17 library(unix): Unix specific operations

See also

library(process) provides a portable high level interface to create and manage processes.

The library(unix) library provides the commonly used Unix primitives to deal with process management. These primitives are useful for many tasks, including server management, parallel computation, exploiting and controlling other processes, etc.

The predicates in this library are modelled closely after their native Unix counterparts.

[det]fork(-Pid)

Clone the current process into two branches. In the child, Pid is unified to child. In the original process, Pid is unified to the process identifier of the created child. Both parent and child are fully functional Prolog processes running the same program. The processes share open I/O streams that refer to Unix native streams, such as files, sockets and pipes. Data is not shared, though on most Unix systems data is initially shared and duplicated only if one of the programs attempts to modify the data.

Unix fork() is the only way to create new processes and fork/1 is a simple direct interface to it.

Errors

permission_error(fork, process, main) is raised if the calling thread is not the only thread in the process. Forking a Prolog process with threads will typically deadlock because only the calling thread is cloned in the fork, while all thread synchronization are cloned.

[det]fork_exec(+Command)

Fork (as fork/1) and exec (using exec/1) the child immediately. This behaves as the code below, but bypasses the check for the existence of other threads because this is a safe scenario.

fork_exec(Command) :-
      (   fork(child)
      ->  exec(Command)
      ;   true
      ).

exec(+Command)

Replace the running program by starting Command. Command is a callable term. The functor is the command and the arguments provide the command-line arguments for the command. Each command-line argument must be atomic and is converted to a string before passed to the Unix call execvp(). Here are some examples:

• exec(ls('-l'))
• exec('/bin/ls'('-l', '/home/jan'))

Unix exec() is the only way to start an executable file executing. It is commonly used together with fork/1. For example to start netscape on an URL in the background, do:

run_netscape(URL) :-
        (    fork(child),
             exec(netscape(URL))
        ;    true
        ).

Using this code, netscape remains part of the process-group of the invoking Prolog process and Prolog does not wait for netscape to terminate. The predicate wait/2 allows waiting for a child, while detach_IO/0 disconnects the child as a deamon process.

[det]wait(?Pid, -Status)

Wait for a child to change status. Then report the child that changed status as well as the reason. If Pid is bound on entry then the status of the specified child is reported. If not, then the status of any child is reported. Status is unified with exited(ExitCode) if the child with pid Pid was terminated by calling exit() (Prolog halt/1). ExitCode is the return status. Status is unified with signaled(Signal) if the child died due to a software interrupt (see kill/2). Signal contains the signal number. Finally, if the process suspended execution due to a signal, Status is unified with stopped(Signal).

[det]kill(+Pid, +Signal)

Deliver a software interrupt to the process with identifier Pid using software-interrupt number Signal. See also on_signal/2. Signals can be specified as an integer or signal name, where signal names are derived from the C constant by dropping the SIG prefix and mapping to lowercase. E.g. int is the same as SIGINT in C. The meaning of the signal numbers can be found in the Unix manual.

[det]pipe(-InSream, -OutStream)

Create a communication-pipe. This is normally used to make a child communicate to its parent. After pipe/2, the process is cloned and, depending on the desired direction, both processes close the end of the pipe they do not use. Then they use the remaining stream to communicate. Here is a simple example:

:- use_module(library(unix)).

fork_demo(Result) :-
        pipe(Read, Write),
        fork(Pid),
        (   Pid == child
        ->  close(Read),
            format(Write, '~q.~n',
                   [hello(world)]),
            flush_output(Write),
            halt
        ;   close(Write),
            read(Read, Result),
            close(Read)
        ).

[det]dup(+FromStream, +ToStream)

Interface to Unix dup2(), copying the underlying filedescriptor and thus making both streams point to the same underlying object. This is normally used together with fork/1 and pipe/2 to talk to an external program that is designed to communicate using standard I/O.

Both FromStream and ToStream either refer to a Prolog stream or an integer descriptor number to refer directly to OS descriptors. See also demo/pipe.pl in the source-distribution of this package.

[det]detach_IO(+Stream)

This predicate is intended to create Unix deamon processes. It performs two actions.

1. The I/O streams user_input, user_output and user_error are closed if they are connected to a terminal (see tty property in stream_property/2). Input streams are rebound to a dummy stream that returns EOF. Output streams are reboud to forward their output to Stream.
2. The process is detached from the current process-group and its controlling terminal. This is achieved using setsid() if provided or using ioctl() TIOCNOTTY on /dev/tty.

To ignore all output, it may be rebound to a null stream. For example:

      ...,
      open_null_stream(Out),
      detach_IO(Out).

The detach_IO/1 should be called only once per process. Subsequent calls silently succeed without any side effects.

See also

detach_IO/0 and library(syslog).

[det]detach_IO

Detach I/O similar to detach_IO/1. The output streams are bound to a file /tmp/pl-out.<pid>. Output is line buffered (see set_stream/2).

See also

library(syslog) allows for sending output to the Unix logging service.

Compatibility

Older versions of this predicate only created this file if there was output.

[det]prctl(+Option)

Access to Linux process control operations. Defines values for Option are:

set_dumpable(+Boolean)

Control whether the process is allowed to dump core. This right is dropped under several uid and gid conditions.

get_dumpable(-Boolean)

Get the value of the dumpable flag.

[semidet]sysconf(+Conf)

Access system configuration. See sysconf(1) for details. Conf is a term Config(Value), where Value is always an integer. Config is the sysconf() name after removing =_SC_= and conversion to lowercase. Currently support the following configuration info: arg_max, child_max, clk_tck, open_max, pagesize, phys_pages, avphys_pages, nprocessors_conf and nprocessors_onln. Note that not all values may be supported on all operating systems.