/* Part of SWI-Prolog
Author: Jan Wielemaker
Copyright (c) 2008-2018, University of Amsterdam
VU University Amsterdam
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[ process_create/3, % +Exe, +Args, +Options
process_wait/2, % +PID, -Status
process_wait/3, % +PID, -Status, +Options
process_id/1, % -PID
process_id/2, % +Process, -PID
is_process/1, % +PID
process_release/1, % +PID
process_kill/1, % +PID
process_group_kill/1, % +PID
process_group_kill/2, % +PID, +Signal
process_kill/2, % +PID, +Signal
process_set_method/1 % +CreateMethod
:- predicate_options(process_create/3, 3,
/** <module> Create processes and redirect I/O
The module library(process) implements interaction with child processes
and unifies older interfaces such as shell/[1,2], open(pipe(command),
...) etc. This library is modelled after SICStus 4.
The main interface is formed by process_create/3. If the process id is
requested the process must be waited for using process_wait/2. Otherwise
the process resources are reclaimed automatically.
In addition to the predicates, this module defines a file search path
(see user:file_search_path/2 and absolute_file_name/3) named =path= that
locates files on the system's search path for executables. E.g. the
following finds the executable for =ls=:
?- absolute_file_name(path(ls), Path, [access(execute)]).
*|Incompatibilities and current limitations|*
* Where SICStus distinguishes between an internal process id and
the OS process id, this implementation does not make this
distinction. This implies that is_process/1 is incomplete and
* SICStus only supports ISO 8859-1 (latin-1). This implementation
supports arbitrary OS multibyte interaction using the default
* It is unclear what the detached(true) option is supposed to do. Disable
signals in the child? Use setsid() to detach from the session? The
current implementation uses setsid() on Unix systems.
* An extra option env([Name=Value, ...]) is added to
process_create/3. As of version 4.1 SICStus added
environment(List) which _modifies_ the environment. A
compatible option was added to SWI-Prolog 7.7.23.
@tbd Implement detached option in process_create/3
@compat SICStus 4
%! process_create(+Exe, +Args:list, +Options) is det.
% Create a new process running the file Exe and using arguments
% from the given list. Exe is a file specification as handed to
% absolute_file_name/3. Typically one use the =path= file alias to
% specify an executable file on the current PATH. Args is a list
% of arguments that are handed to the new process. On Unix
% systems, each element in the list becomes a seperate argument in
% the new process. In Windows, the arguments are simply
% concatenated to form the commandline. Each argument itself is
% either a primitive or a list of primitives. A primitive is
% either atomic or a term file(Spec). Using file(Spec), the system
% inserts a filename using the OS filename conventions which is
% properly quoted if needed.
% * stdin(Spec)
% * stdout(Spec)
% * stderr(Spec)
% Bind the standard streams of the new process. Spec is one of
% the terms below. If pipe(Pipe) is used, the Prolog stream is
% a stream in text-mode using the encoding of the default
% locale. The encoding can be changed using set_stream/2.
% The options =stdout= and =stderr= may use the same stream,
% in which case both output streams are connected to the same
% Prolog stream.
% * std
% Just share with the Prolog I/O streams
% * null
% Bind to a _null_ stream. Reading from such a stream
% returns end-of-file, writing produces no output
% * pipe(-Stream)
% Attach input and/or output to a Prolog stream.
% * cwd(+Directory)
% Run the new process in Directory. Directory can be a
% compound specification, which is converted using
% * env(+List)
% As environment(List), but _only_ the specified variables
% are passed, i.e., no variables are _inherited_.
% * environment(+List)
% Specify _additional_ environment variables for the new process.
% List is a list of `Name=Value` terms, where `Value` is expanded
% the same way as the Args argument. If neither `env` nor
% `environment` is passed the environment is inherited from the
% Prolog process.
% * process(-PID)
% Unify PID with the process id of the created process.
% * detached(+Bool)
% In Unix: If =true=, detach the process from the terminal
% Currently mapped to setsid();
% Also creates a new process group for the child
% In Windows: If =true=, detach the process from the current
% job via the CREATE_BREAKAWAY_FROM_JOB flag. In Vista and beyond,
% processes launched from the shell directly have the 'compatibility
% assistant' attached to them automatically unless they have a UAC
% manifest embedded in them. This means that you will get a
% permission denied error if you try and assign the newly-created
% PID to a job you create yourself.
% * window(+Bool)
% If =true=, create a window for the process (Windows only)
% * priority(+Priority)
% In Unix: specifies the process priority for the newly
% created process. Priority must be an integer between -20
% and 19. Positive values are nicer to others, and negative
% values are less so. The default is zero. Users are free to
% lower their own priority. Only the super-user may _raise_ it
% to less-than zero.
% If the user specifies the process(-PID) option, he *must* call
% process_wait/2 to reclaim the process. Without this option, the
% system will wait for completion of the process after the last
% pipe stream is closed.
% If the process is not waited for, it must succeed with status 0.
% If not, an process_error is raised.
% *|Windows notes|*
% On Windows this call is an interface to the CreateProcess() API.
% The commandline consists of the basename of Exe and the
% arguments formed from Args. Arguments are separated by a single
% space. If all characters satisfy iswalnum() it is unquoted. If
% the argument contains a double-quote it is quoted using single
% quotes. If both single and double quotes appear a domain_error
% is raised, otherwise double-quote are used.
% The CreateProcess() API has many options. Currently only the
% =CREATE_NO_WINDOW= options is supported through the
% window(+Bool) option. If omitted, the default is to use this
% option if the application has no console. Future versions are
% likely to support more window specific options and replace
% First, a very simple example that behaves the same as
% =|shell('ls -l')|=, except for error handling:
% ?- process_create(path(ls), ['-l'], ).
% The following example uses grep to find all matching lines in a
% grep(File, Pattern, Lines) :-
% process_create(path(grep), [ Pattern, file(File) ],
% [ stdout(pipe(Out))
% read_lines(Out, Lines),
% read_lines(Out, Lines) :-
% read_line_to_codes(Out, Line1),
% read_lines(Line1, Out, Lines).
% read_lines(end_of_file, _, ) :- !.
% read_lines(Codes, Out, [Line|Lines]) :-
% atom_codes(Line, Codes),
% read_line_to_codes(Out, Line2),
% read_lines(Line2, Out, Lines).
% @error process_error(Exe, Status) where Status is one of
% exit(Code) or killed(Signal). Raised if the process
% is waited for (i.e., Options does not include
% process(-PID)), and does not exit with status 0.
% @bug On Windows, environment(List) is handled as env(List),
% i.e., the environment is not inherited.
process_create(Exe, Args, Options) :-
absolute_file_name(Exe, PlProg, ExeOptions),
maplist(map_arg, Args, Av),
Term =.. [Prog|Av],
expand_env_option(env, Options1, Options2),
expand_env_option(environment, Options2, Options3),
Options = [ extensions(['',exe,com]), access(read) ].
Options = [ access(execute) ].
expand_cwd_option(Options0, Options) :-
select_option(cwd(Spec), Options0, Options1),
-> absolute_file_name(Spec, PlDir, [file_type(directory), access(read)]),
Options = [cwd(Dir)|Options1]
-> Options = Options0
; existence_error(directory, Spec)
expand_env_option(Name, Options0, Options) :-
Term =.. [Name,Value0],
select_option(Term, Options0, Options1),
maplist(map_env, Value0, Value),
NewOption =.. [Name,Value],
Options = [NewOption|Options1].
expand_env_option(_, Options, Options).
map_env(Name=Value0, Name=Value) :-
%! map_arg(+ArgIn, -Arg) is det.
% Map an individual argument. Primitives are either file(Spec) or
% an atomic value (atom, string, number). If ArgIn is a non-empty
% list, all elements are converted and the results are
map_arg(, ) :- !.
map_arg(List, Arg) :-
maplist(map_arg_prim, List, Prims),
map_arg(Prim, Arg) :-
map_arg_prim(file(Spec), File) :-
-> absolute_file_name(Spec, PlFile)
; PlFile = Spec
%! process_id(-PID) is det.
% True if PID is the process id of the running Prolog process.
% @deprecated Use current_prolog_flag(pid, PID)
%! process_id(+Process, -PID) is det.
% PID is the process id of Process. Given that they are united in
% SWI-Prolog, this is a simple unify.
%! is_process(+PID) is semidet.
% True if PID might be a process. Succeeds for any positive
PID > 0.
% Release process handle. In this implementation this is the same
% as process_wait(PID, _).
%! process_wait(+PID, -Status) is det.
%! process_wait(+PID, -Status, +Options) is det.
% True if PID completed with Status. This call normally blocks
% until the process is finished. Options:
% * timeout(+Timeout)
% Default: =infinite=. If this option is a number, the
% waits for a maximum of Timeout seconds and unifies Status
% with =timeout= if the process does not terminate within
% Timeout. In this case PID is _not_ invalidated. On Unix
% systems only timeout 0 and =infinite= are supported. A
% 0-value can be used to poll the status of the process.
% * release(+Bool)
% Do/do not release the process. We do not support this flag
% and a domain_error is raised if release(false) is provided.
% @arg Status is one of exit(Code) or killed(Signal), where
% Code and Signal are integers. If the `timeout` option
% is used Status is unified with `timeout` after the wait
% timed out.
process_wait(PID, Status) :-
process_wait(PID, Status, ).
%! process_kill(+PID) is det.
%! process_kill(+PID, +Signal) is det.
% Send signal to process PID. Default is =term=. Signal is an
% integer, Unix signal name (e.g. =SIGSTOP=) or the more Prolog
% friendly variation one gets after removing =SIG= and downcase
% the result: =stop=. On Windows systems, Signal is ignored and
% the process is terminated using the TerminateProcess() API. On
% Windows systems PID must be obtained from process_create/3,
% while any PID is allowed on Unix systems.
% @compat SICStus does not accept the prolog friendly version. We
% choose to do so for compatibility with on_signal/3.
%! process_group_kill(+PID) is det.
%! process_group_kill(+PID, +Signal) is det.
% Send signal to the group containing process PID. Default is
% =term=. See process_wait/1 for a description of signal
% handling. In Windows, the same restriction on PID applies: it
% must have been created from process_create/3, and the the group
% is terminated via the TerminateJobObject API.
%! process_set_method(+Method) is det.
% Determine how the process is created on Unix systems. Method is one
% of `spawn` (default), `fork` or `vfork`. If the method is `spawn`
% but this cannot be used because it is either not supported by the OS
% or the cwd(Dir) option is given `fork` is used.
% The problem is to be understood as follows. The official portable
% and safe method to create a process is using the fork() system call.
% This call however copies the process page tables and get seriously
% slow as the (Prolog) process is multiple giga bytes large.
% Alternatively, we may use vfork() which avoids copying the process
% space. But, the safe usage as guaranteed by the POSIX standard of
% vfork() is insufficient for our purposes. On practical systems your
% mileage may vary. Modern posix systems also provide posix_spawn(),
% which provides a safe and portable alternative for the fork() and
% exec() sequence that may be implemented using fork() or may use a
% fast but safe alternative. Unfortunately posix_spawn() doesn't
% support the option to specify the working directory for the child
% and we cannot use working_directory/2 as the working directory is
% shared between threads.
% Summarizing, the default is safe and tries to be as fast as
% possible. On some scenarios and on some OSes it is possible to do
% better. It is generally a good idea to avoid using the cwd(Dir)
% option of process_create/3 as without we can use posix_spawn().
* MESSAGES *
prolog:error_message(process_error(File, exit(Status))) -->
[ 'Process "~w": exit status: ~w'-[File, Status] ].
prolog:error_message(process_error(File, killed(Signal))) -->
[ 'Process "~w": killed by signal ~w'-[File, Signal] ].