Linux IPC System V Shared Memory: An Advanced Approach to High-Speed ​​Data Exchange

Linux system is an operating system that supports concurrent execution of multi-tasks. It can run multiple processes at the same time, thereby improving system utilization and efficiency. However, if data exchange and collaboration are required between these processes, some inter-process communication (IPC) methods need to be used, such as signals, message queues, semaphores, etc. Among them, System V shared memory is a relatively advanced and high-speed IPC method that allows two or more processes to exchange data through a memory area without the need for data copying and conversion. This article will introduce the method of System V shared memory in Linux systems, including the creation, mapping, reading, writing, unmapping and deletion of shared memory.

Model

#include
#include
#include
ftok()      //获取key值            
shmget()    //创建/获取共享内存 
shmat()     //挂接共享内存        
shmdt()     //脱接共享内存        
shmctl()    //删除共享内存        

ftok()

//获取key值, key值是System V IPC的标识符，成功返回key，失败返回-1设errno
//同pathname+同 proj_id==>同key_t;
key_t ftok(const char *pathname, int proj_id);

pathname: File name
proj_id: a number from 1 to 255, representing project_id

key_t key=ftok(".",100);    //“.”就是一个存在且可访问的路径, 100是假设的proj_id
    if(-1==key)
        perror("ftok"),exit(-1);

shmget()

//创建/获取共享内存，成功返回共享内存的标识符shmid，失败返回-1设errno
int shmget(key_t key, size_t size, int shmflg);     //多设为int shmid=...  和shmat()一起用比较好看

key: The return value of ftok()
size: The size of the shared memory will actually be allocated according to the page size (PAGE_SIZE). 0 means to obtain the allocated shared memory
shmflg: Specific operation flag

• IPC_CREAT: If it does not exist, create it. You need to set "|Permission Information" in shmflg, eg: |0664; If it exists, open it
• IPC_EXCL: Used in conjunction with IPC_CREAT, if it exists, the creation will fail ==> an error will be reported, set errno
• 0: Get existing shared memory

//创建shared memory
shmid=shmget(key,4,IPC_CREAT|IPC_EXCL|0664);
if(-1==shmid)
    perror("shmget"),exit(-1);

Q: Since shmget() can create, what is the use of ftok()
A: shmget is used to create shared memory, ftok() is only used to generate A key. In fact, you can also run it by filling in a number at the key position. However, compared with the system-generated one, it is easy to cause conflicts, so it is best to use ftok to generate a key

shmat()

//挂接共享内存，成功返回映射内存的地址，失败返回(void*)-1设errno
void *shmat(int shmid, const void *shmaddr, int shmflg);

shmid: The return value of shmget()
shmaddr

• NULL means selected by the system (same as mmap())
• If it is not NULL and shflg is SHM_RND, it will find the nearest address starting from shmaddr and start allocating points according to the principle of page alignment. Otherwise, the address specified by shmaddr must be page aligned
• shmflg: The flag of the operation, just give it 0
  • SHM_RDONLYIndicates that the process attached to the shared memory must have read permission
  • SHM_REMAP (Linux-specific) means that if the shared memory to be mapped already has existing memory, then replace the old one

//挂接共享内存
void* pv=shmat(shmid,NULL,0);
if((void*)-1==pv)
    perror("shmat"),exit(-1);

shmdt()

//脱接共享内存，成功返回0,失败返回-1设errno
int shmdt(const void *shmaddr);
//脱接shm
int res=shmdt(pv);
if(-1==res)
    perror("shmdt"),exit(-1);

shmctl()

//共享内存管理，成功返回0,失败返回-1设errno
int shmctl(int shmid, int cmd, struct shmid_ds *buf);

shmid: The id of the shared memory, returned by shmget()
buf: Pointer of type shmid_ds

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;      /* Last change time */
    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 */
};

cmd

• IPC_STAT表示从内核中拷贝关于这个shmid的信息到buf指向的shmid_ds中

• IPC_SET 将buf指向的shmid_ds的信息写入到内核的结构体中，同时更新成员shm_ctime

• IPC_RMID销毁共享内存

• IPC_INFO(Linux-specific)返回系统对共享内存的限制写入到buf指向的时shminfo结构体中

  //_GNU_SOURCE
  struct  shminfo {
      unsigned long   shmmax; /* Maximum segment size */
      unsigned long   shmmin; /* Minimum segment size; always 1 */
      unsigned long   shmmni; /* Maximum number of segments */
      unsigned long   shmseg; /* Maximum number of segments that a process 
  can attach; unused within kernel */
      unsigned long   shmall; /* Maximum number of pages of shared memory, 
  system-wide */
   };
   //shmmni, shmmax, and shmall 可以童工/proc里的同名文件进行修改
  

• SHM_INFO(Linux-specific) 返回一个shm_info结构体来表示该共享内存消耗的系统资源

  //_GNU_SOURCE
  struct shm_info {
      int             used_ids;   /* # of currently existing segments */
      unsigned long   shm_tot;    /* Total number of shared memory pages */
      unsigned long   shm_rss;    /* # of resident shared memory pages */
      unsigned long   shm_swp;    /* # of swapped shared memory pages */
      unsigned long   swap_attempts; /* Unused since Linux 2.4 */
      unsigned long   swap_successes;/* Unused since Linux 2.4 */
   };
  

• SHM_STAT(Linux-specific) 为IPC_STAT返回一个shmid_ds结构结构体，不同的是shmid的参数不是一个标识符，而是内核中一个包含了系统中所有共享内存信息的索引

• SHM_LOCK防止系统将共享内存放到swap区，IPC_STAT读到的信息中SHM_LOCKED标记就被设置了

• SHM_UNLOCK 解除锁定，即允许共享内存被系统放到swap区

//使用IPC_RMID删除共享内存
int res=shmctl(shmid,IPC_RMID,NULL);
if(-1==res)
    perror("shmctl"),exit(-1);

例子

//Sys V IPC shm
int shmid;          //定义全局变量记录id
void fa(int signo){
    printf("deleting shared memories...\n");
    sleep(3);//其实没用
    int res=shmctl(shmid,IPC_RMID,NULL);
    if(-1==res)
        perror("shmctl"),exit(-1);
    printf("delete success\n");
    exit(0);    //ctrl+C已经不能结束while(1),用exit(0)来终结
}
int main(){
    //获取key
    key_t key=ftok(".",100);    //.就是一个存在且可访问的路径, 100是随便给的
    if(-1==key)
        perror("ftok"),exit(-1);
    printf("key=%#x\n",key);    //打印出进制的标示,即0x
    //创建shared memory
    shmid=shmget(key,4,IPC_CREAT|IPC_EXCL|0664);
    if(-1==shmid)
        perror("shmget"),exit(-1);
    printf("shmid=%d\n",shmid);
    //挂接shm
    void* pv=shmat(shmid,NULL,0);
    if((void*)-1==pv)
        perror("shmat"),exit(-1);
    printf("link shared memory success\n");
    //访问shm
    int* pi=(int*)pv;
    *pi=100;
    //脱接shm
    int res=shmdt(pv);
    if(-1==res)
        perror("shmdt"),exit(-1);
    printf("unlink success\n");
    //如果不再使用,删除shm
    printf("删除共享内存请按Ctrl C...\n");
    if(SIG_ERR==signal(SIGINT,fa))
        perror("signal"),exit(-1);
    while(1);
    return 0;
}

本文介绍了Linux系统中System V 共享内存的方法，包括共享内存的创建、映射、读写、解除映射和删除等方面。通过了解和掌握这些知识，我们可以更好地使用System V 共享内存来实现进程间通信，提高系统的性能和可靠性。当然，Linux系统中System V 共享内存还有很多其他的特性和用法，需要我们不断地学习和探索。希望本文能给你带来一些启发和帮助。

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