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_wait/[2,3]. Process references are
process_create/[2,3] if explicitly requested with
process/1 option. Process references are required in
order to obtain the exit status of a process
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:popen/3, See unsafe_system.)
datecommand in the standard shell ‘sh’. The output of the command is sent to the terminal:
| ?- process_create(path(sh), ['-c', date]).
datecommand in the standard shell ‘sh’. Wait for the command to terminate before returning to Prolog. Fail if the process gets an error. The output of the command is sent to the terminal:
| ?- process_create(path(sh), ['-c', date], [wait(exit(0))]).
wait/1 option in this way is a convenient way to
ensure that the command has finished before Prolog continues.
datecommand in the standard shell ‘sh’. The output of the command is received by Prolog:
| ?- 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' ?
datecommand to a file:
| ?- 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.
uniqcommand to the
| ?- 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).
makecommand with the -n (dry run) option, discarding output, fail if it does not succeed:
| ?- process_create(path(make), ['-n'], [stdout(null), wait(Exit)]), Exit = exit(0).
By using the
will not return until the subprocess has exited and its exit status is available.
lson a home directory in a subshell using the user’s preferred shell:
| ?- process_create('$SHELL', ['-c', [ls, ' ', file('~/') ]]).
popen('cat > ./myscript.sh',write,S)in SICStus 3. This example also shows one way to create a shell script which is useful when more advanced shell interaction is needed. (The created script outputs the most common line in its input. It is used in the next example.)
| ?- process_create(path(sh), ['-c', 'cat > ./myscript.sh && chmod a+x ./myscript.sh'], [stdin(pipe(S))]), write(S, '#! /bin/sh\n'), write(S, 'sort | uniq -c | sort -nr | head -n 1\n'), close(S).
Please read Quoting and Security for problems with this approach.
popen('./myscript.sh < ./somefile.txt',read,S)in SICStus 3.
| ?- open('somefile.txt',write,OF), write(OF,'hello\nworld\nhello\nhello\n'),close(OF), process_create(path(sh), ['-c', './myscript.sh < ./somefile.txt'], read_line(S, L), atom_codes(Line, L), close(S). …, Line = ' 3 hello' ?
Please read Quoting and Security for problems with this approach.
| ?- process_create(application(sicstus), ['-f', '--noinfo', '--nologo', '--goal', 'read(X), call(X), halt.'], [stdin(pipe(In)), stdout(pipe(Out))]), format(In,'~q .~n', [(length([h,e,l,l,o], Len), format('~q .~n', [Len]))]), close(In), read(Out,Answer), close(Out). …, Answer = 5
notepad.exeon a file C:/foo.txt under Windows:
| ?- process_create('$SYSTEMROOT/notepad.exe', [file('C:/foo.txt')]).
| ?- process_create('$COMSPEC',,[window(true)]).
On Windows, it is not possible to pass multiple parameters to a subprocess. When a subprocess is started, it receives exactly one argument and a quoting convention must be used to encode the parameters as the single argument actually passed to the process.
Unfortunately, there is no such universal quoting convention, every program can interpret its (single) argument in any way it sees fit.
Most programs use a convention established by the Microsoft C library.
This is the convention used by
process_create/[2,3] and it usually works well.
However, the command processor on Windows (
cmd.exe) does not use the common convention and,
except for very simple cases, passing arguments to
cmd.exe will not work reliably.
Please note: Passing arguments to
cmd.exe suffers from the same
security vulnerabilities as those described in Quoting and Security, below.
If you want to run commands using
cmd.exe, it is best to create a batch (‘.bat’)
file with your commands and then tell
cmd.exe to run the batch file.
The following example illustrates how to create a Windows batch file that pipes some output to a file
COMSPEC is an environment variable containing the path to
| ?- BatFileName='test.bat', open(BatFileName, write, S), write(S, 'date /T > "result.txt"\n'), close(S), process_create('$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
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))]).
process_create(+File, +Args, :Options)
Start a new process running the program identified by File
and the arguments specified in Args. The standard streams of
the new process can be redirected to prolog streams. The exit
status of the process can be obtained with
File, is expanded as if by
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
start the UNIX Bash shell you would specify
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:
The atom name is used as the expanded value. Some operating systems only support 7-bit ASCII characters here. Even when some larger subset of Unicode is used it may not work correctly with all programs.
File, an atom, is treated as a file name and subject to an operating system specific transformation to ensure file name syntax and character set is appropriate for the new process. This is especially important under Windows where it ensures that the full Windows Unicode character set can be used.
Please note: The File part of
file(File) is not subject to syntactic rewriting, the
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
Options is a list of options:
Each Spec specifies how the corresponding standard stream of the new process should be created. Spec can be one of:
The new process shares the (OS level) standard stream with the
Prolog process. This is the default.
Note that, especially under Windows, the Prolog process may not have
any OS level standard streams, or the OS streams may not be
connected to a console or terminal. In such a case you need to use
pipe/[1,2] spec, see below, and explicitly read (write) data
from (to) the process.
The stream is redirected to a null stream, i.e. a stream that discards written data and that is always at end of file when read.
pipe(Stream) since release 4.0
pipe(Stream, StreamOptions) since release 4.3.2
A new Prolog stream is created and connected to the corresponding stream
of the new process. StreamOptions is a list of options affecting the
created stream. The supported stream options are:
with the same meaning as for
open/4 (see mpg-ref-open).
The default, if no stream options are specified, is to use a text stream with the OS default character encoding.
This stream must be closed using
close/[1,2], it is not
closed automatically when the new process exits.
wait(-ExitStatus) since release 4.3
The call will not return until the sub-process has terminated.
ExitStatus will be bound to the exit status of the process,
as described for
Proc will be bound to a process reference that can be used
in calls to
process_wait/[2,3] etc.. This process reference
must be released, either explicitly with
It is often easier to use the
wait/1 option if you just want to
wait for the process to terminate.
Bool is either
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,
CWD is expanded as if by
is used as the working directory for the new process.
By default, the working directory is the same as the Prolog working directory.
Bool is either
false (the default). Specifies whether the process should
open in its own window.
window(true) may give unexpected results if the
standard stream options
stderr/1 are specified with anything but their default
Currently only implemented on Windows.
environment(Env) since release 4.1
Env is a list of
VAR=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 in the SICStus Prolog Manual, for more information.
process_wait(+Process, -ExitStatus, +Options)
Wait for a process to exit and obtain the exit status.
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:
The process has exited with exit code ExitCode. By convention processes use exit code zero to signify success and a (positive) non-zero value to specify failure.
UNIX only, the process was killed by signal
timeout/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
Options is a list of options:
Specify a maximum time, in seconds, to wait for the process to
terminate. Seconds should be an integer or floating point
number or the atom
infinite (the default) to specify
infinite wait. If the specified timeout interval passes before the
process_wait/3 exits with ExitStatus
timeout and the process reference is not released.
Currently the UNIX implementation supports only timeout values
0 (zero) and
Bool is either
true (the default) or
false. Specifies whether the process reference should be
process_wait/3 exits successfully.
Obtain the process identifier of the current (i.e. Prolog) process.
Obtain the process identifier of the process reference Process.
Returns true if Thing is a process reference that has not been released.
Release a process reference Process that has previously been
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
Send a signal to the process designated by Process. The signal can either be a non-negative integer or a signal name as an (all uppercase) atom.
The following signal names are accepted under UNIX if the platform
SIGKILL (the default),
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
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.