F_GETFL

Can anyone give me an actual example of how to do this:

The normal return value from fcntl with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags. Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with O_ACCMODE to compare them.

eg, given flags=fcntl(fd,F_GETFL); "flags" is 2050.

also: once I've set O_NONBLOCK, how do I unset it?

Can anyone give me an actual example of how to do this:

Originally Posted by Codeplug

My understanding of the Posix docs is that *if* you are interesting in access modes only (O_RDONLY, O_RDWR, O_WRONLY), then you use equality (==) and not a bit-wise AND. Before you check for equality, you need to mask off everything but the access mode bits using O_ACCMODE.

If you want to check for status flags (O_APPEND, O_DSYNC, O_NONBLOCK, O_RSYNC, O_SYNC), then you can simply do a bit-wise AND.

Code:

int flags;
flags=fcntl(fd,F_GETFL);

if ((flags & O_ACCMODE) == O_RDWR)
    ..

That's how I would write it based on what Codeplug says [and it makes some sense].

--
Mats

Okay. Sorry guys, I just don't understand the use of ints and bitmasks with this kind of information. But now if:

flags=(flags & O_ACCMODE);

returns 2, which I presume is a normative unix file mode (read). What about the other flags (from before the mask)? How should I interpret "2050"?

ps. I just searched cboard for "bitmasks flags" and this is the only post returned, sadly...

2050 is 2048 + 2, so it's got flags 2 and 2048 (0x800) set.

--
Mats

thanks matsp, I found some tutorials and you've given me somewhere to start