library(process)This package contains utilities for process creation.
A process is represented by a process reference, a ground
compound term. Both SICStus and the operating system maintain a
state for each such process reference and they must therefore be
released, either explicitly with process_release/1 or
implicitly by process_wait/[2,3]. Process references are
created with process_create/[2,3] if explicitly requested with
the process/1 option. Process references are required in
order to obtain the exit status of a process
after process_create/[2,3] has returned.
Many of the predicates can accept a numeric operating system process id (“PID”) but since process ids are subject to re-use by the OS this is less reliable and does not work if the process has already exited.
The following illustrates some common tasks. The process library is portable and works on all supported platforms, including UNIX, Linux and Windows. However, the examples are by necessity platform dependent. Unless otherwise noted, the examples will work on UNIX and similar systems only.
(If you are looking for something like the old SICStus 3 system:system/1
and system:popen/3, See unsafe_system.)
| ?- process_create(path(sh),
['-c', date]).
| ?- process_create(path(sh),
['-c', date], [wait(exit(0))]).
Using wait/1 option in this way is a convenient way to
ensure that the command has finished before Prolog continues.
| ?- process_create(path(sh),
['-c', date], [stdout(pipe(S))]),
read_line(S,L), close(S), atom_codes(Date,L).
...,
Date = 'Fri Jan 24 12:59:26 CET 2014' ?
| ?- process_create(path(sh),
['-c', [date, '>', file('/tmp/foo.txt')]]).
| ?- process_create(path(wc), ['-w'],
[stdin(pipe(In)), stdout(pipe(Out))]),
write(In, 'a b c\n'), close(In),
read_line(Out, L), close(Out), number_codes(N, L).
...
N = 3
It may be preferable to let the input or output go via a file. This avoids deadlock in case the stream buffers fill up.
| ?- process_create(path(sh),
['-c', ['uniq ', file('/tmp/foo.txt'), ' | wc -w']],
[stdout(pipe(Out))]),
read_line(Out, L), close(Out), number_codes(N, L).
...
N = 6
Note that quoting is a problem (and potential security issue), so never pass untrusted data, like file names, to the shell using -c (see Quoting and Security).
| ?- process_create(path(make), ['-n'],
[stdout(null), wait(Exit)]),
Exit = exit(0).
By using the wait/1 option, process_create/3
will not return until the subprocess has exited and its exit status is available.
| ?- process_create('$SHELL', ['-c', [ls, ' ', file('~/') ]]).
popen('cat > ./myscript.sh',write,S) in SICStus 3.
This example also shows one way to create a shell script which is useful when
more advanced shell interaction is needed.
(The created script outputs the most common line in its input.
It is used in the next example.)
| ?- process_create(path(sh),
['-c',
'cat > ./myscript.sh && chmod a+x ./myscript.sh'],
[stdin(pipe(S))]),
write(S, '#! /bin/sh\n'),
write(S, 'sort | uniq -c | sort -nr | head -n 1\n'),
close(S).
Please read Quoting and Security for problems with this approach.
popen('./myscript.sh < ./somefile.txt',read,S) in SICStus 3.
| ?- open('somefile.txt',write,OF),
write(OF,'hello\nworld\nhello\nhello\n'),close(OF),
process_create(path(sh),
['-c', './myscript.sh < ./somefile.txt'],
read_line(S, L), atom_codes(Line, L), close(S).
...,
Line = ' 3 hello' ?
Please read Quoting and Security for problems with this approach.
| ?- process_create(application(sicstus),
['-f', '--noinfo', '--nologo',
'--goal', 'read(X), call(X), halt.'],
[stdin(pipe(In)), stdout(pipe(Out))]),
format(In,'~q .~n', [(length([h,e,l,l,o], Len),
format('~q .~n', [Len]))]),
close(In), read(Out,Answer), close(Out).
...,
Answer = 5
| ?- process_create('$SYSTEMROOT/notepad.exe',
[file('C:/foo.txt')]).
| ?- process_create('$COMSPEC',[],[window(true)]).
On Windows, it is not possible to pass multiple parameters to a subprocess. When a subprocess is started, it receives exactly one argument and a quoting convention must be used to encode the parameters as the single argument actually passed to the process.
Unfortunately, there is no such universal quoting convention, every program can interpret its (single) argument in any way it sees fit.
Most programs use a convention established by the Microsoft C library.
This is the convention used by process_create/[2,3] and it usually works well.
However, the command processor on Windows (cmd.exe) does not use the common convention and, except for very simple cases, passing arguments to cmd.exe will not work reliably.
Please note: Passing arguments to cmd.exe suffers from the same security vulnerabilities as those described in Quoting and Security, below.
If you want to run commands using cmd.exe, it is best to create a batch (‘.bat’) file with your commands and then tell cmd.exe to run the batch file.
The following example illustrates how to create a Windows batch file that pipes some output to a file (COMSPEC is an environment variable containing the path to cmd.exe):
| ?- BatFileName='test.bat',
open(BatFileName, write, S),
write(S, 'date /T > "result.txt"\n'), close(S),
process_create('$COMSPEC', ['/Q', '/C', file(BatFileName)],
[wait(exit(0))]),
open('result.txt', read, R),
read_line(R,L),close(R),atom_codes(Date,L).
...,
Date = '2014-01-27 ',
... ?
More recent versions of Windows come with a redesigned command line processor, ‘PowerShell’, which solves the problems associated with the traditional cmd.exe command line processor. In particular, it has a very general way to encode command line arguments, using ‘base-64’ encoding. Currently, there is no direct support for PowerShell in this library, but the following example shows how to get the current week day both using a plain text command and with a base-64-encoded command
| ?- Command = '(get-date).DayOfWeek',
process_create(path(powershell),
['-Command', Command],
[stdout(pipe(S))]),
read_line(S,L),atom_codes(Day,L).
...,
Day = 'Monday',
... ?
| ?- EncodedCommand =
'KABnAGUAdAAtAGQAYQB0AGUAKQAuAEQAYQB5AE8AZgBXAGUAZQBrAA==',
process_create(path(powershell),
['-encodedCommand', EncodedCommand],
[stdout(pipe(S))]),
read_line(S,L),atom_codes(Day,L).
...,
Day = 'Monday',
... ?
where the EncodedCommand value was created by encoding the string
'(get-date).DayOfWeek' using Base 64. See the PowerShell documentation for details.
It easy to get undesired, and possibly harmful, effects if arbitrary data is passed without proper quoting to a shell. For instance, accepting arbitrary file names and passing them as part of a command line to a subshell can cause the shell to execute arbitrary, possibly malicious, code.
The following, vulnerable, predicates suffer from this problem. They are similar to predicates that existed in SICStus 3, and their fragility is one of the reasons process interaction was redesigned in SICStus 4.
% DO NOT USE. This code is vulnerable.
% Similar to system:system/1 in SICStus 3.
unsafe_system(Cmd) :-
% pass Cmd to shell, wait for exit, fail on error.
process_create(path(sh), ['-c', Cmd], [wait(exit(0))]).
% DO NOT USE. This code is vulnerable.
% Similar to system:popen/3 in SICStus 3.
unsafe_popen(Cmd, Direction, Pipe) :-
% pass Cmd to shell, do not wait for exit,
% connect to stdin or stdout of subprocess.
( Direction == read ->
process_create(path(sh), ['-c', Cmd], [stdout(pipe(Pipe))])
; Direction == write ->
process_create(path(sh), ['-c', Cmd], [stdin(pipe(Pipe))])
).
Now consider the task of passing the contents of some file File to a command mycommand. You may think the following is a good idea (it is not!):
% DO NOT USE. This code is vulnerable.
unsafe_command(File, S) :-
atom_concat('./mycommand < ', File, Cmd),
unsafe_popen(Cmd, read, S).
That works as expected if the the File argument is a plain
file with no characters that has special meaning to the shell, e.g.
File = './somefile.txt',
unsafe_command(File, S), read_line(S,L),close(S).
However, assume that the file name was obtained from some untrusted source and consider the following example:
File = '$(say bohoo)',
unsafe_command(File, S), read_line(S,L),close(S).
depending on the system this can have a quite scary effect, and illustrates how shell meta characters in the constructed command line can lead to potentially dangerous results.
The safest way to interact with the shell is to create shell scripts and pass arguments to the scripts as separate arguments to the shell. E.g.
% A safer version
safer_command(File, S) :-
% pass the file as the first argument to mycommand.
process_create(path(sh),
['-c', file('./mycommand'), file(File)],
[stdout(pipe(S))]).
Exported predicates:
process_create(+File, +Args)process_create(+File, +Args, :Options)process_wait/[2,3].
File, is expanded as if by absolute_file_name/2
(with arguments access(execute) and file_type(executable)) and
is used to locate the file to execute.
The predefined file search path path/1 (see ref-fdi)
is especially useful here since it makes it easy to look up the
names of an executable in the directories mentioned by the
PATH environment variable. To run the Windows command shell
cmd you would simply specify path('cmd.exe') (or path(cmd)), to
start the UNIX Bash shell you would specify path(bash).
Args is a list of argument specifications. Each argument specification is either a simple argument specification, see below, or a non-empty list of simple argument specifications. The expanded value of each element of Args is concatenated to produce a single argument to the new process. A simple argument specification can be one of:
file(File)Please note: The File part of
file(File) is not subject to syntactic rewriting, the
argument specification file/1 only adjusts for differences
in file name syntax and character
encoding between SICStus and the operating system. You
must explicitly call
absolute_file_name/[2,3] if you want to expand file search
paths etc.
stdin(Spec)stdout(Spec)stderr(Spec)stdpipe/1 spec, see below, and explicitly read (write) data
from (to) the process.
nullpipe(Stream)close/[1,2], it is not
closed automatically when the new process exits.
wait(-ExitStatus) since release 4.3process_wait/2.
process(Proc)process_wait/[2,3] etc.. This process reference
must be released, either explicitly with process_release/1 or
implicitly by process_wait/[2,3].
It is often easier to use the wait/1 option if you just want to
wait for the process to terminate.
detached(Bool)true or false. Specifies
whether the new process should be “detached”, i.e. whether it
should be notified of terminal events such as ^C
interrupts. By default a new process is created detached if none
of the standard streams are specified, explicitly or implicitly,
as std.
cwd(CWD)absolute_file_name/2 and
is used as the working directory for the new process.
By default, the working directory is the same as the Prolog
working directory.
window(Bool)true or
false (the default). Specifies whether the process should
open in its own window.
Specifying window(true) may give unexpected results if the
standard stream options stdin/1, stdout/1 and
stderr/1 are specified with anything but their default
value std.
Currently only implemented on Windows.
environment(Env) since release 4.1=VALUE for extra
environment variables to pass to the sub-process in addition to the
default process environment.
VAR should be an atom.
VALUE should be an argument specification, as described above. The VALUE
is typically an atom but, especially on the Windows platform, it may be necessary to
wrap file names in file/1 to ensure file paths are converted to the native format.
See System Properties and Environment Variables, for more information.
process_wait(+Process, -ExitStatus)process_wait(+Process, -ExitStatus, +Options)Process is either a process reference obtained from
process_create/3 or an OS process identifier. Specifying a
process identifier is not reliable. The process identifier may
have been re-used by the operating system. Under Windows, it is not
possible to obtain the exit status using a process identifier if
the process has already exited.
ExitStatus is one of:
exit(ExitCode)killed(SignalNumber)SignalNumber (a
positive integer).
timeouttimeout/1 option was specified and the process did not
exit within the specified interval. In this case the process
reference is not released, even if the release/1 option is
specified.
timeout(Seconds)infinite (the default) to specify
infinite wait. If the specified timeout interval passes before the
process exits, process_wait/3 exits with ExitStatus
set to timeout and the process reference is not released.
Currently the UNIX implementation supports only timeout values
0 (zero) and infinite.
release(Bool)true (the default) or
false. Specifies whether the process reference should be
released when process_wait/3 exits successfully.
process_id(-PID)process_id(+Process, -PID)is_process(+Thing)process_release(+Process)process_create/3. This ensures that Prolog
and the operating system can reclaim any resources associated with
the process reference.
Usually you would not call this. Either do not request the process
reference when calling process_create/3 or let
process_wait/[2,3] reclaim the process reference when the
process terminates.
process_kill(+Process)process_kill(+Process, +SignalSpec)The following signal names are accepted under UNIX if the platform
defines them: SIGABRT, SIGALRM, SIGBUS,
SIGCHLD, SIGCONT, SIGFPE, SIGHUP,
SIGILL, SIGINT, SIGKILL (the default),
SIGPIPE, SIGPOLL, SIGPROF, SIGQUIT,
SIGSEGV, SIGSTOP, SIGSYS, SIGTERM,
SIGTRAP, SIGTSTP, SIGTTIN, SIGTTOU,
SIGURG, SIGUSR1, SIGUSR2, SIGVTALRM,
SIGXCPU and SIGXFSZ. However, many of these do not
make sense to send as signals.
Under Windows, which does not have the signal
concept, the signal name SIGKILL (the default) is treated
specially and terminates the process with
TerminateProcess(Process, -1).
Please note: Using process_kill/[2,3] on Windows
is not recommended. Also, on Windows, the call may throw an error
if the process has already exited.