OS55.【Linux】System V消息队列的简单了解

目录

1.知识回顾: 进程间通信的前提

2.消息队列的原理

3.消息队列的系统调用

msgget

msgctl

msgsnd

查看消息队列的命令

删除消息队列的命令

4.shmid_ds、msqid_ds、semqid_ds共同点

有统一的结构

如何管理struct ipc_perm?

1.知识回顾: 进程间通信的前提

之前在前面的几篇文章都反复强调了进程间通信的前提:必须先让不同进程间看到同一份资源

,同一份资源可以是文件缓冲区也可以是内存块也可以是:对于System V消息队列,即必须先让不同进程间看到同一个队列,如下图:

2.消息队列的原理

进程间以数据块的形式发送数据,这些数据块需要排队形成队列

这个数据块是带类型的数据块

假设进程A和进程B使用消息队列进行通信,那么进程A和要区分是自己发送的数据块还是进程B发给它的

其次,该队列肯定由操作系统提供,不可能是进程提供的,因为进程间具有独立性

3.消息队列的系统调用

类比OS52.【Linux】System V 共享内存(1)文章讲的:shmget、shmat、shmdt、shmctl系统调用

msgget

作用: 获取或创建消息队列

和之前一样, key仍然用ftok算

若要创建全新的消息队列,则msgflg为IPC_CREAT | IPC_EXCL

返回值也和shmget类似:

注意: 消息队列的msgget和共享内存的shmget有区别,消息队列不用提供大小

msgctl

若要关闭消息队列,buf传空指针即可

msgsnd

msgsnd的snd是send的缩写,作用: 发送消息,换句话说,是发送数据块

msqid、msgflg分别类似shmid、shmflg

msgp指向要发送的数据块,msgsz为要发送数据块的大小

注意到: msgp的类型是const void*,用户可以自己定义类型进行传递

手册指出"caller-defined structure": 大致定义为struct msgbuf

查看消息队列的命令

ipcs -q

删除消息队列的命令

ipcrm -M key #或 ipcrm -m shmid

4.shmid_ds、msqid_ds、semqid_ds共同点

shmid_ds负责描述共享内存:

struct shmid_ds { struct ipc_perm shm_perm; /* Ownership and permissions */ size_t shm_segsz; /* Size of segment (bytes) */ time_t shm_atime; /* Last attach time */ time_t shm_dtime; /* Last detach time */ time_t shm_ctime; /* Creation time/time of last modification via shmctl() */ pid_t shm_cpid; /* PID of creator */ pid_t shm_lpid; /* PID of last shmat(2)/shmdt(2) */ shmatt_t shm_nattch; /* No. of current attaches */ ... }; struct ipc_perm { key_t __key; /* Key supplied to shmget(2) */ uid_t uid; /* Effective UID of owner */ gid_t gid; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short mode; /* Permissions + SHM_DEST and SHM_LOCKED flags */ unsigned short __seq; /* Sequence number */ };

msqid_ds负责描述消息队列:

struct msqid_ds { struct ipc_perm msg_perm; /* Ownership and permissions */ time_t msg_stime; /* Time of last msgsnd(2) */ time_t msg_rtime; /* Time of last msgrcv(2) */ time_t msg_ctime; /* Time of creation or last modification by msgctl() */ unsigned long msg_cbytes; /* # of bytes in queue */ msgqnum_t msg_qnum; /* # number of messages in queue */ msglen_t msg_qbytes; /* Maximum # of bytes in queue */ pid_t msg_lspid; /* PID of last msgsnd(2) */ pid_t msg_lrpid; /* PID of last msgrcv(2) */ }; struct ipc_perm { key_t __key; /* Key supplied to msgget(2) */ uid_t uid; /* Effective UID of owner */ gid_t gid; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short mode; /* Permissions */ unsigned short __seq; /* Sequence number */ };

进程间通信其实还可以用信号量,semqid_ds负责描述信号量:

struct semid_ds { struct ipc_perm sem_perm; /* Ownership and permissions */ time_t sem_otime; /* Last semop time */ time_t sem_ctime; /* Creation time/time of last modification via semctl() */ unsigned long sem_nsems; /* No. of semaphores in set */ }; struct ipc_perm { key_t __key; /* Key supplied to semget(2) */ uid_t uid; /* Effective UID of owner */ gid_t gid; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short mode; /* Permissions */ unsigned short __seq; /* Sequence number */ };

有统一的结构

System V的进程间通信被精心设计过,三个结构体的第一个字段类型都是struct ipc_perm,尽管shm_perm msg_perm sem_perm的名字不一样

struct xxxid_ds { struct ipc_perm xxx_perm; /* Ownership and permissions */ //...... }; struct ipc_perm { key_t __key; uid_t uid; gid_t gid; uid_t cuid; gid_t cgid; unsigned short mode; unsigned short __seq; };

这里面有面向对象的思想,struct xxxid_ds含有struct ipc_perm xxx_perm,显然后者是基类,前者是子类

如何管理struct ipc_perm?

答:先描述再组织

先描述: ipc权限使用struct ipc_perm

再组织: 使用数据结构将struct ipc_perm串起来

描述ipc权限有两种,struct ipc_perm是用户层的描述,struct kern_ipc_perm是内核层的描述

例如linux 2.6.18内核的/include/linux/ipc.h定义了struct kern_ipc_perm:

/* used by in-kernel data structures */ struct kern_ipc_perm { spinlock_t lock; int deleted; key_t key; uid_t uid; gid_t gid; uid_t cuid; gid_t cgid; mode_t mode; unsigned long seq; void *security; };

在/ipc/util.h中定义了struct ipc_id_ary ,其使用变长数组织ipc权限:

struct ipc_id_ary { int size; struct kern_ipc_perm *p[0]; //写在结构体末尾,是变长数组 };

将shm_perm、msg_perm、sem_perm的地址写入struct kern_ipc_perm *p[0],那么就能很方便地使用箭头->和强制类型转换访问struct shmid_ds msqid_ds semid_ds的每一个字段:

因为三个结构体的第一个字段类型都是struct ipc_perm,即struct ipc_perm字段的地址等同于struct xxxid_ds的地址

//addr1、addr2、addr3的类型都为struct ipc_perm* ((struct shmid_ds*)addr1)->??? ((struct msqid_ds*)addr2)->??? ((struct semqid_ds*)addr3)->???

管理ipc权限就相当于对struct kern_ipc_perm *p[0]进行增删改查

描述xxid有两种,struct xxxid_ds是用户层的描述,struct xxxid_kernel是内核层的描述

例如Linux 6.18.6中的/ipc/shm.c:

struct shmid_kernel /* private to the kernel */ { struct kern_ipc_perm shm_perm; struct file *shm_file; unsigned long shm_nattch; unsigned long shm_segsz; time64_t shm_atim; time64_t shm_dtim; time64_t shm_ctim; struct pid *shm_cprid; struct pid *shm_lprid; struct ucounts *mlock_ucounts; /* * The task created the shm object, for * task_lock(shp->shm_creator) */ struct task_struct *shm_creator; /* * List by creator. task_lock(->shm_creator) required for read/write. * If list_empty(), then the creator is dead already. */ struct list_head shm_clist; struct ipc_namespace *ns; } __randomize_layout;

补: shmid msqid semid是数组的下标,是线性递增的