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[PATCH] (perlfaq/bleadperl) append mode and locking

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Gwyn Judd
April 28, 2001 12:36
[PATCH] (perlfaq/bleadperl) append mode and locking
Message ID:
This is my first patch so go easy if it doesn't apply or something. This
is from a discussion on clpm. I didn't write any of it, Abigail and
nobull did, so whatever is right in here give them credit, whatever is
blatantly wrong, flame me mercilessly.

--- pod/perlfaq5.pod.old	Sat Apr 28 11:03:44 2001
+++ pod/perlfaq5.pod	Sun Apr 29 01:28:01 2001
@@ -723,6 +723,35 @@
 If the count doesn't impress your friends, then the code might.  :-)
+=head2 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 above code.
+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 garanteed 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 systems 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.
 =head2 How do I randomly update a binary file?
 If you're just trying to patch a binary, in many cases something as
--- pod/perlfaq.pod.old	Sat Apr 28 17:45:02 2001
+++ pod/perlfaq.pod	Sat Apr 28 17:48:54 2001
@@ -692,6 +692,11 @@
 =item *
+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?
+=item *
 How do I randomly update a binary file?
 =item *

Gwyn Judd (
Wives are young men's mistresses, companions for middle age, and old
men's nurses.
-Francis Bacon

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