我正在尝试使用单独的流程来完成一些工作。在Windows版本中，我使用CreateProcess启动它并正常重定向stdin / stdout（继承匿名管道）。管道工作;但是，当子进程退出时，父进程将挂起读取调用。
父母如何知道子进程已退出？我认为当孩子退出时，ReadFile应该立即返回一个破裂的管道错误。
用于创建具有重定向输入和输出的子进程的Windows示例代码具有相同的问题，因此我将其作为MCVE发布。它也可以在https://msdn.microsoft.com/library/windows/desktop/ms682499
获得#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>
#define BUFSIZE 4096
HANDLE g_hChildStd_IN_Rd = NULL;
HANDLE g_hChildStd_IN_Wr = NULL;
HANDLE g_hChildStd_OUT_Rd = NULL;
HANDLE g_hChildStd_OUT_Wr = NULL;
HANDLE g_hInputFile = NULL;
void CreateChildProcess(void);
void WriteToPipe(void);
void ReadFromPipe(void);
void ErrorExit(PTSTR);
int _tmain(int argc, TCHAR *argv[])
{
SECURITY_ATTRIBUTES saAttr;
printf("\n->Start of parent execution.\n");
// Set the bInheritHandle flag so pipe handles are inherited.
saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
saAttr.bInheritHandle = TRUE;
saAttr.lpSecurityDescriptor = NULL;
// Create a pipe for the child process's STDOUT.
if ( ! CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr, 0) )
ErrorExit(TEXT("StdoutRd CreatePipe"));
// Ensure the read handle to the pipe for STDOUT is not inherited.
if ( ! SetHandleInformation(g_hChildStd_OUT_Rd, HANDLE_FLAG_INHERIT, 0) )
ErrorExit(TEXT("Stdout SetHandleInformation"));
// Create a pipe for the child process's STDIN.
if (! CreatePipe(&g_hChildStd_IN_Rd, &g_hChildStd_IN_Wr, &saAttr, 0))
ErrorExit(TEXT("Stdin CreatePipe"));
// Ensure the write handle to the pipe for STDIN is not inherited.
if ( ! SetHandleInformation(g_hChildStd_IN_Wr, HANDLE_FLAG_INHERIT, 0) )
ErrorExit(TEXT("Stdin SetHandleInformation"));
// Create the child process.
CreateChildProcess();
// Get a handle to an input file for the parent.
// This example assumes a plain text file and uses string output to verify data flow.
if (argc == 1)
ErrorExit(TEXT("Please specify an input file.\n"));
g_hInputFile = CreateFile(
argv[1],
GENERIC_READ,
0,
NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_READONLY,
NULL);
if ( g_hInputFile == INVALID_HANDLE_VALUE )
ErrorExit(TEXT("CreateFile"));
// Write to the pipe that is the standard input for a child process.
// Data is written to the pipe's buffers, so it is not necessary to wait
// until the child process is running before writing data.
WriteToPipe();
printf( "\n->Contents of %s written to child STDIN pipe.\n", argv[1]);
// Read from pipe that is the standard output for child process.
printf( "\n->Contents of child process STDOUT:\n\n", argv[1]);
ReadFromPipe();
printf("\n->End of parent execution.\n");
// The remaining open handles are cleaned up when this process terminates.
// To avoid resource leaks in a larger application, close handles explicitly.
return 0;
}
void CreateChildProcess()
// Create a child process that uses the previously created pipes for STDIN and STDOUT.
{
TCHAR szCmdline[]=TEXT("child");
PROCESS_INFORMATION piProcInfo;
STARTUPINFO siStartInfo;
BOOL bSuccess = FALSE;
// Set up members of the PROCESS_INFORMATION structure.
ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );
// Set up members of the STARTUPINFO structure.
// This structure specifies the STDIN and STDOUT handles for redirection.
ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
siStartInfo.cb = sizeof(STARTUPINFO);
siStartInfo.hStdError = g_hChildStd_OUT_Wr;
siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
siStartInfo.hStdInput = g_hChildStd_IN_Rd;
siStartInfo.dwFlags |= STARTF_USESTDHANDLES;
// Create the child process.
bSuccess = CreateProcess(NULL,
szCmdline, // command line
NULL, // process security attributes
NULL, // primary thread security attributes
TRUE, // handles are inherited
0, // creation flags
NULL, // use parent's environment
NULL, // use parent's current directory
&siStartInfo, // STARTUPINFO pointer
&piProcInfo); // receives PROCESS_INFORMATION
// If an error occurs, exit the application.
if ( ! bSuccess )
ErrorExit(TEXT("CreateProcess"));
else
{
// Close handles to the child process and its primary thread.
// Some applications might keep these handles to monitor the status
// of the child process, for example.
CloseHandle(piProcInfo.hProcess);
CloseHandle(piProcInfo.hThread);
}
}
void WriteToPipe(void)
// Read from a file and write its contents to the pipe for the child's STDIN.
// Stop when there is no more data.
{
DWORD dwRead, dwWritten;
CHAR chBuf[BUFSIZE];
BOOL bSuccess = FALSE;
for (;;)
{
bSuccess = ReadFile(g_hInputFile, chBuf, BUFSIZE, &dwRead, NULL);
if ( ! bSuccess || dwRead == 0 ) break;
bSuccess = WriteFile(g_hChildStd_IN_Wr, chBuf, dwRead, &dwWritten, NULL);
if ( ! bSuccess ) break;
}
// Close the pipe handle so the child process stops reading.
if ( ! CloseHandle(g_hChildStd_IN_Wr) )
ErrorExit(TEXT("StdInWr CloseHandle"));
}
void ReadFromPipe(void)
// Read output from the child process's pipe for STDOUT
// and write to the parent process's pipe for STDOUT.
// Stop when there is no more data.
{
DWORD dwRead, dwWritten;
CHAR chBuf[BUFSIZE];
BOOL bSuccess = FALSE;
HANDLE hParentStdOut = GetStdHandle(STD_OUTPUT_HANDLE);
for (;;)
{
bSuccess = ReadFile( g_hChildStd_OUT_Rd, chBuf, BUFSIZE, &dwRead, NULL);
if( ! bSuccess || dwRead == 0 ) break;
bSuccess = WriteFile(hParentStdOut, chBuf,
dwRead, &dwWritten, NULL);
if (! bSuccess ) break;
}
}
void ErrorExit(PTSTR lpszFunction)
// Format a readable error message, display a message box,
// and exit from the application.
{
LPVOID lpMsgBuf;
LPVOID lpDisplayBuf;
DWORD dw = GetLastError();
FormatMessage(
FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL,
dw,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR) &lpMsgBuf,
0, NULL );
lpDisplayBuf = (LPVOID)LocalAlloc(LMEM_ZEROINIT,
(lstrlen((LPCTSTR)lpMsgBuf)+lstrlen((LPCTSTR)lpszFunction)+40)*sizeof(TCHAR));
StringCchPrintf((LPTSTR)lpDisplayBuf,
LocalSize(lpDisplayBuf) / sizeof(TCHAR),
TEXT("%s failed with error %d: %s"),
lpszFunction, dw, lpMsgBuf);
MessageBox(NULL, (LPCTSTR)lpDisplayBuf, TEXT("Error"), MB_OK);
LocalFree(lpMsgBuf);
LocalFree(lpDisplayBuf);
ExitProcess(1);
}
这是子进程代码：
#include <windows.h>
#include <stdio.h>
#define BUFSIZE 4096
int main(void)
{
CHAR chBuf[BUFSIZE];
DWORD dwRead, dwWritten;
HANDLE hStdin, hStdout;
BOOL bSuccess;
hStdout = GetStdHandle(STD_OUTPUT_HANDLE);
hStdin = GetStdHandle(STD_INPUT_HANDLE);
if (
(hStdout == INVALID_HANDLE_VALUE) ||
(hStdin == INVALID_HANDLE_VALUE)
)
ExitProcess(1);
// Send something to this process's stdout using printf.
printf("\n ** This is a message from the child process. ** \n");
// This simple algorithm uses the existence of the pipes to control execution.
// It relies on the pipe buffers to ensure that no data is lost.
// Larger applications would use more advanced process control.
for (;;)
{
// Read from standard input and stop on error or no data.
bSuccess = ReadFile(hStdin, chBuf, BUFSIZE, &dwRead, NULL);
if (! bSuccess || dwRead == 0)
break;
// Write to standard output and stop on error.
bSuccess = WriteFile(hStdout, chBuf, dwRead, &dwWritten, NULL);
if (! bSuccess)
break;
}
return 0;
}
这是执行的输出。问题是父亲从未检测到子退出，因此永远不会完成。
->Start of parent execution.
->Contents of E:\Development\test\redirect\Debug/data.txt written to child STDIN
pipe.
->Contents of child process STDOUT:
+ This is a test of windows pipes.
+ This is only a test.
** This is a message from the child process. **
答案 0 :(得分：2)
我认为当孩子退出时，ReadFile应该立即返回一个损坏的管道错误。
不完全。当管道的另一端关闭时立即返回，一旦最后一个句柄关闭就会发生。
父进程未关闭其句柄副本。就Windows而言，父进程中的另一个线程可能正在计划写入该句柄，因此ReadFile会继续等待它发生。
因此，在创建子进程后，关闭句柄：
CloseHandle(g_hChildStd_OUT_Wr);
PS：您可以通过等待进程句柄显式检查进程是否已退出。但是使用管道通常更容易，更有效。