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### 8.14.11 Start a child process, connected with pipes, also on Dos

The following listing contains the interface of the Pipe unit.

This unit provides routines to start a child process and write to/read from its Input/Output/StdErr via pipes. All of this is emulated transparently under Dos as far as possible.

    { Piping data from and to processes

      Copyright (C) 1998-2002 Free Software Foundation, Inc.

      Author: Frank Heckenbach <frank@pascal.gnu.de>

      This file is part of GNU Pascal.

      GNU Pascal is free software; you can redistribute it and/or modify
      it under the terms of the GNU General Public License as published
      by the Free Software Foundation; either version 2, or (at your
      option) any later version.

      GNU Pascal is distributed in the hope that it will be useful, but
      WITHOUT ANY WARRANTY; without even the implied warranty of
      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
      General Public License for more details.

      You should have received a copy of the GNU General Public License
      along with GNU Pascal; see the file COPYING. If not, write to the
      Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA
      02111-1307, USA.

      As a special exception, if you link this file with files compiled
      with a GNU compiler to produce an executable, this does not cause
      the resulting executable to be covered by the GNU General Public
      License. This exception does not however invalidate any other
      reasons why the executable file might be covered by the GNU
      General Public License. }

    {$gnu-pascal,I-}
    {$if __GPC_RELEASE__ < 20011202}
    {$error This unit requires GPC release 20011202 or newer.}
    {$endif}

    { Keep this consistent with the one in pipec.c }
    {$if defined (MSDOS) or defined (__MINGW32__)}
    {$define NOFORK}
    {$endif}

    unit Pipe;

    interface

    uses GPC;

    const
      PipeForking = {$ifdef NOFORK} False {$else} True {$endif};

    type
      TProcedure = procedure;

      PWaitPIDResult = ^TWaitPIDResult;
      TWaitPIDResult = (PIDNothing, PIDExited, PIDSignaled, PIDStopped,
      PIDUnknown);

      PPipeProcess = ^TPipeProcess;
      TPipeProcess = record
        PID      : Integer;         { Process ID of process forked }
        SignalPID: Integer;         { Process ID to send the signal to.
                                      Equals PID by default }
        OpenPipes: Integer;         { Number of pipes to/from the
                                      process, for internal use }
        Signal   : Integer;         { Send this signal (if not 0) to the
                                      process after all pipes have been
                                      closed after some time }
        Seconds  : Integer;         { Wait so many seconds before
                                      sending the signal if the process
                                      has not terminated by itself }
        Wait     : Boolean;         { Wait for the process, even longer
                                      than Seconds seconds, after
                                      sending the signal (if any) }
        Result   : PWaitPIDResult;  { Default nil. If a pointer to a
                                      variable is stored here, its
                                      destination will contain the
                                      information whether the process
                                      terminated by itself, or was
                                      terminated or stopped by a signal,
                                      when waiting after closing the
                                      pipes }
        Status   : ^Integer;        { Default nil. If a pointer to a
                                      variable is stored here, its
                                      destination will contain the exit
                                      status if the process terminated
                                      by itself, or the number of the
                                      signal otherwise, when waiting
                                      after closing the pipes }
      end;

    var
      { Default values for TPipeProcess records created by Pipe }
      DefaultPipeSignal : Integer = 0;
      DefaultPipeSeconds: Integer = 0;
      DefaultPipeWait   : Boolean = True;

    { The procedure Pipe starts a process whose name is given by
      ProcessName, with the given parameters (can be Null if no
      parameters) and environment, and create pipes from and/or to the
      process' standard input/output/error. ProcessName is searched for
      in the PATH with FSearchExecutable. Any of ToInputFile,
      FromOutputFile and FromStdErrFile can be Null if the corresponding
      pipe is not wanted. FromOutputFile and FromStdErrFile may be
      identical, in which case standard output and standard error are
      redirected to the same pipe. The behaviour of other pairs of files
      being identical is undefined, and useless, anyway. The files are
      Assigned and Reset or Rewritten as appropriate. Errors are
      returned in IOResult. If Process is not Null, a pointer to a
      record is stored there, from which the PID of the process created
      can be read, and by writing to which the action after all pipes
      have been closed can be changed. (The record is automatically
      Dispose'd of after all pipes have been closed.) If automatic
      waiting is turned off, the caller should get the PID from the
      record before it's Dispose'd of, and wait for the process sometime
      in order to avoid zombies. If no redirections are performed (i.e.,
      all 3 files are Null), the caller should wait for the process with
      WaitPipeProcess. When an error occurs, Process is not assigned to,
      and the state of the files is undefined, so be sure to check
      IOResult before going on.

      ChildProc, if not nil, is called in the child process after
      forking and redirecting I/O, but before executing the new process.
      It can even be called instead of executing a new process
      (ProcessName can be empty then).

      The procedure even works under Dos, but, of course, in a limited
      sense: if ToInputFile is used, the process will not actually be
      started until ToInputFile is closed. Signal, Seconds and Wait of
      TPipeProcess are ignored, and PID and SignalPID do not contain a
      Process ID, but an internal value without any meaning to the
      caller. Result will always be PIDExited. So, Status is the only
      interesting field (but Result should also be checked). Since there
      is no forking under Dos, ChildProc, if not nil, is called in the
      main process before spawning the program. So, to be portable, it
      should not do any things that would influence the process after
      the return of the Pipe function.

      The only portable way to use "pipes" in both directions is to call
      `Pipe', write all the Input data to ToInputFile, close
      ToInputFile, and then read the Output and StdErr data from
      FromOutputFile and FromStdErrFile. However, since the capacity of
      pipes is limited, one should also check for Data from
      FromOutputFile and FromStdErrFile (using CanRead, IOSelect or
      IOSelectRead) while writing the Input data (under Dos, there
      simply won't be any data then, but checking for data doesn't do
      any harm). Please see pipedemo.pas for an example. }
    { @@ IO critical } procedure Pipe (var ToInputFile, FromOutputFile,
      FromStdErrFile: AnyFile; const ProcessName: String; protected var
      Parameters: TPStrings; ProcessEnvironment: PCStrings; var Process:
      PPipeProcess; ChildProc: TProcedure);

    { Waits for a process created by Pipe as determined in the Process
      record. (Process is Dispose'd of afterwards.) Returns True if
      successful. }
    function WaitPipeProcess (Process: PPipeProcess): Boolean;

    { Alternative interface from PExecute }

    const
      PExecute_First   = 1;
      PExecute_Last    = 2;
      PExecute_One     = PExecute_First or PExecute_Last;
      PExecute_Search  = 4;
      PExecute_Verbose = 8;

    { PExecute: execute a chain of processes.

      Program and Arguments are the arguments to execv/execvp.

      Flags and PExecute_Search is non-zero if $PATH should be searched.
      Flags and PExecute_First is nonzero for the first process in
      chain. Flags and PExecute_Last is nonzero for the last process in
      chain.

      The result is the pid on systems like Unix where we fork/exec and
      on systems like MS-Windows and OS2 where we use spawn. It is up to
      the caller to wait for the child.

      The result is the exit code on systems like MSDOS where we spawn
      and wait for the child here.

      Upon failure, ErrMsg is set to the text of the error message,
      and -1 is returned. `errno' is available to the caller to use.

      PWait: cover function for wait.

      PID is the process id of the task to wait for. Status is the
      `status' argument to wait. Flags is currently unused (allows
      future enhancement without breaking upward compatibility). Pass 0
      for now.

      The result is the process ID of the child reaped, or -1 for
      failure.

      On systems that don't support waiting for a particular child, PID
      is ignored. On systems like MSDOS that don't really multitask
      PWait is just a mechanism to provide a consistent interface for
      the caller. }
    function  PExecute (ProgramName: CString; Arguments: PCStrings; var
      ErrMsg: String; Flags: Integer): Integer; asmname '_p_pexecute';
    function  PWait (PID: Integer; var Status: Integer; Flags: Integer):
      Integer; asmname '_p_pwait';

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This document was generated by Frank Heckenbach on May, 10 2002 using texi2html