#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 0 to set the new position to POSITION; 1 to set it to the current position plus POSITION; and 2 to 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.

sysseek bypasses normal buffered IO, so mixing it with reads other than sysread (for example readline or read), print, write, seek, tell, or eof may cause confusion.

For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR, and 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:

use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }

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.

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