Windows System Programming Third Edition [Electronic resources] نسخه متنی

Example: I/O Redirection Using an Anonymous Pipe

Chapter 6.

The location of WriteFile in Program2 on the right side of Figure 11-1 assumes that the program reads a large amount of data, processes it, and then writes out results. Alternatively, the write could be inside the loop, putting out results after each read.

The pipe and thread handles are closed at the earliest possible point. Figure 11-1 does not show the handle closings, but Program 11-1 does. It is necessary for the parent to close the standard output handle immediately after creating the first child process so that the second process will be able to recognize an end of file when the first process terminates. If there were still an open handle, the second process might not terminate because the system would not indicate an end of file.

Program 11-1 uses an unusual syntax; the = sign is the pipe symbol separating the two commands. The vertical bar ( | ) would conflict with the command processor. Figure 11-1 schematically shows the execution of the command:

$ pipe Program1 arguments = Program2 arguments

In UNIX or the Windows command prompt, the corresponding command would be:

$ Program1 arguments | Program2 arguments

Program 11-1. pipe: Interprocess Communication with Anonymous Pipes

#include "EvryThng.h"
int _tmain (int argc, LPTSTR argv [])
/* Pipe together two programs on the command line:
pipe command1 = command2 */
{
DWORD i = 0;
HANDLE hReadPipe, hWritePipe;
TCHAR Command1 [MAX_PATH];
SECURITY_ATTRIBUTES PipeSA = /* For inheritable handles. */
{sizeof (SECURITY_ATTRIBUTES), NULL, TRUE};
PROCESS_INFORMATION ProcInfo1, ProcInfo2;
STARTUPINFO StartInfoCh1, StartInfoCh2;
LPTSTR targv = SkipArg (GetCommandLine ());
GetStartupInfo (&StartInfoCh1);
GetStartupInfo (&StartInfoCh2);
/* Find the = separating the two commands. */
while (*targv != '=' && *targv != '\0') {
Command1 [i] = *targv;
targv++;
i++;
}
Command1 [i] = '\0';
/* Skip to start of second command. */
targv = SkipArg (targv);
CreatePipe (&hReadPipe, &hWritePipe, &PipeSA, 0);
/* Redirect standard output & create first process. */
StartInfoCh1.hStdInput = GetStdHandle (STD_INPUT_HANDLE);
StartInfoCh1.hStdError = GetStdHandle (STD_ERROR_HANDLE);
StartInfoCh1.hStdOutput = hWritePipe;
StartInfoCh1.dwFlags = STARTF_USESTDHANDLES;
CreateProcess (NULL, (LPTSTR)Command1, NULL, NULL,
TRUE /* Inherit handles. */, 0, NULL, NULL,
&StartInfoCh1, &ProcInfo1);
CloseHandle (ProcInfo1.hThread);
/* Close the pipe's write handle as it is no longer needed
and to ensure the second command detects a file end. */
CloseHandle (hWritePipe);
/* Repeat (symmetrically) for the second process. */
StartInfoCh2.hStdInput = hReadPipe;
StartInfoCh2.hStdOutput = GetStdHandle (STD_OUTPUT_HANDLE);
StartInfoCh2.hStdError = GetStdHandle (STD_ERROR_HANDLE);
StartInfoCh2.dwFlags = STARTF_USESTDHANDLES;
CreateProcess (NULL, (LPTSTR) targv, NULL, NULL, TRUE, 0, NULL,
NULL, &StartInfoCh2, &ProcInfo2);
CloseHandle (ProcInfo2.hThread);
CloseHandle (hReadPipe);
/* Wait for the first and second processes to terminate. */
WaitForSingleObject (ProcInfo1.hProcess, INFINITE);
CloseHandle (ProcInfo1.hProcess);
WaitForSingleObject (ProcInfo2.hProcess, INFINITE);
CloseHandle (ProcInfo2.hProcess);
return 0;
}