This is an old revision of the document!
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:
'Value Definition'
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.
Example Code
C Binding
<PRE> #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 */ </PRE> This example opens file test.dat, writes some data, and resets the file pointer to the beginning of the file. <PRE> #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 */
</PRE>
MASM Binding
<PRE> 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 </PRE>
Note
Text based on http://www.edm2.com/index.php/DosChgFilePtr