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Int 21 Fn 3D - DOS 2+ - "open" - Open Existing File [D]
AH = 3Dh
AL = access and sharing modes (see below)
DS:DX -> ASCIZ filename
CL = attribute mask of files to look for (server call only)
Return: CF clear if successful
AX = file handle
CF set on error
AX = error code (01h,02h,03h,04h,05h,0Ch,56h) (see AH=59h)
Notes: file pointer is set to start of file
file handles which are inherited from a parent also inherit sharing
and access restrictions
files may be opened even if given the hidden or system attributes
under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
DR-DOS checks the system password or explicitly supplied password at
the end of the filename against the reserved field in the directory
entry before allowing access
sharing modes are only effective on local drives if SHARE is loaded
See Also: AH=0Fh,AH=3Ch,AX=4301h,AX=5D00h,INT 2F/AX=1116h,INT 2F/AX=1226h
Bitfields for access and sharing modes:
bits 2-0 access mode
000 read only
001 write only
010 read/write
011 (DOS 5+ internal) passed to redirector on EXEC to allow
case-sensitive filenames
bit 3 reserved (0)
bits 6-4 sharing mode (DOS 3+)
000 compatibility mode
001 "DENYALL" prohibit both read and write access by others
010 "DENYWRITE" prohibit write access by others
011 "DENYREAD" prohibit read access by others
100 "DENYNONE" allow full access by others
111 network FCB (only available during server call)
bit 7 inheritance
if set, file is private to current process and will not be inherited
by child processes
File sharing behavior:
| Second and subsequent Opens
First |Compat Deny Deny Deny Deny
Open | All Write Read None
|R W RW R W RW R W RW R W RW R W RW
- - - - -| - - - - - - - - - - - - - - - - -
Compat R |Y Y Y N N N 1 N N N N N 1 N N
W |Y Y Y N N N N N N N N N N N N
RW|Y Y Y N N N N N N N N N N N N
- - - - -|
Deny R |C C C N N N N N N N N N N N N
All W |C C C N N N N N N N N N N N N
RW|C C C N N N N N N N N N N N N
- - - - -|
Deny R |2 C C N N N Y N N N N N Y N N
Write W |C C C N N N N N N Y N N Y N N
RW|C C C N N N N N N N N N Y N N
- - - - -|
Deny R |C C C N N N N Y N N N N N Y N
Read W |C C C N N N N N N N Y N N Y N
RW|C C C N N N N N N N N N N Y N
- - - - -|
Deny R |2 C C N N N Y Y Y N N N Y Y Y
None W |C C C N N N N N N Y Y Y Y Y Y
RW|C C C N N N N N N N N N Y Y Y
Legend: Y = open succeeds, N = open fails with error code 05h
C = open fails, INT 24 generated
1 = open succeeds if file read-only, else fails with error code
2 = open succeeds if file read-only, else fails with INT 24
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