10.27 Process Utilities—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.

10.27.1 Examples

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.)

  1. Run the date command in the standard shell ‘sh’. The output of the command is sent to the terminal:
    | ?- process_create(path(sh),
         ['-c', date]).
  2. Run the date command 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))]).

    Using wait/1 option in this way is a convenient way to ensure that the command has finished before Prolog continues.

  3. Run the date command 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' ? 
  4. Pipe the output of the date command to a file:
    | ?- process_create(path(sh),
         ['-c', [date, '>', file('/tmp/foo.txt')]]).
  5. Count the number of words in an atom, using the wc command:
    | ?- 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.

  6. Count the number of unique words in a file, piping the output of the uniq command to the wc command:
    | ?- process_create(path(sh),
         ['-c', ['uniq ', file('/tmp/foo.txt'), ' | wc -w']],
         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).

  7. Run the make command 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 wait/1 option, process_create/3 will not return until the subprocess has exited and its exit status is available.

  8. Run ls on a home directory in a subshell using the user’s preferred shell:
    | ?- process_create('$SHELL', ['-c', [ls, ' ', file('~/') ]]).
  9. Run a command with output piped from a file and input provided by Prolog. This is similar to 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),
         'cat > ./myscript.sh && chmod a+x ./myscript.sh'],
         write(S, '#! /bin/sh\n'),
         write(S, 'sort | uniq -c | sort -nr | head -n 1\n'),

    Please read Quoting and Security for problems with this approach.

  10. Run a shell script with input piped from a file and output read by Prolog. This is similar to popen('./myscript.sh < ./somefile.txt',read,S) in SICStus 3.
    | ?- open('somefile.txt',write,OF),
         ['-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.

  11. Run a goal in a SICStus subprocess (UNIX and Windows):
    | ?- 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
  12. Run notepad.exe on a file C:/foo.txt under Windows:
    | ?- process_create('$SYSTEMROOT/notepad.exe',
  13. Open a command shell in a separate window under Windows:
    | ?- process_create('$COMSPEC',[],[window(true)]). Microsoft Windows Shell

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)],
     open('result.txt', read, R),
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',
     ['-Command', Command],
Day = 'Monday',
… ?
| ?- EncodedCommand =
     ['-encodedCommand', EncodedCommand],
Day = 'Monday',
… ? 

where the EncodedCommand value was created by encoding the string '(get-date).DayOfWeek' using Base 64. See the PowerShell documentation for details.

10.27.2 Quoting and Security

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.
                  ['-c', file('./mycommand'), file(File)],

Exported predicates:

process_create(+File, +Args)
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 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:

an atom

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 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.

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: type/1, eol/1, and encoding/1, 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 process_wait/2.


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 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.


Bool is either 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 is expanded as if by 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.


Bool is either 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

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)
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 SignalNumber (a positive integer).


The 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 specified.

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 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.


Bool is either true (the default) or false. Specifies whether the process reference should be released when process_wait/3 exits successfully.


Obtain the process identifier of the current (i.e. Prolog) process.

process_id(+Process, -PID)

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 obtained from 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, +SignalSpec)

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 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.

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