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27754 plus NEWS change: add "zsystem flock"

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Peter Stephenson 2010-02-24 21:37:24 +00:00
parent 48315b019b
commit bec3de98df
7 changed files with 284 additions and 6 deletions
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43
Doc/Zsh/mod_system.yo
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@ -1,8 +1,8 @@
COMMENT(!MOD!zsh/system
A builtin interface to various low-level system features.
!MOD!)
The tt(zsh/system) module makes available three builtin commands and
two parameters.
The tt(zsh/system) module makes available various builtin commands and
parameters.
subsect(Builtins)
@ -109,6 +109,45 @@ to the command, or 2 for an error on the write; no error message is
printed in the last case, but the parameter tt(ERRNO) will reflect
the error that occurred.
)
xitem(tt(zsystem flock [ -t) var(timeout) tt(] [ -f) var(var) tt(] [-er]) var(file))
item(tt(zsystem flock -u) var(fd_expr))(
The builtin tt(zsystem)'s subcommand tt(flock) performs advisory file
locking (via the manref(fcntl)(2) system call) over the entire contents
of the given file. This form of locking requires the processes
accessing the file to cooperate; its most obvious use is between two
instances of the shell itself.
In the first form the named var(file), which must already exist, is
locked by opening a file descriptor to the file and applying a lock to
the file descriptor. The lock terminates when the shell process that
created the lock exits; it is therefore often convenient to create file
locks within subshells, since the lock is automatically released when
the subshell exits. Status 0 is returned if the lock succeeds, else
status 1.
In the second form the file descriptor given by the arithmetic
expression tt(fd_expr) is closed, releasing a lock. The file descriptor
can be queried by using the `tt(-f) var(var)' form during the lock;
on a successful lock, the shell variable var(var) is set to the file
descriptor used for locking. The lock will be released if the
file descriptor is closed by any other means, for example using
`tt(exec {)var(var)tt(}>&-)'; however, the form described here performs
a safety check that the file descriptor is in use for file locking.
By default the shell waits indefinitely for the lock to succeed.
The option tt(-t) var(timeout) specifies a timeout for the lock in
seconds; currently this must be an integer. The shell will attempt
to lock the file once a second during this period. If the attempt
times out, status 2 is returned.
If the option tt(-e) is given, the file descriptor for the lock is
preserved when the shell uses tt(exec) to start a new process;
otherwise it is closed at that point and the lock released.
If the option tt(-r) is given, the lock is only for reading, otherwise
it is for reading and writing. The file descriptor is opened
accordingly.
)
enditem()
subsect(Parameters)