DosFileLocks

#define INCL_DOSFILEMGR

USHORT  rc = DosFileLocks(FileHandle, UnLockRange, LockRange);

HFILE            FileHandle;    /* File handle */
PLONG            UnLockRange;   /* UnLock range */
PLONG            LockRange;     /* Lock range */

USHORT           rc;            /* return code */

Example 

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

#define INCL_DOSFILEMGR

#define OPEN_FILE 0x01
#define CREATE_FILE 0x10
#define FILE_ARCHIVE 0x20
#define FILE_EXISTS OPEN_FILE
#define FILE_NOEXISTS CREATE_FILE
#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 FILE_ATTRIBUTE FILE_ARCHIVE
#define RESERVED 0L
#define NULL_RANGE 0L

HFILE   FileHandle;
USHORT  Wrote;
USHORT  Action;
PSZ     FileData[100];
USHORT  rc;

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 */
      }