- sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are
to set the new position to POSITION;
1to set the it to the current position plus POSITION; and
2to set it to EOF plus POSITION, typically negative.
Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the
:encoding(UTF-8)I/O layer), the seek, tell, and sysseek family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file.
For WHENCE, you may also use the constants
SEEK_END(start of the file, current position, end of the file) from the Fcntl module. Use of the constants is also more portable than relying on 0, 1, and 2. For example to define a "systell" function:
Returns the new position, or the undefined value on failure. A position of zero is returned as the string
"0 but true"; thus sysseek returns true on success and false on failure, yet you can still easily determine the new position.