FAQ 5.24 All I want to do is append a small amount of text to the end of a file. Do I still have to

Discussion in 'Perl Misc' started by PerlFAQ Server, Mar 31, 2011.

  1. This is an excerpt from the latest version perlfaq5.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    5.24: All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?

    If you are on a system that correctly implements "flock" and you use the
    example appending code from "perldoc -f flock" everything will be OK
    even if the OS you are on doesn't implement append mode correctly (if
    such a system exists.) So if you are happy to restrict yourself to OSs
    that implement "flock" (and that's not really much of a restriction)
    then that is what you should do.

    If you know you are only going to use a system that does correctly
    implement appending (i.e. not Win32) then you can omit the "seek" from
    the code in the previous answer.

    If you know you are only writing code to run on an OS and filesystem
    that does implement append mode correctly (a local filesystem on a
    modern Unix for example), and you keep the file in block-buffered mode
    and you write less than one buffer-full of output between each manual
    flushing of the buffer then each bufferload is almost guaranteed to be
    written to the end of the file in one chunk without getting intermingled
    with anyone else's output. You can also use the "syswrite" function
    which is simply a wrapper around your system's write(2) system call.

    There is still a small theoretical chance that a signal will interrupt
    the system level "write()" operation before completion. There is also a
    possibility that some STDIO implementations may call multiple system
    level "write()"s even if the buffer was empty to start. There may be
    some systems where this probability is reduced to zero, and this is not
    a concern when using ":perlio" instead of your system's STDIO.



    --------------------------------------------------------------------

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    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
    operating system or platform, so please include relevant details for
    corrections to examples that do not work on particular platforms.
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    PerlFAQ Server, Mar 31, 2011
    #1
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