If rogue chunk is e.g. 7 bytes long, then unpacking with the proposed template will die on "X", so wrapping into eval is required anyway if data are unreliable.

However, 'x' doesn't have a direct effect on the output, so a partial match involving 'x' must be communicated through an error.

Looks to me like an attempt to whitewash inconsistent Perl's behaviour :-), By similar reasoning, failure to unpack e.g. Pascal strings (as "unpack 'C/a', qq(\03ab)") should be fatal, I think.

Side-note: PNG tags were made human-readable for a good reason, so perhaps "A4" instead of "N" (or "L") will serve better. E.g., if data are super-reliable (CRC sums to be ignored), then chunks can be read into a hash:

my ( $head, %chunks ) = unpack 'a8 (x4 A4 X8 N x4 /a x4)*', $input;
say for keys %chunks;

[download]

In reply to Re^4: Seeking with 'x' in unpack and out of bounds reads by vr
in thread Seeking with 'x' in unpack and out of bounds reads by mxb

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.