Programming Perl, 4th Edition

sysseek

sysseek FILEHANDLE, POSITION, WHENCE

This function sets FILEHANDLE’s system position using the syscall lseek(2). It bypasses standard I/O, so mixing this with reads (other than sysread), print, write, seek, tell, or eof may (and probably shall) cause confusion. 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 bytes into the file, 1 to set it to the current position plus POSITION, and 2 to set it to EOF plus POSITION bytes (typically negative). For WHENCE, you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END from the standard IO::Seekable and POSIX modules, or from the Fcntl module, which is more portable and convenient.

This function returns the new position in bytes, or undef on failure. A position of zero is returned as the special string “0 but true”, which can be used numerically without producing warnings or having to mess around with // die instead of the more customary || die.

Warning: POSITION is in bytes not characters, no matter whether there should happen to be any encoding layer on the filehandle. However, all functions in Perl that read from files do go through any encoding layer, and you can therefore read a partial “character” and wind up with an invalid Perl string. ...