DosChgFilePtr

This call moves the read/write pointer in accordance with the type of move specified.

DosChgFilePtr (FileHandle, Distance, MoveType, NewPointer)

• FileHandle (HFILE) - input : Handle returned by a previous DosOpen call.
• Distance (LONG) - input : The offset to move, in bytes.
• MoveType (USHORT) - input : Method of moving. Specifies a location in the file from where Distance to move the read/write pointer starts. Values and their meanings are:

• NewPointer (PULONG) - output : Address of the new pointer location.

rc (USHORT) - return

Return code descriptions are:

• 0 NO_ERROR
• 1 ERROR_INVALID_FUNCTION
• 6 ERROR_INVALID_HANDLE

The read/write pointer in a file is a signed 32-bit number. A negative value moves the pointer backward in the file. A positive value moves the pointer forward. DosChgFilePtr cannot be used to seek to a negative position in the file.

This call may not be used for a character device or pipe.

#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 */

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 */

EXTRN  DosChgFilePtr:FAR
INCL_DOSFILEMGR     EQU 1
 
PUSH   WORD    FileHandle    ;File handle
PUSH   DWORD   Distance      ;Distance to move in bytes
PUSH   WORD    MoveType      ;Method of moving (0, 1, 2)
PUSH@  DWORD   NewPointer    ;New Pointer Location (returned)
CALL   DosChgFilePtr

Returns WORD

Text based on http://www.edm2.com/index.php/DosChgFilePtr