System V共享内存区


1.shmget 函数：创建一个新的共享内存区，或者访问一个已经存在的共享内存区

        int shmget(key_t key, size_t size, int oflag);

        key 可以是ftok返回的值，也可以是IPC_PRIVATE

        size 共享内存区的大小

        oflag 共享内存去的操作及读写权限(IPC_CREAT、或者IPC_CREAT|IPC_EXCL，还可以与读写权限按位与)

2.shmat 函数：通过该函数附接到共享内存区(及返回其起始地址)

        void* shmat(int shmid, const char* shmaddr, int flag);

        shmid 创建的共享内存标示符

        shmaddr 一般填NULL，由系统决定起始地址

        flag  可以指定只读(SHM_RDONLY)，一般赋值为0，读写权限使用shmget指定的

3.shmdt 函数：断开连接共享内存区

        int shmdt(const void* shmaddr);

4.shmctl 函数：提供对共享内存区的多种操作

        int shmctl(int shmid, int cmd, struct shm-d_ds* buff);

        cmd：IPC_RMID、IPC_SET、IPC_STAT

        IPC_RMID：从系统中删除共享内存

        IPC_SET  ：设置共享内存shmid_ds结构的成员

        IPC_STAT：返回共享内存区当前的shmid_ds结构

==============================================================

/**************************************
*Date:Fri Apr 25 10:32:47 CST 2014
*Target:test System V shared memory
***************************************/
#include <iostream>
extern "C"{
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/errno.h>
#include <unistd.h>
#include <fcntl.h>
}

#define SHM_KEY 0x10002
#define SHM_SIZE 1024*1024*1
using namespace std;
#pragma pack(push,1)
typedef struct Q30MSG{
char MsgType[3+1];
char Iverstd[3+1];
int ID;
char Reverse[1];
}Q30MSG;
#pragma pack(pop)

void write(void* ptr,int size)
{
Q30MSG* temp = (Q30MSG*)ptr;
for(int i = 0;i < size;i++){
Q30MSG msg;
strcpy(msg.MsgType,"Q30");
strcpy(msg.Iverstd,"999");
msg.ID = i + 1;
*temp++ = msg;
}
}

void read(void* ptr,int size)
{
Q30MSG* temp = (Q30MSG*)ptr;
while(size-- != 0){
Q30MSG msg = *temp;
cout << msg.ID << "\t" << msg.MsgType << "\t" << msg.Iverstd << endl;
temp++;
}
}

Q30MSG* readValue(void* ptr,int position)
{
Q30MSG* temp = (Q30MSG*)ptr;
return temp + position;
}

string getValue(void* ptr,int offset)
{
char* addr = (char*)ptr;
char msgType[3+1] = {0};
strncpy(msgType,addr+offset,3);
cout << "getValue:" << msgType << endl;
return msgType;
}

void delShm(void* ptr)
{
int res = shmdt(ptr);
if(res == -1){
cout << "failed to disconnect shm,error:" << strerror(errno) << endl;
}
}

void desShm(int shm_id)
{
int res = shmctl(shm_id,IPC_RMID,NULL);
if(res == -1){
cout << "failed to destroy shm,shm_id:" << shm_id << ",error:" << strerror(errno) << endl;
}
}

int main(int argc,char* argv[])
{
int oflag = IPC_CREAT;//|IPC_EXCL;
oflag |= 0666;
int shm_id = shmget(SHM_KEY,SHM_SIZE,oflag);
if(shm_id == -1){
if(errno == EEXIST){
cout << "this shm has exited" << endl;
}
else{
cout << "failed to create the shm,error:" << strerror(errno) << endl;
return -1;
}
}
void* ptr = NULL;
cout << "shm_id:" << shm_id << endl;
ptr = shmat(shm_id,NULL,0);
if((void*)-1 == ptr){
cout << "failed to at shm,error:" << strerror(errno) << endl;
return -1;
}
write(ptr,5);
read(ptr,5);

cout << "####################" << endl;
Q30MSG* msg = readValue(ptr,4);
if(msg != NULL){
cout << msg->ID << "\t" << msg->MsgType << "\t" << msg->Iverstd << endl;
}
cout << "####################" << endl;
string msgType = getValue(ptr,0);
msgType = getValue(ptr,0+sizeof(Q30MSG));
string iverstd = getValue(ptr,4);
cout << "####################" << endl;
delShm(ptr);
desShm(shm_id);
return 0;
}