fgetpos/ftello without system call

[Carlos O'Donell]

On 09/01/2017 04:17 AM, Florian Weimer wrote:
> Is it possible to obtain the current read offset of a stream without
> triggering an lseek system call?

Hard to say without looking at a very specific use case.

The answer to that is actually incredibly complicated, and requires a
table (which may now be out of date), and a bunch of rules from POSIX:

https://sourceware.org/glibc/wiki/File%20offsets%20in%20a%20stdio%20stream%20and%20ftell

> I know that nothing manipulates the underlying file descriptor directly.

That's good, because otherwise you *would* need a seek to allow the active
handle to change between the fd and the FILE* as required by POSIX.
 
> The actual goal here is to read the same line multiple times using
> fgets, enlarging the buffer if necessary.

You should be able to do that, and it shouldn't trigger a seek.

> It seems that libio maintains an _offset member in the underlying FILE
> object, but never actually uses it for descriptor-based files (mapped
> files are a different story).

It should.

-- 
Cheers,
Carlos.