Libuv库(探讨)---第六节:进程相关

索引目录：https://blog.youkuaiyun.com/knowledgebao/article/details/84776754

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libuv offers considerable child process management, abstracting the platform differences and allowing communication with the child process using streams or named pipes.（libuv提供子进程的管理工作，抽象平台的差异性，允许通过stream和pipe进行线程之间的交互）

A common idiom in Unix is for every process to do one thing and do it well. In such a case, a process often uses multiple child processes to achieve tasks (similar to using pipes in shells). A multi-process model with messages may also be easier to reason about compared to one with threads and shared memory.（unix建议每个进程只做一件事并且做好。所以一个进程常常使用多个子进程来完成各种任务，类似于shell中使用多个pipe。与线程共享内存类似，含有消息的多进程模型也很简单。）

A common refrain against event-based programs is that they cannot take advantage of multiple cores in modern computers. In a multi-threaded program the kernel can perform scheduling and assign different threads to different cores, improving performance. But an event loop has only one thread. The workaround can be to launch multiple processes instead, with each process running an event loop, and each process getting assigned to a separate CPU core.（基于事件的程序想利用现代计算机的多核功能是有一定挑战性的。在多线程程序里，内核可以通过调度把不同的线程分配到不同的系统核core上来提供core的利用率。但是libuv的loop只有一个线程，通过变通的使用多进程，每个进程上运行一个loop，每个进程关联到不同的core上以达到利用多核系统。）

创建进程：

The simplest case is when you simply want to launch a process and know when it exits. This is achieved using uv_spawn.

使用uv_spawn来创建一个进程。代码如下：

UV_EXTERN int uv_spawn(uv_loop_t* loop,uv_process_t* handle,const uv_process_options_t* options);

uv_loop_t *loop;
uv_process_t child_req;
uv_process_options_t options;
void on_exit(uv_process_t *req, int64_t exit_status, int term_signal) {
    fprintf(stderr, "Process exited with status %" PRId64 ", signal %d\n", exit_status, term_signal);
    uv_close((uv_handle_t*) req, NULL);
}
int main() {
    loop = uv_default_loop();

    char* args[3];
    args[0] = "mkdir";
    args[1] = "test-dir";
    args[2] = NULL;

    options.exit_cb = on_exit;
    options.file = "mkdir";
    options.args = args;

    int r;
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));
        return 1;
    } else {
        fprintf(stderr, "Launched process with ID %d\n", child_req.pid);
    }

    return uv_run(loop, UV_RUN_DEFAULT);
}

options必须被初始化位0，上边代码因为options是全局的，所以默认会被置为0.

args的数组最后一个参数必须是NULL。

完成调用之后，uv_process_t.pid包含的就是进程ID.

on_exit()在进程退出时，会触发。

改变参数：

在进程运行前，可以改变参数uv_process_options_t。

Set uv_process_options_t.cwd to the corresponding directory.设置运行目录。

uv_process_options_t.env is a null-terminated array of strings, each of the form VAR=VALUE used to set up the environment variables for the process. Set this to NULL to inherit the environment from the parent (this) process.运行环境。

按位或flags参数。

Setting uv_process_options_t.flags to a bitwise OR of the following flags, modifies the child process behaviour:

• UV_PROCESS_SETUID - sets the child’s execution user ID to uv_process_options_t.uid.
• UV_PROCESS_SETGID - sets the child’s execution group ID to uv_process_options_t.gid.

Changing the UID/GID is only supported on Unix, uv_spawn will fail on Windows with UV_ENOTSUP.

• UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS - No quoting or escaping of uv_process_options_t.args is done on Windows. Ignored on Unix.
• UV_PROCESS_DETACHED - Starts the child process in a new session, which will keep running after the parent process exits. See example below.

分离过程

可以创建和父进程无关的子进程，使用UV_PROCESS_DETACHED标记。不受父进程的影响，父进程关闭不影响子进程的运行。可以用来写守候等不受父进程影响的子进程。举例如下：

int main() {
    loop = uv_default_loop();

    char* args[3];
    args[0] = "sleep";
    args[1] = "100";
    args[2] = NULL;

    options.exit_cb = NULL;
    options.file = "sleep";
    options.args = args;
    options.flags = UV_PROCESS_DETACHED;

    int r;
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));
        return 1;
    }
    fprintf(stderr, "Launched sleep with PID %d\n", child_req.pid);
    uv_unref((uv_handle_t*) &child_req);

    return uv_run(loop, UV_RUN_DEFAULT);

虽然使用了UV_PROCESS_DETACHED，但是句柄还在父进程那里，如果想完全失去读子进程的控制，需要调用uv_unref接口，完全释放对子进程的控制。

向进程发送信号

在进程执行的过程中，可以通过发送信号来控制进程的执行。当然 SIGTERM, SIGINT and SIGKILL也会导致线程终止，具体参考https://blog.youkuaiyun.com/qq562029186/article/details/70132719

UV_EXTERN int uv_process_kill(uv_process_t*, int signum);
UV_EXTERN int uv_kill(int pid, int signum);

如果调用uv_process_kill关闭进程的话，别忘记调用uv_close函数。

信号

libuv针对Unix提供了一些信号，有些也适用于Windows.

使用uv_signal_init()初始化，它与一个循环关联。要侦听该处理程序上的特定信号，使用uv_signal_start()处理程序函数。每个处理程序只能与一个信号编号相关联，连续调用 uv_signal_start()后边的覆盖先前信号。使用uv_signal_stop()停止信号监听。下边提供一个小例子供参考：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <uv.h>

uv_loop_t* create_loop()
{
    uv_loop_t *loop = malloc(sizeof(uv_loop_t));
    if (loop) {
      uv_loop_init(loop);
    }
    return loop;
}

void signal_handler(uv_signal_t *handle, int signum)
{
    printf("Signal received: %d\n", signum);
    uv_signal_stop(handle);
}

// two signal handlers in one loop
void thread1_worker(void *userp)
{
    uv_loop_t *loop1 = create_loop();

    uv_signal_t sig1a, sig1b;
    uv_signal_init(loop1, &sig1a);
    uv_signal_start(&sig1a, signal_handler, SIGUSR1);

    uv_signal_init(loop1, &sig1b);
    uv_signal_start(&sig1b, signal_handler, SIGUSR1);

    uv_run(loop1, UV_RUN_DEFAULT);
}

// two signal handlers, each in its own loop
void thread2_worker(void *userp)
{
    uv_loop_t *loop2 = create_loop();
    uv_loop_t *loop3 = create_loop();

    uv_signal_t sig2;
    uv_signal_init(loop2, &sig2);
    uv_signal_start(&sig2, signal_handler, SIGUSR1);

    uv_signal_t sig3;
    uv_signal_init(loop3, &sig3);
    uv_signal_start(&sig3, signal_handler, SIGUSR1);

    while (uv_run(loop2, UV_RUN_NOWAIT) || uv_run(loop3, UV_RUN_NOWAIT)) {
    }
}

int main()
{
    printf("PID %d\n", getpid());

    uv_thread_t thread1, thread2;

    uv_thread_create(&thread1, thread1_worker, 0);
    uv_thread_create(&thread2, thread2_worker, 0);

    uv_thread_join(&thread1);
    uv_thread_join(&thread2);
    return 0;
}

上述监听回调会触发4次监听。

子进程I/O

子进程有其自身的一组文件描述符，用0，1和2的分别表示stdin，stdout和stderr。有时我们可能希望子进程与父进程共享文件描述符。libuv支持 继承文件描述符。

int main() {
    loop = uv_default_loop();

    /* ... */

    options.stdio_count = 3;
    uv_stdio_container_t child_stdio[3];
    child_stdio[0].flags = UV_IGNORE;
    child_stdio[1].flags = UV_IGNORE;
    child_stdio[2].flags = UV_INHERIT_FD;
    child_stdio[2].data.fd = 2;
    options.stdio = child_stdio;

    options.exit_cb = on_exit;
    options.file = args[0];
    options.args = args;

    int r;
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));
        return 1;
    }

    return uv_run(loop, UV_RUN_DEFAULT);
}

UV_IGNORE被重定向到/dev/null

UV_INHERIT_FD继承父进程的I/O,需要在data.fd中，设置父进程的描述符。

同理，子进程可以继承父进程的Stream，举例如下：

    args[1] = NULL;

    /* ... finding the executable path and setting up arguments ... */

    options.stdio_count = 3;
    uv_stdio_container_t child_stdio[3];
    child_stdio[0].flags = UV_IGNORE;
    child_stdio[1].flags = UV_INHERIT_STREAM;
    child_stdio[1].data.stream = (uv_stream_t*) client;
    child_stdio[2].flags = UV_IGNORE;
    options.stdio = child_stdio;

    options.exit_cb = cleanup_handles;
    options.file = args[0];
    options.args = args;

    // Set this so we can close the socket after the child process exits.
    child_req.data = (void*) client;
    int r;
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));

管道

管道不像linux下的符号“|”或者pipe，管道是用于两个进程之间进行通信的方法。具体如下：

父子管道：

父进程在创建子进程的时候uv_spawn，可以通过设置 uv_stdio_container_t.flags 参数，来设置父子进程之间的单向或双向通信， uv_stdio_container_t.flags可以是UV_CREATE_PIPE 、 UV_READABLE_PIPE or UV_WRITABLE_PIPE的一个或多个。可以同时设置多个，占不同的二进制位。UV_CREATE_PIPE 、 UV_READABLE_PIPE or UV_WRITABLE_PIPE都是从子进程的角度来看。

Arbitrary process IPC

Since domain sockets [1] can have a well known name and a location in the file-system they can be used for IPC between unrelated processes. The D-BUS system used by open source desktop environments uses domain sockets for event notification. Various applications can then react when a contact comes online or new hardware is detected. The MySQL server also runs a domain socket on which clients can interact with it.

When using domain sockets, a client-server pattern is usually followed with the creator/owner of the socket acting as the server. After the initial setup, messaging is no different from TCP, so we’ll re-use the echo server example.

pipe-echo-server/main.c

void remove_sock(int sig) {
    uv_fs_t req;
    uv_fs_unlink(loop, &req, PIPENAME, NULL);
    exit(0);
}

int main() {
    loop = uv_default_loop();

    uv_pipe_t server;
    uv_pipe_init(loop, &server, 0);

    signal(SIGINT, remove_sock);

    int r;
    if ((r = uv_pipe_bind(&server, PIPENAME))) {
        fprintf(stderr, "Bind error %s\n", uv_err_name(r));
        return 1;
    }
    if ((r = uv_listen((uv_stream_t*) &server, 128, on_new_connection))) {
        fprintf(stderr, "Listen error %s\n", uv_err_name(r));
        return 2;
    }
    return uv_run(loop, UV_RUN_DEFAULT);
}

We name the socket echo.sock which means it will be created in the local directory. This socket now behaves no different from TCP sockets as far as the stream API is concerned. You can test this server using socat:

$ socat - /path/to/socket

A client which wants to connect to a domain socket will use:

void uv_pipe_connect(uv_connect_t *req, uv_pipe_t *handle, const char *name, uv_connect_cb cb);

where name will be echo.sock or similar. On Unix systems, name must point to a valid file (e.g. /tmp/echo.sock). On Windows, name follows a \\?\pipe\echo.sock format.

进程相关API

/* uv_spawn() options. */
typedef enum {
  UV_IGNORE         = 0x00,
  UV_CREATE_PIPE    = 0x01,
  UV_INHERIT_FD     = 0x02,
  UV_INHERIT_STREAM = 0x04,

  /*
   * When UV_CREATE_PIPE is specified, UV_READABLE_PIPE and UV_WRITABLE_PIPE
   * determine the direction of flow, from the child process' perspective. Both
   * flags may be specified to create a duplex data stream.
   */
  UV_READABLE_PIPE  = 0x10,
  UV_WRITABLE_PIPE  = 0x20,

  /*
   * Open the child pipe handle in overlapped mode on Windows.
   * On Unix it is silently ignored.
   */
  UV_OVERLAPPED_PIPE = 0x40
} uv_stdio_flags;

typedef struct uv_stdio_container_s {
  uv_stdio_flags flags;

  union {
    uv_stream_t* stream;
    int fd;
  } data;
} uv_stdio_container_t;

typedef struct uv_process_options_s {
  uv_exit_cb exit_cb; /* Called after the process exits. */
  const char* file;   /* Path to program to execute. */
  /*
   * Command line arguments. args[0] should be the path to the program. On
   * Windows this uses CreateProcess which concatenates the arguments into a
   * string this can cause some strange errors. See the note at
   * windows_verbatim_arguments.
   */
  char** args;
  /*
   * This will be set as the environ variable in the subprocess. If this is
   * NULL then the parents environ will be used.
   */
  char** env;
  /*
   * If non-null this represents a directory the subprocess should execute
   * in. Stands for current working directory.
   */
  const char* cwd;
  /*
   * Various flags that control how uv_spawn() behaves. See the definition of
   * `enum uv_process_flags` below.
   */
  unsigned int flags;
  /*
   * The `stdio` field points to an array of uv_stdio_container_t structs that
   * describe the file descriptors that will be made available to the child
   * process. The convention is that stdio[0] points to stdin, fd 1 is used for
   * stdout, and fd 2 is stderr.
   *
   * Note that on windows file descriptors greater than 2 are available to the
   * child process only if the child processes uses the MSVCRT runtime.
   */
  int stdio_count;
  uv_stdio_container_t* stdio;
  /*
   * Libuv can change the child process' user/group id. This happens only when
   * the appropriate bits are set in the flags fields. This is not supported on
   * windows; uv_spawn() will fail and set the error to UV_ENOTSUP.
   */
  uv_uid_t uid;
  uv_gid_t gid;
} uv_process_options_t;

/*
 * These are the flags that can be used for the uv_process_options.flags field.
 */
enum uv_process_flags {
  /*
   * Set the child process' user id. The user id is supplied in the `uid` field
   * of the options struct. This does not work on windows; setting this flag
   * will cause uv_spawn() to fail.
   */
  UV_PROCESS_SETUID = (1 << 0),
  /*
   * Set the child process' group id. The user id is supplied in the `gid`
   * field of the options struct. This does not work on windows; setting this
   * flag will cause uv_spawn() to fail.
   */
  UV_PROCESS_SETGID = (1 << 1),
  /*
   * Do not wrap any arguments in quotes, or perform any other escaping, when
   * converting the argument list into a command line string. This option is
   * only meaningful on Windows systems. On Unix it is silently ignored.
   */
  UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS = (1 << 2),
  /*
   * Spawn the child process in a detached state - this will make it a process
   * group leader, and will effectively enable the child to keep running after
   * the parent exits.  Note that the child process will still keep the
   * parent's event loop alive unless the parent process calls uv_unref() on
   * the child's process handle.
   */
  UV_PROCESS_DETACHED = (1 << 3),
  /*
   * Hide the subprocess console window that would normally be created. This
   * option is only meaningful on Windows systems. On Unix it is silently
   * ignored.
   */
  UV_PROCESS_WINDOWS_HIDE = (1 << 4)
};

/*
 * uv_process_t is a subclass of uv_handle_t.
 */
struct uv_process_s {
  UV_HANDLE_FIELDS
  uv_exit_cb exit_cb;
  int pid;
  UV_PROCESS_PRIVATE_FIELDS
};

UV_EXTERN int uv_spawn(uv_loop_t* loop,
                       uv_process_t* handle,
                       const uv_process_options_t* options);
UV_EXTERN int uv_process_kill(uv_process_t*, int signum);
UV_EXTERN int uv_kill(int pid, int signum);
UV_EXTERN uv_pid_t uv_process_get_pid(const uv_process_t*);

uv_spawn：创建子进程。
uv_process_kill：杀掉子进程，通过进程结构体。
uv_kill：杀掉子进程，通过进程ID。
uv_process_get_pid：获取进程号。

参考文章：

http://docs.libuv.org/en/v1.x/process.html

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