% This LaTeX document was generated using the LaTeX backend of PlDoc, % The SWI-Prolog documentation system \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 \file{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 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 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 \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}, or by using the two-argument form of \const{pipe}, which accepts an \verb$encoding(Encoding)$ option. 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. On Unix, if the \verb$user_input$, 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. \termitem{null}{} Bind to a \textit{null} stream. Reading from such a stream returns end-of-file, writing produces no output \termitem{pipe}{-Stream} \termitem{pipe}{-Stream, +StreamOptions} Attach input and/or output to a Prolog stream. The optional \arg{StreamOptions} argument is a list of options that affect the stream. Currently only the options \verb$type(+Type)$ and \verb$encoding(+Encoding)$ are supported, which have the same meaning as the stream properties of the same name (see \predref{stream_property}{2}). \arg{StreamOptions} is provided mainly for SICStus compatibility - the SWI-Prolog predicate \predref{set_stream}{2} can be used for the same purpose. \termitem{stream}{+Stream} Attach input or output to an existing Prolog stream. This stream must be associated with an OS file handle (see \predref{stream_property}{2}, property \verb$file_no$). This option is \textbf{not} provided by the SICStus implementation. \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}. See also \predref{process_set_method}{1}. \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. At most one \verb$env(List)$ or \verb$environment(List)$ term may appear in the options. If multiple appear a \verb$permission_error$ is raised for the second option. \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}