进程间通信 IPC 系列一 消息队列

本系列主要根据内核代码，分析 消息队列 共享内存 信号量 三种 进程间通信的方法。
内核 目录下有个ipc文件夹， 实现代码在这个里面

linux/ipc$ ls
built-in.a  ipc_sysctl.c  modules.order  mqueue.c  msgutil.c    sem.c  syscall.c  util.h
compat.c    Makefile      mq_sysctl.c    msg.c     namespace.c  shm.c  util.c

ipc_sysctl.c

static struct ctl_table ipc_kern_table[] = {
	{
		.procname	= "shmmax",                         #  共享内存最多的条数
		.data		= &init_ipc_ns.shm_ctlmax,
		.maxlen		= sizeof(init_ipc_ns.shm_ctlmax),
		.mode		= 0644,
		.proc_handler	= proc_ipc_doulongvec_minmax,
	},

device_initcall(ipc_sysctl_init);

这个文件主要是 系统启动的时候调用 device_initcall 这篇文章介绍的不错 device_initcall
register_sysctl_table(ipc_root_table); 这里会去注册ipc_kern_table
系统启动的时候会注册ipc 的配置， 初始化一些ipc的权限，权限，ipc最多的条数，超过的时候的处理函数。

syscall.c

/*定义系统调用 ipc 六个参数*/
SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
		unsigned long, third, void __user *, ptr, long, fifth)
{
	int version, ret;

	version = call >> 16; /* hack for backward compatibility */
	call &= 0xffff;    /* 解析出系统调用的类型*/

	switch (call) {
	case SEMOP:
		return ksys_semtimedop(first, (struct sembuf __user *)ptr,
				       second, NULL);
	case SEMTIMEDOP:
		if (IS_ENABLED(CONFIG_64BIT) || !IS_ENABLED(CONFIG_64BIT_TIME))
			return ksys_semtimedop(first, ptr, second,
			        (const struct __kernel_timespec __user *)fifth);
		else if (IS_ENABLED(CONFIG_COMPAT_32BIT_TIME))
			return compat_ksys_semtimedop(first, ptr, second,
			        (const struct old_timespec32 __user *)fifth);
		else
			return -ENOSYS;

	case SEMGET:
		return ksys_semget(first, second, third);
	case SEMCTL: {
		unsigned long arg;
		if (!ptr)
			return -EINVAL;
		if (get_user(arg, (unsigned long __user *) ptr))
			return -EFAULT;
		return ksys_semctl(first, second, third, arg);
	}

	case MSGSND:
		return ksys_msgsnd(first, (struct msgbuf __user *) ptr,
				  second, third);
	case MSGRCV:
		switch (version) {
		case 0: {
			struct ipc_kludge tmp;
			if (!ptr)
				return -EINVAL;

			if (copy_from_user(&tmp,
					   (struct ipc_kludge __user *) ptr,
					   sizeof(tmp)))
				return -EFAULT;
			return ksys_msgrcv(first, tmp.msgp, second,
					   tmp.msgtyp, third);
		}
		default:
			return ksys_msgrcv(first,
					   (struct msgbuf __user *) ptr,
					   second, fifth, third);
		}
		/**/
	case MSGGET:
		return ksys_msgget((key_t) first, second);
	case MSGCTL:
		return ksys_msgctl(first, second,
				   (struct msqid_ds __user *)ptr);

	case SHMAT:
		switch (version) {
		default: {
			unsigned long raddr;
			ret = do_shmat(first, (char __user *)ptr,
				       second, &raddr, SHMLBA);
			if (ret)
				return ret;
			return put_user(raddr, (unsigned long __user *) third);
		}
		case 1:
			/*
			 * This was the entry point for kernel-originating calls
			 * from iBCS2 in 2.2 days.
			 */
			return -EINVAL;
		}
	case SHMDT:
		return ksys_shmdt((char __user *)ptr);
	case SHMGET:
		return ksys_shmget(first, second, third);
	case SHMCTL:
		return ksys_shmctl(first, second,
				   (struct shmid_ds __user *) ptr);
	default:
		return -ENOSYS;
	}
}

msg.c

long ksys_msgget(key_t key, int msgflg)
{
	struct ipc_namespace *ns;
	static const struct ipc_ops msg_ops = {
		.getnew = newque,
		.associate = security_msg_queue_associate,
	};
	struct ipc_params msg_params;

	ns = current->nsproxy->ipc_ns;   /* 获取当前的 namespace */

	msg_params.key = key;
	msg_params.flg = msgflg;

	return ipcget(ns, &msg_ids(ns), &msg_ops, &msg_params);
}

ipcget ------> ipcget_public ---->

	struct kern_ipc_perm *ipcp;
	int flg = params->flg;
	int err;

	/*
	 * Take the lock as a writer since we are potentially going to add
	 * a new entry + read locks are not "upgradable"
	 */
	ipcp = ipc_findkey(ids, params->key);  /*查看ipc id 是否已经被使用*/
	if (ipcp == NULL) {
		/* key not used */
		if (!(flg & IPC_CREAT))
			err = -ENOENT;
		else
			err = ops->getnew(ns, params);   /* 创建一个新的msg id*/

/**
 * newque - Create a new msg queue
 * @ns: namespace
 * @params: ptr to the structure that contains the key and msgflg
 *
 * Called with msg_ids.rwsem held (writer)
 */
static int newque(struct ipc_namespace *ns, struct ipc_params *params)
{
	struct msg_queue *msq;
	int retval;
	key_t key = params->key;
	int msgflg = params->flg;

	msq = kvmalloc(sizeof(*msq), GFP_KERNEL);
	if (unlikely(!msq))
		return -ENOMEM;

	msq->q_perm.mode = msgflg & S_IRWXUGO;
	msq->q_perm.key = key;

	msq->q_perm.security = NULL;
	retval = security_msg_queue_alloc(&msq->q_perm);
	if (retval) {
		kvfree(msq);
		return retval;
	}

	msq->q_stime = msq->q_rtime = 0;
	msq->q_ctime = ktime_get_real_seconds();
	msq->q_cbytes = msq->q_qnum = 0;
	msq->q_qbytes = ns->msg_ctlmnb;
	msq->q_lspid = msq->q_lrpid = NULL;
	INIT_LIST_HEAD(&msq->q_messages);
	INIT_LIST_HEAD(&msq->q_receivers);
	INIT_LIST_HEAD(&msq->q_senders);

	/* ipc_addid() locks msq upon success. */
	retval = ipc_addid(&msg_ids(ns), &msq->q_perm, ns->msg_ctlmni);
	if (retval < 0) {
		ipc_rcu_putref(&msq->q_perm, msg_rcu_free);
		return retval;
	}

	ipc_unlock_object(&msq->q_perm);
	rcu_read_unlock();

	return msq->q_perm.id;
}

最后

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