使用Java的stdin/out与C程序通信

Question

我希望我的Java程序与一个C程序通信。这只是一个简单的例子，但我无法让它工作。Java程序应该运行C程序并写入其输入流。C程序应该看到这一点并写入标准输出作为响应。最后，Java程序应该从C程序的标准输出中读取这个响应并将其打印到屏幕上。

从命令行运行 C 程序，我得到了所需的行为。但是，当从 Java 程序运行时，它只是“挂起”并且不执行任何操作。Java 程序似乎已经将其消息写入了 C 程序的 stdin，但在 C 程序中看不到此消息。

我将C程序设置为将它读取的消息写入文件，只是为了检查它是否读取了消息，但它没有这样做。

以下是C程序：

#include <stdio.h>
#include <string.h>

void hello();
void wrong();

int main() {
    char buff[256];

    /* 1. read stdin */
    fscanf(stdin, "%s", buff);

    /* side effect - if message was received it should be 
        printed to file */
    FILE *fp = fopen("file.txt", "w");
    fprintf(fp, "%s", buff);
    fclose(fp);

    /* 2. depending on message, write something to stdout */
    if(strcmp(buff, "hello") == 0) {
        hello();
    } else {
        wrong();
    }
}

void hello() {
    printf("Hello World!");
}

void wrong() {
    printf("WRONG!");
}

下面是Java程序：

import java.io.*;

public class Main {
    public static void main(String[] args) {
        try {
            // 1. run C program
            Process proc = Runtime.getRuntime().exec("./hello");
            InputStream in = proc.getInputStream();
            OutputStream out = proc.getOutputStream();
            // 2. write 'hello' to 'hello' program
            writeToProc(out, "hello");
            // 3. read response
            readFromProc(in);
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    // write message to process
    public static void writeToProc(OutputStream out, String msg) throws IOException {
        byte[] buff = msg.getBytes();
        out.write(buff);
        out.flush();
        System.out.println("done writing: " + new String(buff));
    }

    // read stdin of process
    public static void readFromProc(InputStream in) throws IOException {
        byte[] buff = new byte[256];
        int read = in.read();
        for(int i=0; read != -1; i++) {
            read = in.read();
            buff[i] = (byte) read;
        }
        String str = new String(buff);
        System.out.println("proc says: " + str);
    }
}

当我运行 Main 时，我得到以下输出：

$ java Main
  done writing: hello

然后只是一个闪烁的光标和文件“file.txt”没有被写入，说明C程序没有从stdin中读取“hello”。

这是一个简单的例子，所以我想我错过了一些简单的东西，或者以某种方式走错了方向。

Answer 1

问题是您忘记了在从Java发送的字符串中添加新行字符< code>\n，因此< code>fscanf会无限期地等待进程< code >“按enter”。我对代码做了一些修改，修改记录在注释中。

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class Main {
    public static void main(String[] args) {
        try {
            // 1. run C program
            Process proc = Runtime.getRuntime().exec("./hello");
            InputStream in = proc.getInputStream();
            OutputStream out = proc.getOutputStream();
            // 2. write 'hello' to 'hello' program
            // <change>
            // don't forget to add the new line here or in the 
            // writeToProc method
            writeToProc(out, "hello\n");
            // 3. read response
            readFromProc(in);
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    // write message to process
    public static void writeToProc(OutputStream out, String msg) throws IOException {
        // <change>
        // Using UTF-8 encoding since all chars in C are byte sized
        byte[] buff = msg.getBytes("UTF-8");
        out.write(buff);
        out.flush();
        System.out.println("done writing: " + new String(buff));
    }

    // read stdin of process
    public static void readFromProc(InputStream in) throws IOException {
        byte[] buff = new byte[256];
        int read = in.read();
        int index = 0;
        while(read != -1) {
            buff[index] = (byte) read;
            read = in.read();
            ++index;
        }
        String str = new String(buff, 0, index);
        System.out.println("proc says: " + str);
    }
}