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\section{library(process): Create processes and redirect I/O}

\label{sec:process}

\begin{tags}
    \tag{Compatibility}
SICStus 4
    \tag{To be done}
Implement detached option in \predref{process_create}{3}
\end{tags}

The module \verb$library(process)$ implements interaction with child processes
and unifies older interfaces such as shell/[1,2], \verb$open(pipe(command), ...)$ etc. This library is modelled after SICStus 4.

The main interface is formed by \predref{process_create}{3}. If the process id is
requested the process must be waited for using \predref{process_wait}{2}. Otherwise
the process resources are reclaimed automatically.

In addition to the predicates, this module defines a file search path
(see \qpredref{user}{file_search_path}{2} and \predref{absolute_file_name}{3}) named \const{path} that
locates files on the system's search path for executables. E.g. the
following finds the executable for \const{ls}:

\begin{code}
?- absolute_file_name(path(ls), Path, [access(execute)]).
\end{code}

\textbf{Incompatibilities and current limitations}

\begin{itemize}
    \item Where SICStus distinguishes between an internal process id and
the OS process id, this implementation does not make this
distinction. This implies that \predref{is_process}{1} is incomplete and
unreliable.
    \item SICStus only supports ISO 8859-1 (latin-1). This implementation
supports arbitrary OS multibyte interaction using the default
locale.
    \item It is unclear what the \verb$detached(true)$ option is supposed to do. Disable
signals in the child? Use \verb$setsid()$ to detach from the session? The
current implementation uses \verb$setsid()$ on Unix systems.
    \item An extra option \verb$env([Name=Value, ...])$ is added to
\predref{process_create}{3}. As of version 4.1 SICStus added
\verb$environment(List)$ which \textit{modifies} the environment. A
compatible option was added to SWI-Prolog 7.7.23.
\end{itemize}

\vspace{0.7cm}

\begin{description}
    \predicate[det]{process_create}{3}{+Exe, +Args:list, +Options}
Create a new process running the file \arg{Exe} and using arguments
from the given list. \arg{Exe} is a file specification as handed to
\predref{absolute_file_name}{3}. Typically one use the \const{path} file alias to
specify an executable file on the current PATH. \arg{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 \verb$file(Spec)$. Using \verb$file(Spec)$, the system
inserts a filename using the OS filename conventions which is
properly quoted if needed.

\arg{Options}:

\begin{description}
    \termitem{stdin}{Spec}
    \termitem{stdout}{Spec}
    \termitem{stderr}{Spec}
Bind the standard streams of the new process. \arg{Spec} is one of
the terms below. If \verb$pipe(Pipe)$ is used, the Prolog stream is
a stream in text-mode using the encoding of the default
locale. The encoding can be changed using \predref{set_stream}{2}.
The options \const{stdout} and \const{stderr} may use the same stream,
in which case both output streams are connected to the same
Prolog stream.

\begin{description}
    \termitem{std}{}
Just share with the Prolog I/O streams
    \termitem{null}{}
Bind to a \textit{null} stream. Reading from such a stream
returns end-of-file, writing produces no output
    \termitem{pipe}{-Stream}
Attach input and/or output to a Prolog stream.
\end{description}

    \termitem{cwd}{+Directory}
Run the new process in \arg{Directory}. \arg{Directory} can be a
compound specification, which is converted using
\predref{absolute_file_name}{3}.
    \termitem{env}{+List}
As \verb$environment(List)$, but \textit{only} the specified variables
are passed, i.e., no variables are \textit{inherited}.
    \termitem{environment}{+List}
Specify \textit{additional} environment variables for the new process.
\arg{List} is a list of \verb$Name=Value$ terms, where \arg{Value} is expanded
the same way as the \arg{Args} argument. If neither \const{env} nor
\const{environment} is passed the environment is inherited from the
Prolog process.
    \termitem{process}{-PID}
Unify \arg{PID} with the process id of the created process.
    \termitem{detached}{+Bool}
In Unix: If \const{true}, detach the process from the terminal
Currently mapped to \verb$setsid()$;
Also creates a new process group for the child
In Windows: If \const{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.
    \termitem{window}{+Bool}
If \const{true}, create a window for the process (Windows only)
    \termitem{priority}{+Priority}
In Unix: specifies the process priority for the newly
created process. \arg{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 \textit{raise} it
to less-than zero.
\end{description}

If the user specifies the \verb$process(-PID)$ option, he \textbf{must} call
\predref{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.

\textbf{Windows notes}

On Windows this call is an interface to the CreateProcess() API.
The commandline consists of the basename of \arg{Exe} and the
arguments formed from \arg{Args}. Arguments are separated by a single
space. If all characters satisfy \verb$iswalnum()$ 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
\verb$CREATE_NO_WINDOW$ options is supported through the
\verb$window(+Bool)$ 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
\predref{win_exec}{2}.

\textbf{Examples}

First, a very simple example that behaves the same as
\verb$shell('ls -l')$, except for error handling:

\begin{code}
?- process_create(path(ls), ['-l'], []).
\end{code}

The following example uses grep to find all matching lines in a
file.

\begin{code}
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).
\end{code}

\begin{tags}
    \tag{Errors}
\verb$process_error(Exe, Status)$ where Status is one of
\verb$exit(Code)$ or \verb$killed(Signal)$. Raised if the process
is waited for (i.e., \arg{Options} does not include
\verb$process(-PID)$), and does not exit with status 0.
    \tag{bug}
On Windows, \verb$environment(List)$ is handled as \verb$env(List)$,
i.e., the environment is not inherited.
\end{tags}

    \predicate[det]{process_id}{1}{-PID}
True if \arg{PID} is the process id of the running Prolog process.

\begin{tags}
    \tag{deprecated}
Use \verb$current_prolog_flag(pid, PID)$
\end{tags}

    \predicate[det]{process_id}{2}{+Process, -PID}
\arg{PID} is the process id of \arg{Process}. Given that they are united in
SWI-Prolog, this is a simple unify.

    \predicate[semidet]{is_process}{1}{+PID}
True if \arg{PID} might be a process. Succeeds for any positive
integer.

    \predicate{process_release}{1}{+PID}
Release process handle. In this implementation this is the same
as \verb$process_wait(PID, _)$.

    \predicate[det]{process_wait}{2}{+PID, -Status}
\nodescription
    \predicate[det]{process_wait}{3}{+PID, -Status, +Options}
True if \arg{PID} completed with \arg{Status}. This call normally blocks
until the process is finished. \arg{Options}:

\begin{description}
    \termitem{timeout}{+Timeout}
Default: \const{infinite}. If this option is a number, the
waits for a maximum of \arg{Timeout} seconds and unifies \arg{Status}
with \const{timeout} if the process does not terminate within
\arg{Timeout}. In this case \arg{PID} is \textit{not} invalidated. On Unix
systems only timeout 0 and \const{infinite} are supported. A
0-value can be used to poll the status of the process.
    \termitem{release}{+Bool}
Do/do not release the process. We do not support this flag
and a domain_error is raised if \verb$release(false)$ is provided.
\end{description}

\begin{arguments}
\arg{Status} & is one of \verb$exit(Code)$ or \verb$killed(Signal)$, where
Code and Signal are integers. If the \const{timeout} option
is used \arg{Status} is unified with \const{timeout} after the wait
timed out. \\
\end{arguments}

    \predicate[det]{process_kill}{1}{+PID}
\nodescription
    \predicate[det]{process_kill}{2}{+PID, +Signal}
Send signal to process \arg{PID}. Default is \const{term}. \arg{Signal} is an
integer, Unix signal name (e.g. \verb$SIGSTOP$) or the more Prolog
friendly variation one gets after removing \verb$SIG$ and downcase
the result: \const{stop}. On Windows systems, \arg{Signal} is ignored and
the process is terminated using the TerminateProcess() API. On
Windows systems \arg{PID} must be obtained from \predref{process_create}{3},
while any \arg{PID} is allowed on Unix systems.

\begin{tags}
    \tag{Compatibility}
SICStus does not accept the prolog friendly version. We
choose to do so for compatibility with \predref{on_signal}{3}.
\end{tags}

    \predicate[det]{process_group_kill}{1}{+PID}
\nodescription
    \predicate[det]{process_group_kill}{2}{+PID, +Signal}
Send signal to the group containing process \arg{PID}. Default is
\const{term}. See \predref{process_wait}{1} for a description of signal
handling. In Windows, the same restriction on \arg{PID} applies: it
must have been created from \predref{process_create}{3}, and the the group
is terminated via the TerminateJobObject API.

    \predicate[det]{process_set_method}{1}{+Method}
Determine how the process is created on Unix systems. \arg{Method} is one
of \const{spawn} (default), \const{fork} or \const{vfork}. If the method is \const{spawn}
but this cannot be used because it is either not supported by the OS
or the \verb$cwd(Dir)$ option is given \const{fork} is used.

The problem is to be understood as follows. The official portable
and safe method to create a process is using the \verb$fork()$ 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 \verb$vfork()$ which avoids copying the process
space. But, the safe usage as guaranteed by the POSIX standard of
\verb$vfork()$ is insufficient for our purposes. On practical systems your
mileage may vary. Modern posix systems also provide \verb$posix_spawn()$,
which provides a safe and portable alternative for the \verb$fork()$ and
\verb$exec()$ sequence that may be implemented using \verb$fork()$ or may use a
fast but safe alternative. Unfortunately \verb$posix_spawn()$ doesn't
support the option to specify the working directory for the child
and we cannot use \predref{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 \verb$cwd(Dir)$
option of \predref{process_create}{3} as without we can use \verb$posix_spawn()$.
\end{description}

