葫芦里大有乾坤：不止是“葫芦娃”，更是无限可能！

目录

知识回顾: 进程间通信的前提

消息队列的原理

消息队列的系统调用msggetmsgctlmsgsnd查看消息队列的命令删除消息队列的命令

shmid_ds、msqid_ds、semqid_ds共同点有统一的结构如何管理struct ipc_perm?

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:

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

消息队列的系统调用类比OS

【Linux】System V 共享内存(

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msgsndmsgsnd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、shmflgmsgp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

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(

/shmdt(

*/ shmatt_t shm_nattch; /* No. of current attaches */ ... }; struct ipc_perm { key_t __key; /* Key supplied to shmget(

*/ 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(

*/ time_t msg_rtime; /* Time of last msgrcv(

*/ 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(

*/ pid_t msg_lrpid; /* PID of last msgrcv(

*/ }; struct ipc_perm { key_t __key; /* Key supplied to msgget(

*/ 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(

*/ 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.

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//addr

addr

addr3的类型都为struct ipc_perm* ((struct shmid_ds*)addr

-??? ((struct msqid_ds*)addr

-??? ((struct semqid_ds*)addr

-???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.

1

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

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