Title: Capture Output of a Console Application - Revised
Question: How do you start a DOS or Console Application and
capture the output while it is running ? For example,
how do you capture the output of the FileCompare (FC)
Command ?
Answer:
There are already two articles about this problem,
http://www.delphi3000.com/articles/article_2112.asp
http://www.delphi3000.com/articles/article_2298.asp
but they get the output when the process is finished.
There are also two Microsoft Articles about Redirection of
DOS Applications :
Microsoft Knowledge Base Article - Q190351
HOWTO: Spawn Console Processes with Redirected Standard Handles
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q190351
Microsoft Knowledge Base Article - Q150956
INFO: Redirection Issues on Windows 95 MS-DOS Applications
http://support.microsoft.com/default.aspx?scid=KB;EN-US;Q150956
The basic idea is to start the console application with
CreateProcess and redirect the input and output with
pipes, enabling the calling process to access the output
of the console application.
It is important to capture the output while the process is
still running. If the output-pipe is blocked by an overflow,
new information can not be written from the console app to
the output-pipe, and the program stops. In this case we have
a classic deadlock situation: the user (parent process) is
waiting for the child-process to finish, and the child-process
is waiting for the user to clear the buffer.
procedure CaptureConsoleOutput(DosApp : string;AMemo : TMemo);
const
ReadBuffer = 1048576; // 1 MB Buffer
var
Security : TSecurityAttributes;
ReadPipe,WritePipe : THandle;
start : TStartUpInfo;
ProcessInfo : TProcessInformation;
Buffer : Pchar;
TotalBytesRead,
BytesRead : DWORD;
Apprunning,n,
BytesLeftThisMessage,
TotalBytesAvail : integer;
begin
with Security do
begin
nlength := SizeOf(TSecurityAttributes);
binherithandle := true;
lpsecuritydescriptor := nil;
end;
if CreatePipe (ReadPipe, WritePipe, @Security, 0) then
begin
// Redirect In- and Output through STARTUPINFO structure
Buffer := AllocMem(ReadBuffer + 1);
FillChar(Start,Sizeof(Start),#0);
start.cb := SizeOf(start);
start.hStdOutput := WritePipe;
start.hStdInput := ReadPipe;
start.dwFlags := STARTF_USESTDHANDLES + STARTF_USESHOWWINDOW;
start.wShowWindow := SW_HIDE;
// Create a Console Child Process with redirected input and output
if CreateProcess(nil ,PChar(DosApp),
@Security,@Security,
true ,CREATE_NO_WINDOW or NORMAL_PRIORITY_CLASS,
nil ,nil,
start ,ProcessInfo) then
begin
n:=0;
TotalBytesRead:=0;
repeat
// Increase counter to prevent an endless loop if the process is dead
Inc(n,1);
// wait for end of child process
Apprunning := WaitForSingleObject(ProcessInfo.hProcess,100);
Application.ProcessMessages;
// it is important to read from time to time the output information
// so that the pipe is not blocked by an overflow. New information
// can be written from the console app to the pipe only if there is
// enough buffer space.
if not PeekNamedPipe(ReadPipe ,@Buffer[TotalBytesRead],
ReadBuffer ,@BytesRead,
@TotalBytesAvail,@BytesLeftThisMessage) then break
else if BytesRead 0 then
ReadFile(ReadPipe,Buffer[TotalBytesRead],BytesRead,BytesRead,nil);
TotalBytesRead:=TotalBytesRead+BytesRead;
until (Apprunning WAIT_TIMEOUT) or (n 150);
Buffer[TotalBytesRead]:= #0;
OemToChar(Buffer,Buffer);
AMemo.Text := AMemo.text + StrPas(Buffer);
end;
FreeMem(Buffer);
CloseHandle(ProcessInfo.hProcess);
CloseHandle(ProcessInfo.hThread);
CloseHandle(ReadPipe);
CloseHandle(WritePipe);
end;
end;