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 use locking?
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From: PerlFAQ Server on 24 Jun 2010 18:00 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. -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, 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. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |