[PATCH] Avoid ARM SWI Seek when querying file position

[Andy Koppe]

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

[Andy Koppe]

Post by Andy Koppe
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.)

Slightly amended patch attached: Use off_t rather than int for the poslog.

[Corinna Vinschen]

Post by Andy Koppe

Post by Andy Koppe
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.)

Slightly amended patch attached: Use off_t rather than int for the poslog.

Pushed.


Thanks,
Corinna

[Richard Earnshaw (lists)]

Post by Andy Koppe

Post by Andy Koppe
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.)

Slightly amended patch attached: Use off_t rather than int for the poslog.
0001-Avoid-ARM-SWI-Seek-when-querying-file-position.patch
From 7fd1cd9e81fe5ab8f2c3524384431ba0d58ca151 Mon Sep 17 00:00:00 2001
Date: Fri, 31 Aug 2018 12:26:02 +0100
Subject: [PATCH] Avoid ARM SWI Seek when querying file position
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.
Also, use off_t rather than int for the poslog.
---
newlib/libc/sys/arm/syscalls.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/newlib/libc/sys/arm/syscalls.c b/newlib/libc/sys/arm/syscalls.c
index e0cf0ac65..6e70467ea 100644
--- a/newlib/libc/sys/arm/syscalls.c
+++ b/newlib/libc/sys/arm/syscalls.c
@@ -76,7 +76,7 @@ static int monitor_stderr;
typedef struct
{
int handle;
- int pos;
+ off_t pos;
}
poslog;
@@ -234,9 +234,16 @@ _swilseek (int file, off_t ptr, int dir)
if (dir == SEEK_CUR)
{
+ off_t pos;
if (slot == MAX_OPEN_FILES)
return -1;
- ptr = openfiles[slot].pos + ptr;
+ pos = openfiles[slot].pos;
+
+ /* Avoid SWI SEEK command when just querying file position. */
+ if (ptr == 0)
+ return pos;
+
+ ptr += pos;
dir = SEEK_SET;
}

We probably need the same patch in libgloss/arm/syscalls.c

R.