If you open a file with O_APPEND (which is what >> does) then each write to the file is preceded with a seek to the current end of the file. The seek and write are atomic, provided that you are writing less than a block of data (might work for more than a block, I disremember). The size of a block varies from one filesystem to another and depends on the platform, the size of the filesystem, the options with which the fs was created, the phase of the moon, and so on. It is *typically* of the order of 1 to 4 K.

Be aware that this race condition *does* exist in NFS. Which doesn't provide adequate locking to get around it. Yay.

In reply to Re: Re: Re: Add a line to the end of a file by DrHyde
in thread Add a line to the end of a file by ecuguru

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.