FS_CHGFILEPTR
Purpose
Move a file's logical read/write position pointer.
Calling Sequence
int far pascal FS_CHGFILEPTR(psffsi, psffsd, offset, type, IOflag) struct sffsi far * psffsi; struct sffsd far * psffsd; long offset; unsigned short type; unsigned short IOflag;
Where
psffsi is a pointer to the file-system-independent portion of an open file instance.
The FSD uses the current file size or sfi_position along with offset and type to compute a new sfi_position. This is updated by the system.
psffsd is a pointer to the file-system-dependent portion of an open file instance. The FSD may store or adjust data as appropriate in this structure.
offset is the signed offset to be added to the current file size or position to form the new position within the file.
| type | indicates the basis of a seek operation. |
|---|---|
| type == 0 | indicates seek relative to beginning of file. |
| type == 1 | indicates seek relative to current position within the file. |
| type == 2 | indicates seek relative to end of file. |
The value of type passed to the FSD will be valid.
| IOflag | indicates information about the operation on the handle. |
|---|---|
| IOflag == 0x0010 | indicates write-through. |
| IOflag == 0x0020 | indicates no-cache. |
Remarks
The file system may want to take the seek operation as a hint that an I/O operation is about to take place at the new position and initiate a positioning operation on sequential access media or read-ahead operation on other media.
Some DOS mode programs expect to be able to do a negative seek. OS/2 passes these requests on to the FSD and returns an error for OS/2 mode negative seek requests. Because a seek to a negative position is, effectively, a seek to a very large offset, it is suggested that the FSD return end-of-file for subsequent read requests.
FSDs must allow seeks to positions beyond end-of-file.
The information passed in IOflag is what was set for the handle during a DosOpen/DosOpen2 operation, or by a DosSetFHandState call.