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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.

Example Code

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

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