Int 21H, AH=3DH

2 and higher

“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)

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

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

(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

(Table 01404) Values for DOS 7.x file sharing behavior:

        |     Second and subsequent Opens

First |Compat Deny Deny Deny Deny Open | All Write Read None

        |R W RW A  R W RW A  R W RW A  R W RW A  R W RW A

- - - - -| - - - - - - - - - - - - - - - - - - - - - - - - Compat R |Y Y Y Y N N N N Y N N Y N N N Y Y N N Y

      W |Y Y Y C   N N N N   N N N N   N N N Y   Y N N Y
      RW|Y Y Y C   N N N N   N N N N   N N N Y   Y N N Y
      NA|Y C C Y   N N N N   Y N N Y   N N N Y   Y N N Y

- - - - -| Deny R |C C C C N N N N N N N N N N N N N N N N All W |C C C C N N N N N N N N N N N N N N N N

      RW|C C C C   N N N N   N N N N   N N N N   N N N N
      NA|C C C C   N N N N   N N N N   N N N N   N N N N

- - - - -| Deny R |Y C C Y N N N N Y N N Y N N N Y Y N N Y Write W |C C C C N N N N N N N N Y N N Y Y N N Y

      RW|C C C C   N N N N   N N N N   N N N Y   Y N N Y
      NA|Y C C Y   N N N N   Y N N Y   N N N Y   Y N N Y

- - - - -| Deny R |C C C C N N N N N Y N N N N N N N Y N N Read W |C C C C N N N N N N N N N Y N N N Y N N

      RW|C C C C   N N N N   N N N N   N N N N   N Y N N
      NA|Y Y Y Y   N N N N   Y Y Y Y   N N N Y   Y Y Y Y

- - - - -| Deny R |Y Y Y Y N N N N Y Y Y Y N N N Y Y Y Y Y None W |C C C C N N N N N N N N Y Y Y Y Y Y Y Y

      RW|C C C C   N N N N   N N N N   N N N Y   Y Y Y Y
      NA|Y Y Y Y   N N N N   Y Y Y Y   N N N Y   Y Y Y Y

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
        

AH=0Fh,AH=3Ch,AX=4301h,AX=5D00h,INT 2F/AX=1116h,INT 2F/AX=1226h

Text based on Ralf Brown Interrupt List Release 61