Question

So I have a java program that calls a C program through ProcessBuilder, and I need the C program to inform tha Java program when something happens. I have the following code for the java prog:

    String cmd[] = //string to run the c program in the terminal, no probs here
    ProcessBuilder builder = new ProcessBuilder(cmd);
    builder.redirectErrorStream(true);
    Process process = builder.start();

    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getInputStream()));
    System.out.println(bufferedReader.ready());
    System.out.println(bufferedReader.readLine());

The c program at a given point will have to inform the java program of something. I have tried many things like

char Buff[] = "output";
write(0, Buff, strlen(Buff)+1);
write(1, Buff, strlen(Buff)+1);
printf("output\n");

But I cant get the java program to read this, the only output I get is

false
null

Answer 1

The Java program won’t see the output until buffers are flushed.

Buffering at the level of write is OS dependent and even within the same OS, different kinds of streams may have different default buffering modes. In Linux the documents imply a write to a pipe will immediately be readable by the other process, and ProcessBuilder uses a pipe at least in Android.

It’s likely that if you use stdio.h that fflush will portably push data all the way out to the socket or pipe. E.g. I have had success with fflush for this purpose in Android using ProcessBuilder.

Line buffering is another possible choice for the OS. In this case appending \n to your messages may have an effect.

By the way, mixing write and printf calls in the same program is asking for trouble. And as has been mentioned, write(0 is an attempt to write to stdin, and strlen(buf)+1 is causing a final zero byte to be sent to the Java program, which is unlikely to be what you want.