Int 21/AH=3Dh

DOS 2+ - OPEN - OPEN EXISTING FILE

AH = 3Dh
AL = access and sharing modes (see #01402)
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 #01680 at AH=59h)

Notes: File pointer is set to start of file. If SHARE or a network is loaded, the file open may fail if the file is already open, depending on the combination of sharing modes (see #01403,#01404). 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 (following a semicolon) against the reserved field in the directory entry before allowing access. Sharing modes are only effective on local drives if SHARE is loaded

BUG: Novell DOS 7 SHARE v1.00 would refuse file access in the cases in #01403 marked with [1] (read-only open of a read-only file which had previously been opened in compatibility mode); this was fixed in SHARE v1.01 of 09/29/94

See Also: AH=0Fh - AH=3Ch - AX=4301h - AX=5D00h -

Bitfields for access and sharing modes: Bit(s) Description (Table 01402) 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 3 reserved (0) 6-4 sharing mode (DOS 3.0+) (see #01403). 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) 7 inheritance. If set, file is private to current process and will not be inherited by child processes

See Also: #01782 - #01403



(Table 01403)
Values of DOS 2-6.22 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

See Also: #01636 - #01404

Legend:
R -> reading, W -> writing, RW -> both reading & writing,. A/NA -> reading without access time update. Y = open succeeds, N = open fails with error code 05h. C = open fails, INT 24 generated

See Also: #01403 - #01636

Category: DOS Kernel - Int 21h - D

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