file descriptor and file reference number mapping

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piyuesh@gmail.com - 28 Jun 2006 08:01 GMT

There are times when you want to use low level unix system calls in
your carbon application. It's trivial but some syscalls needs file
descriptor
and you only have file reference number (got from HOpenDF, FSpOpenDF
etc..)
at hand. There is no official way to map this filerefnum to file
descriptor.
There is a hack available to do the mapping between these 2 numbers.
{
    int fd = open(filename, flags);
    close(fd);
    short int fileRef= HOpenDF()/FSOpenFork()/FSpOpenDF;
    // now fd corresponds to fileRef.
    fstat(fd); // It will work.
}
The reason for this behavior is open() syscall always utilizes
lowest file descriptor available in the process's file descriptor table.

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Ben Artin - 29 Jun 2006 05:56 GMT

> There are times when you want to use low level unix system calls in
> your carbon application. It's trivial but some syscalls needs file
[quoted text clipped - 3 lines]
> at hand. There is no official way to map this filerefnum to file
> descriptor.

Can you give us some example when your code (which relies on undocumented
behavior that might change in a new OS release) is better than simply getting
the path from the Carbon file ref and using that path with POSIX calls?

Ben

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piyuesh@gmail.com - 30 Jun 2006 10:22 GMT

> > There are times when you want to use low level unix system calls in
> > your carbon application. It's trivial but some syscalls needs file
[quoted text clipped - 11 lines]
>
> --

for example fcntl, flock etc..works on file descriptor not on
pathnames, also there may be some other usage for this method. I'm not
able to think every possible scenario right now.

Piyuesh

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Ben Artin - 30 Jun 2006 17:16 GMT

> > > There are times when you want to use low level unix system calls in your
> > > carbon application. It's trivial but some syscalls needs file descriptor
[quoted text clipped - 13 lines]
> there may be some other usage for this method. I'm not able to think every
> possible scenario right now.

OK, so once you have the path, call fopen() yourself. That's a much better
idea than relying on the undocumented implementation details of Apple APIs.

Ben

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Mac Forum / Programming / Mac Programming / June 2006

file descriptor and file reference number mapping