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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
#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 */
MASM Binding
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
Note
Text based on http://www.edm2.com/index.php/DosChgFilePtr