DosChgFilePtr

#define INCL_DOSFILEMGR

USHORT  rc = DosChgFilePtr(FileHandle, Distance, MoveType, NewPointer);

HFILE            FileHandle;    /* File handle */
LONG             Distance;      /* Distance to move in bytes */
USHORT           MoveType;      /* Method of moving (0, 1, 2) */
PULONG           NewPointer;    /* New Pointer Location */

USHORT           rc;            /* return code */

Example 

This example opens file test.dat, writes some data, and resets the file 
pointer to the beginning of the file. 

#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

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


   Action = 2;
   strcpy(FileData, "Data...");
   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) */
      if(!DosWrite(FileHandle,           /* File handle */
                   (PVOID) FileData,     /* User buffer */
                   sizeof(FileData),     /* Buffer length */
                   &Wrote))              /* Bytes written */
         rc = DosChgFilePtr(FileHandle,  /* File handle */
                            MOVE_DIST,   /* Distance to move in bytes */
                            FILE_BEG,    /* Method of moving */
                            &Local);     /* New pointer location */