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
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.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.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
separate 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.
Options:
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,
or by using the two-argument form of pipe, which accepts an
encoding(Encoding) option. The options stdout
and stderr may use the same stream, in which case both
output streams are connected to the same Prolog stream.
user_input,
etc. are bound to a file handle but not to 0,1,2 the process I/O is
bound to the file handles of these streams.type(+Type) and encoding(+Encoding) are
supported, which have the same meaning as the stream properties of the
same name (see stream_property/2).
StreamOptions is provided mainly for SICStus compatibility -
the SWI-Prolog predicate set_stream/2 can
be used for the same purpose.file_no). This option is not provided by
the SICStus implementation.environment(List), but only the specified
variables are passed, i.e., no variables are inherited.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. At most one env(List) or environment(List)
term may appear in the options. If multiple appear a
permission_error is raised for the second option.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.true, create a window for the process (Windows only)
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
win_exec/2.
Examples
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 file.
grep(File, Pattern, Lines) :-
setup_call_cleanup(
process_create(path(grep), [ Pattern, file(File) ],
[ stdout(pipe(Out))
]),
read_lines(Out, Lines),
close(Out)).
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).
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.environment(List) is handled as env(List),
i.e., the environment is not inherited.current_prolog_flag(pid, PID)process_wait(PID, _).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(false) is provided.| 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. |
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.
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.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().