DosChgFilePtr

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

Syntax

DosChgFilePtr (FileHandle, Distance, MoveType, NewPointer)

Parameters

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:

0 - Beginning of the file.

1 - Current location of the read/write pointer.

2 - End of the file. Use this method to determine a file's size.

NewPointer (PULONG) - output

Address of the new pointer location.

Return Code

rc (USHORT) - return

Return code descriptions are:

• 0 NO_ERROR
• 1 ERROR_INVALID_FUNCTION
• 6 ERROR_INVALID_HANDLE

Remarks

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.

Bindings

C

#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 */

MASM

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

Example 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 */