Author image Nicholas Clark
and 1 contributors


PerlIO::subfile - Perl extension to provide a PerlIO layer to pretend a subsection of a file is a whole regular file.


  use PerlIO::subfile;
  open FOO, "<:subfile(start=123,end=+43)", "bigfile" or die $!;
  print while <FOO>; # Just prints the specified subsection of the file
  seek (FOO, 0, SEEK_SET) # Takes you to offset 123 in bigfile.


zip files (and other archive files) contain multiple other files within them. If the contained file is stored uncompressed then it would be nice to save having to copying it out before a program accesses it. Instead, it would be nice to give a file handle onto the subfile which behaves as if the temporary copy has taken place, but actually reads from the original zip. Basically all you need to do is nobble seek and tell so that file offsets are given from the start of the contained file (and you can't seek your way outside the bounds), and nobble read so that eof happens in the right place.


PerlIO::subfile exports no subroutines or symbols, just a perl layer subfile


The subfile layer takes a comma separated list of arguments. The value will be treated as a hexadecimal number if it starts Ox, octal if it starts with O followed by a digit, decimal in other cases. (Or whatever your C library's strtoul function things is valid) Values can be preceded with + or - to treat them as relative, otherwise they are taken as absolute.


Start of the subfile within the whole file. An absolute value is taken as a file position, and immediately causes seek to that position (using SEEK_SET)). A relative value is taken as a value relative to the current position, and immediately causes a seek using SEEK_CUR)).

Omitting start will cause the subfile to start at the current file position.


End of the subfile within the whole file. An absolute value is taken as an absolute file position (in the parent file). A relative value is taken as relative to the (current) start.

The absolute value 0 (zero) is taken as "unbounded" - you can read to the end of file on the parent file.

Arguments are parsed left to right, so it's possible to specify a range as

    # end 8 bytes beyond current file position, start at byte 4 of file

If you're writing a perl extension in XS, calling PerlIOSubfile_pushed with an SV that is SvIOK and not SvPOK, then the argument is treated as a relative end= value. This behaviour probably isn't accessible from the perl language level. ex::lib::zip uses this.


This is a lazy implementation. It adds a whole extra (unneeded) layer of buffering. There ought to be a total re-write to make most methods just call the parent, with (probably) only read and seek suitably clipped. Then again, as I'm using it for zip files, how many files in zips are stored rather than deflated?

It also doesn't do write. Mainly because I have no need for writes at this time.


Nicholas Clark, <>