From 505f1c2804dcb5f1930a199a3bdf6fd32e605883 Mon Sep 17 00:00:00 2001
From: Stephen Heumann <stephenheumann@gmail.com>
Date: Thu, 14 Jul 2022 18:33:31 -0500
Subject: [PATCH] lseek: make seeking to an offset before end of file work
 properly.

The direction specified by the offset was essentially reversed when calling lseek with whence==2 (seek to offset from end of file). Therefore, specifying a negative offset with whence==2 would fail, rather than seeking before the end of the file as it should.

(The ORCA/C manual is not totally clear about this behavior, but the new behavior is consistent with the POSIX spec and all other implementations I'm aware of, including traditional Unix and APW C. Note that Unix/POSIX allows seeking beyond the end of the file, but GS/OS does not.)

There are also improvements to error handling, so lseek consistently reports EINVAL for invalid offsets.
---
 fcntl.asm | 55 ++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 34 insertions(+), 21 deletions(-)

diff --git a/fcntl.asm b/fcntl.asm
index 2a4186d..e159175 100644
--- a/fcntl.asm
+++ b/fcntl.asm
@@ -380,43 +380,56 @@ mark     equ   1                        new file mark
          tax
          lda   >files,X
          bne   lb2
-lb1      lda   #EBADF                   bad refnum error
-         sta   >errno
-         bra   lb4
+lb1      bra   lb4a                     bad refnum error
 
 lb2      sta   >smRefnum                set the file refnum
          sta   >gmRefnum
          lda   whence                   convert from UNIX whence to GS/OS base
-         beq   lb3
-         eor   #$0003
-         cmp   #4
-         bge   lb2a
-         cmp   #2
-         bne   lb3
-         sta   >smBase
-         lda   offset+2
-         bpl   lb3a
-         sub4  #0,offset,offset
-         lda   #3
+         cmp   #SEEK_SET                if whence == 0 (SEEK_SET)
+         bne   lb2a
+         lda   offset+2                   if offset is negative
+         bmi   lb4                          fail with EINVAL
+         lda   #0                         set mark to offset
          bra   lb3
-lb2a     lda   #EINVAL                  invalid whence flag
-         sta   >errno
-         bra   lb4
+lb2a     cmp   #SEEK_END                else if whence == 2 (SEEK_END)
+         bne   lb2c
+         lda   offset+2                   if offset > 0
+         bmi   lb2b
+         ora   offset
+         bne   lb4                          fail with EINVAL
+lb2b     sub4  #0,offset,offset           negate offset
+         lda   #1                         set mark to EOF - offset
+         bra   lb3
+lb2c     cmp   #SEEK_CUR                else if whence == 1 (SEEK_CUR)
+         bne   lb4
+         lda   offset                     if offset is positive or 0
+         bmi   lb2d
+         lda   #2                           set mark to old mark + offset
+         bra   lb3                        else
+lb2d     sub4  #0,offset,offset             negate offset
+         lda   #3                           set mark to old mark - offset
 lb3      sta   >smBase                  save the base parameter
 lb3a     lda   offset                   set the displacement
          sta   >smDisplacement
          lda   offset+2
          sta   >smDisplacement+2
          OSSet_Mark smRec               set the file mark
-         bcs   lb1
-         OSGet_Mark gmRec               get the new mark
-         bcs   lb1
+         bcc   lb5
+         cmp   #$4D                     out of range error => fail with EINVAL
+         bne   lb4a
+lb4      lda   #EINVAL
+         bra   lb4b           
+lb4a     lda   #EBADF                   bad refnum error
+lb4b     sta   >errno
+         bra   lb6
+lb5      OSGet_Mark gmRec               get the new mark
+         bcs   lb4a
          lda   >gmDisplacement
          sta   mark
          lda   >gmDisplacement+2
          sta   mark+2
 
-lb4      creturn 4:mark
+lb6      creturn 4:mark
 
 smRec    dc    i'3'                     SetMark record
 smRefnum ds    2