[PATCH] Avoid ARM SWI Seek when querying file position

[Andy Koppe <andy dot koppe at gmail dot com>]

Issuing an ARM semi-hosting Seek command when just querying file
position with SEEK_CUR and offset zero is unnecessary, because unlike
the lseek() Unix system call the Seek command does not actually return
the file position. For that reason, syscalls.c for ARM keeps track of
file position in the 'poslog', so we can just return that.

Moreover, since the Seek command only accepts an absolute file position,
SEEK_CUR operations are implemented by adding the relative offset to the
position in the poslog. If the host implements non-binary files with
implicit carriage return characters but doesn't discount those implicit
CRs when implementing Seek (by just mapping straight to Windows file
operations), this actually ended up wrongly changing file position when
using SEEK_CUR with offset zero or functions like ftell() or fgetpos()
that are based on that.

The ARM semi-hosting Seek command is documented at
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0040d/BACDFGCG.html

(There's a wider problem in that using the position returned by the
likes of ftell() in a later seek operation gets you to the wrong place
if the host adds implicit CRs without discounting them in Seek
commands. Not sure there's anything that can be done about it on the
newlib side though.)

Regards,
Andy

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