This is part of Family API which allow to create dual-os version of program runs under OS/2 and DOS

Note: This is legacy API call. It is recommended to use 32-bit equivalent

2021/09/17 04:47 · prokushev · 0 Comments
2021/08/20 03:18 · prokushev · 0 Comments


This call locks and unlocks a range in an opened file.


DosFileLocks (FileHandle, UnLockRange, LockRange) 


  • FileHandle (HFILE) - input : File handle.
  • UnLockRange (PLONG) - input : Address of the structure containing the offset and length of a range to be unlocked. A doubleword of zero indicates that unlocking is not required.
  • FileOffset (ULONG) : The offset to the beginning of the range to be unlocked.
  • RangeLength (ULONG) : The length of the range to be unlocked.
  • LockRange (PLONG) - input : Address of the structure containing the offset and length of a range to be locked. A doubleword of zero indicates that locking is not required.
  • FileOffset (ULONG) : The offset to the beginning of the range to be locked.
  • RangeLength (ULONG) : The length of the range to be locked.

Return Code

rc (USHORT) - return

Return code descriptions are:

  • 0 NO_ERROR


DosFileLocks provides a mechanism that allows a process to lock a region in a file for read/write access. The time a region is locked should be short.

Instead of denying another process read/write access to the entire file by means of access and sharing modes specified with DosOpen or DosOpen2 requests, a process attempts to lock only the range needed for read/write access and examines the error code returned.

A range to be locked must first be cleared of any locked subranges or overlapping ranges. The locked region can be located anywhere in the file, and locking beyond end-of-file is not considered an error.

Once a lock is successful, read/write access by another process to the specified range is denied until the range is unlocked. If both unlocking and locking are specified by a DosFileLocks request, the unlocking operation is performed first. After unlocking is completed, locking is done.

Duplicating the handle duplicates access to any locked regions; however, access to locked regions is not duplicated across the DosExecPgm call.

If a file is closed (either by a DosClose request or by a process terminating) and locks are still in effect, the locks are released in no defined order.

Family API Considerations

Some options operate differently in the DOS mode than in OS/2 mode. Therefore, the following restrictions apply to DosFileLocks when coding for the DOS mode:

  • If Block = 1 is specified, an “invalid range lock list” or “invalid unlock list” error is returned.
  • NewLockIDList is not supported.


C Binding

USHORT  rc = DosFileLocks(FileHandle, UnLockRange, LockRange);
HFILE            FileHandle;    /* File handle */
PLONG            UnLockRange;   /* UnLock range */
PLONG            LockRange;     /* Lock range */
USHORT           rc;            /* return code */


This example opens a file, writes some data to it, locks a block of the data, and then unlocks it.

#define OPEN_FILE 0x01
#define CREATE_FILE 0x10
#define FILE_ARCHIVE 0x20
#define DASD_FLAG 0
#define INHERIT 0x80
#define WRITE_THRU 0
#define FAIL_FLAG 0
#define SHARE_FLAG 0x10
#define ACCESS_FLAG 0x02
#define FILE_NAME "test.dat"
#define FILE_SIZE 800L
#define RESERVED 0L
#define NULL_RANGE 0L
HFILE   FileHandle;
USHORT  Wrote;
USHORT  Action;
PSZ     FileData[100];
struct LockStrc
   long Offset;
   long Range;
   } Area;
int i;
   Action = 2;
   strcpy(FileData, "Data...");
   Area.Offset = 4;
   Area.Range = 100;
   if(!DosOpen(FILE_NAME,                /* File path name */
                &FileHandle,             /* File handle */
                &Action,                 /* Action taken */
                FILE_SIZE,               /* File primary allocation */
                FILE_ATTRIBUTE,          /* File attribute */
                FILE_EXISTS | FILE_NOEXISTS,              /* Open function
                                                                    type */
                DASD_FLAG | INHERIT |            /* Open mode of the file */
                WRITE_THRU | FAIL_FLAG |
                SHARE_FLAG | ACCESS_FLAG,
                RESERVED))               /* Reserved (must be zero) */
      for(i=0; i<200; ++i)
         DosWrite(FileHandle,            /* File handle */
                  FileData,              /* User buffer */
                  sizeof(FileData),      /* Buffer length */
                  &Wrote);               /* Bytes written */
      rc = DosFileLocks(FileHandle,            /* File handle */
                        NULL_RANGE,            /* Unlock range */
                        (PLONG) &Area);       /* Lock range */
      rc = DosFileLocks(FileHandle,            /* File handle */
                        (PLONG) &Area,        /* Unlock range */
                        NULL_RANGE);           /* Lock range */

MASM Binding

EXTRN  DosFileLocks:FAR
PUSH   WORD    FileHandle    ;File handle
PUSH@  OTHER   UnLockRange   ;UnLock range
PUSH@  OTHER   LockRange     ;Lock range
CALL   DosFileLocks
Returns WORD