[PATCH] Avoid ARM SWI Seek when querying file position
Andy Koppe <[email protected]>
| Newsgroups | gmane.comp.lib.newlib |
|---|---|
| Message-ID | <CAHWeT-aJcma4z1sXPrJO7xUV1b+AyKWUcNFkLmheK4J_hKDrDA@mail.gmail.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
0001-Avoid-ARM-SWI-Seek-when-querying-file-position.patch
(application/octet-stream, 1.7 KB)
From ccf650927dfb7d3357a2e45f8b8c6568e82567f1 Mon Sep 17 00:00:00 2001 From: Andy Koppe <[email protected]> Date: Fri, 31 Aug 2018 11:36:03 +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. --- newlib/libc/sys/arm/syscalls.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/newlib/libc/sys/arm/syscalls.c b/newlib/libc/sys/arm/syscalls.c index e0cf0ac65..c9950fe09 100644 --- a/newlib/libc/sys/arm/syscalls.c +++ b/newlib/libc/sys/arm/syscalls.c @@ -234,9 +234,16 @@ _swilseek (int file, off_t ptr, int dir) if (dir == SEEK_CUR) { + int 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; } -- 2.17.0