SetFPos
SetFPos Position file mark for random-access read/write short fRefNum ; file reference, as obtained via FSOpen short posMode ; 1=from start, 2=from end, 3=from mark
long posOffset ; file offset, absolute pos depends on posMode
SetFPos sets the position of the file mark (the file position at which the next read or write operation will start).
fRefNum is the reference number of an open file. See FSOpen and OpenRF. posMode specifies the method by which the file pointer will be moved. The
following constants are defined in Files.h:
fsAtMark 0 Remain at current mark (posOffset is ignored)
fsFromStart 1 Move to absolute file position in posOffset
fsFromLEOF 2 Move posOffset bytes from logical end of file
fsFromMark 3 Move posOffset bytes from current position
posOffset is a signed long integer (positive or negative); it identifies how far
to move the file mark. The resulting absolute file position will
depend upon the method specified by posMode.
noErr (0) No error
eofErr (-39) Attempt tp position past the end-of-file
extFSErr (-58) External file system
fnOpnErr (-38) File not open
ioErr (-36) I/O error
posErr (-40) Can't position to before start of file
rfNumErr (-51) Bad fRefNum
Notes: SetFPos (LSEEK to UNIX fans) is used in random-access disk operations to position the file mark to a specified position in order to read from or
write to a selected position in the file. This function is not needed in
sequential file I/O, since the file mark is updated automatically via FSRead The posOffset parameter may be positive or negative. If the combination of posMode and a positive posOffset would move past the end of the file, the
mark is set to the EOF and eofErr is returned. If you attempt to position the
file mark before the start of the file (i.e., while using a negative value in
posOffset ) posErr is returned.
If you wish to append records to the end of the file, simply use
posMode=fsFromLEOF and posOffset=0 (see FSWrite for an example). If you need to seek beyond the end of the file, you can use Allocate or SetEOF to add empty space to the end of the file before using SetFPos. Some examples:
FSOpen( "\pHardDisk:MyFile", 0, &fRef ); FSRead( fRef, 20, myBuf ); /* mark now points to 21st byte */ SetFPos( fRef,fsFromStart,0 ); /* rewind to start of file */ SetFPos( fRef,fsFromLEOF,0 ); /* prepare to append to file */ SetFPos( fRef,fsFromMark,-10 ); /* rewind by 10 bytes */ newRecAddr = GetNameHashAddr( theName ); /* calc record address */
err = SetFPos( fRef, fsFromStart, newRecAddr ); if (err == eofErr) { /* file is too small */
SetEOF( fRef,newRecAddr+REC_SIZE ); /* increase file size */ SetFPos( fRef, fsFromStart, newRecAddr ); }
Note: The fsAtMark constant is meaningless here, but it is needed in the