PRB: Process.waitFor() Enters a Deadlock When Parent Process Does Not Read Child Process Data (326709)

CAUSE

The Runtime.exec() method creates a pipe for the standard output. When the child process writes a large amount of data to this pipe so that the buffer is full, it blocks on the pipe until the data in the pipe buffer is read by the parent process. If the parent process never reads the standard output, Process.waitFor() does not return.

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The following sample program starts a child process, WriteALot.exe, and then waits for it to return. Note that code has been added to read the output from WriteALot.exe so that Process.waitFor() does not deadlock.

import java.io.*;

public class Class1
{
	public static void main(String[] args)
	{
		String command = "WriteALot.exe";
		try 
		{
			Process pr = Runtime.getRuntime().exec(command);
			System.out.print("Waiting...");
			new PrintStream(pr.getInputStream()).start();
			pr.waitFor();
			System.out.println("Done!");
			int exitValue = pr.exitValue();
			System.out.println("The exit value is " + exitValue);
		} 
		catch (Throwable t) 
		{
			System.out.println("Caught " + t);
		}
	}
}

class PrintStream extends Thread 
{
	java.io.InputStream __is = null;
	public PrintStream(java.io.InputStream is) 
	{
		__is = is;
	} 

	public void run() 
	{
		try 
		{
			while(this != null) 
			{
				int _ch = __is.read();
				if(_ch != -1) 
					System.out.print((char)_ch); 
				else break;
			}
		} 
		catch (Exception e) 
		{
			e.printStackTrace();
		} 
	}
}

The following is code for WriteALot, a Microsoft Visual C++ .NET console application:

//WriteALot.cpp
#include "stdafx.h"
#include "stdio.h"

int _tmain(int argc, _TCHAR* argv[])
{
	for (int i=0; i<1000; i++)
	{
		printf("Writing %dth record\n", i);
	}
	return 0;
}

PRB: Process.waitFor() Enters a Deadlock When Parent Process Does Not Read Child Process Data (326709)

SYMPTOMS

CAUSE

RESOLUTION

STATUS

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