Sun Aug 31 18:52:20 CEST 2003
Vance Shipley <> wrote:
>File descriptors are opaque data types. You shouldn't make
>any assumptions about what values they take other than that
>they fit in an int. That file descriptors are assigned
>incrementally increasing from stdio is only a well known
>practice not a guarantee.
POSIX requires not only that file descriptors are "small integers", and
not only that they are assigned incrementally, but that any system call
returning a file descriptor will return the lowest-numbered free one.
fd = open("/dev/null", O_RDWR);
is guaranteed to set fd to 0 (or -1 in case /dev/null couldn't be
opened:-). And there is *lots* of "core" Unix code that depends on this
> I'm currently working with devices
>which start at 257 and assign incrementally from there.
I guess they're not POSIX-compliant then. And of course "file
descriptor" can be taken in an "abstract" sense, but then it's not
something you can pass to select() (or poll()).
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