【Linux學習筆記】進程間通信之共享內存

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文章目錄

【Linux學習筆記】進程間通信之共享內存
- 前言
- 一. system V共享內存
- - 1.1 共享內存數據結構
  - 1.2 共享內存函數
  - 1.3 共享內存實現通信
  - 1.4 借助管道實現訪問控制版的共享內存
- 后言

前言

哈嘍，各位小伙伴大家好!上期我們講了進程間通信之管道今天我們講的是進程間通信之共享內存。話不多說，我們進入正題！向大廠沖鋒！

一. system V共享內存

共享內存區是最快的IPC形式。?旦這樣的內存映射到共享它的進程的地址空間，這些進程間數據傳遞不再涉及到內核，換句話說是進程不再通過執?進?內核的系統調?來傳遞彼此的數據

共享內存示意圖

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1.1 共享內存數據結構

struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
int shm_segsz; /* size of segment
(bytes) */
__kernel_time_t shm_atime; /* last attach time */
__kernel_time_t shm_dtime; /* last detach time */
__kernel_time_t shm_ctime; /* last change time */
__kernel_ipc_pid_t shm_cpid; /* pid of creator */
__kernel_ipc_pid_t shm_lpid; /* pid of last operator */
unsigned short shm_nattch; /* no. of current
attaches */
unsigned short shm_unused; /* compatibility */
void shm_unused2; / ditto - used by
DIPC */
void shm_unused3; / unused */
};

1.2 共享內存函數

shmget函數

功能：?來創建共享內存
原型
int shmget(key_t key, size_t size, int shmflg);
參數
key:這個共享內存段名字
size:共享內存??
shmflg:由九個權限標志構成，它們的?法和創建?件時使?的mode模式標志是?樣的
取值為IPC_CREAT：共享內存不存在，創建并返回；共享內存已存在，獲取并返回。
取值為IPC_CREAT | IPC_EXCL：共享內存不存在，創建并返回；共享內存已存在，出
錯返回。
返回值：成功返回?個?負整數，即該共享內存段的標識碼；失敗返回-1

shmat函數

功能：將共享內存段連接到進程地址空間
原型
void *shmat(int shmid, const void *shmaddr, int shmflg);
參數
shmid: 共享內存標識
shmaddr:指定連接的地址
shmflg:它的兩個可能取值是SHM_RND和SHM_RDONLY
返回值：成功返回?個指針，指向共享內存第?個節；失敗返回-1

說明：

shmaddr為NULL，核??動選擇?個地址
shmaddr不為NULL且shmflg?SHM_RND標記，則以shmaddr為連接地址。
shmaddr不為NULL且shmflg設置了SHM_RND標記，則連接的地址會?動向下調整為SHMLBA的整數倍。
公式：shmaddr - (shmaddr % SHMLBA)
shmflg=SHM_RDONLY，表?連接操作?來只讀共享內存

shmdt函數

功能：將共享內存段與當前進程脫離
原型
int shmdt(const void *shmaddr);
參數
shmaddr: 由shmat所返回的指針
返回值：成功返回0；失敗返回-1
注意：將共享內存段與當前進程脫離不等于刪除共享內存段

shmctl函數

功能：?于控制共享內存
原型
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
參數
shmid:由shmget返回的共享內存標識碼
cmd:將要采取的動作（有三個可取值）
buf:指向?個保存著共享內存的模式狀態和訪問權限的數據結構
返回值：成功返回0；失敗返回-1

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1.3 共享內存實現通信

測試代碼結構

# ls
client.c comm.c comm.h Makefile server.c
# cat Makefile
.PHONY:all
all:server client
client:client.c comm.c
gcc -o $@ $^
server:server.c comm.c
gcc -o $@ $^
.PHONY:clean
clean:
rm -f client server
1

comm.h

#ifndef _COMM_H_
#define _COMM_H_
# include <stdio.h>
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/shm.h>
# define PATHNAME "."
# define PROJ_ID 0x6666
int createShm(int size);
int destroyShm(int shmid);
int getShm(int size);
# endif

comm.c

#include "comm.h"
static int commShm(int size, int flags)
{
key_t key = ftok(PATHNAME, PROJ_ID);
if(key < 0){
perror("ftok");
return -1;
}
int shmid = 0;
if( (shmid = shmget(key, size, flags)) < 0){
perror("shmget");
return -2;
}
return shmid;
}
int destroyShm(int shmid)
{
if(shmctl(shmid, IPC_RMID, NULL) < 0){
perror("shmctl");
return -1;
}
return 0;
}
int createShm(int size)
{
return commShm(size, IPC_CREAT|IPC_EXCL|0666);
}
int getShm(int size)
{
return commShm(size, IPC_CREAT);
}

server.c

#include "comm.h"
int main()
{
int shmid = createShm(4096);
char *addr = shmat(shmid, NULL, 0);
sleep(2);
int i = 0;
while(i++<26){
printf("client# %s\n", addr);
sleep(1);
}
shmdt(addr);
sleep(2);
destroyShm(shmid);
return 0;
}

client.c

#include "comm.h"
int main()
{
int shmid = getShm(4096);
sleep(1);
char *addr = shmat(shmid, NULL, 0);
sleep(2);
int i = 0;
while(i<26){
addr[i] = 'A'+i;
i++;
addr[i] = 0;
sleep(1);
}
shmdt(addr);
sleep(2);
return 0;
}

1.4 借助管道實現訪問控制版的共享內存

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Comm.hpp

以下是提取的代碼：```cpp
#pragma once#include <fcntl.h>#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <cassert>
#include <cstdio>
#include <ctime>
#include <cstring>
#include <iostream>
#include <string>using namespace std;#define Debug 0
#define Notice 1
#define Warning 2
#define Error 3
const std::string msg[] = {"Debug","Notice","Warning","Error"
};std::ostream &Log(std::string message, int level) {std::cout << " | " << (unsigned)time(nullptr) << " | " << msg[level] << " | " << message;return std::cout;
}#define PATH_NAME "/home/hyb"
#define PROJ_ID 0x66
#define SHM_SIZE 4096  // 共享內存的大小，最好是頁(PAGE: 4096)的整數倍
#define FIFO_NAME "./fifo"class Init {
public:Init() {umask(0);int n = mkfifo(FIFO_NAME, 0666);assert(n == 0);(void)n;Log("create fifo success", Notice) << "\n";}~Init() {unlink(FIFO_NAME);Log("remove fifo success", Notice) << "\n";}
};#define READ O_RDONLY
#define WRITE O_WRONLY
int OpenFIFO(std::string pathname, int flags) {int fd = open(pathname.c_str(), flags);assert(fd >= 0);return fd;
}void CloseFifo(int fd) {close(fd);
}void Wait(int fd) {Log("等待中....", Notice) << "\n";uint32_t temp = 0;ssize_t s = read(fd, &temp, sizeof(uint32_t));assert(s == sizeof(uint32_t));(void)s;
}void Signal(int fd) {uint32_t temp = 1;ssize_t s = write(fd, &temp, sizeof(uint32_t));assert(s == sizeof(uint32_t));(void)s;Log("喚醒中....", Notice) << "\n";
}string TransToHex(key_t k) {char buffer[32];sprintf(buffer, sizeof buffer, "0x%x", k);return buffer;
}

ShmServer.cc

以下是提取的代碼：```cpp
#include "Comm.hpp"
Init init;int main() {// 1. 創建公共的Key值key_t k = ftok(PATH_NAME, PROJ_ID);assert(k != -1);Log("create key done", Debug) << " server key : " << TransToHex(k) << endl;// 2. 創建共享內存 -- 建議要創建一個全新的共享內存 -- 通信的發起者int shmid = shmget(k, SHM_SIZE, IPC_CREAT | IPC_EXCL | 0666);if (shmid == -1) {perror("shmget");exit(1);}Log("create shm done", Debug) << " shmid : " << shmid << endl;// 3. 將指定的共享內存，掛接到自己的地址空間char *shmaddr = (char*)shmat(shmid, nullptr, 0);Log("attach shm done", Debug) << " << shmid << endl;// 4. 訪問控制int fd = OpenFIFO(FIFO_NAME, O_RDONLY);while (true) {// 阻塞Wait(fd);// 臨界區printf("%s\n", shmaddr);if (strcmp(shmaddr, "quit") == 0)break;}CloseFifo(fd);// 5. 將指定的共享內存，從自己的地址空間中去關聯int n = shmdt(shmaddr);assert(n != -1);(void)n;Log("detach shm done", Debug) << " << shmid << endl;// 6. 刪除共享內存, IPC_RMID即便是有進程和當下的shm掛接，依舊刪除共享內存n = shmctl(shmid, IPC_RMID, nullptr);assert(n != -1);(void)n;Log("delete shm done", Debug) << " << shmid << endl;return 0;
}

ShmClient.cc

以下是提取的代碼：```cpp
#include "Comm.hpp"int main() {// 1. 創建公共的key值key_t k = ftok(PATH_NAME, PROJ_ID);if (k < 0) {Log("create key failed", Error) << " client key : " << TransToHex(k) << endl;exit(1);}Log("create key done", Debug) << " client key : " << TransToHex(k) << endl;// 2. 獲取共享內存int shmid = shmget(k, SHM_SIZE, 0);if (shmid < 0) {Log("create shm failed", Error) << " client key : " << TransToHex(k) << endl;exit(2);}Log("create shm success", Error) << " client key : " << TransToHex(k) << endl;// 3. 掛接共享內存char* shmaddr = (char*)shmat(shmid, nullptr, 0);if (shmaddr == nullptr) {Log("attach shm failed", Error) << " client key : " << TransToHex(k) << endl;exit(3);}Log("attach shm success", Error) << " client key : " << TransToHex(k) << endl;// 4. 寫int fd = OpenFIFO(FIFO_NAME, O_WRONLY);while (true) {ssize_t s = read(0, shmaddr, SHM_SIZE - 1);if (s > 0) {shmaddr[s - 1] = 0;Signal(fd);if (strcmp(shmaddr, "quit") == 0)break;}}CloseFifo(fd);// 5. 去關聯int n = shmdt(shmaddr);assert(n != -1);Log("detach shm success", Error) << " client key : " << TransToHex(k) << endl;return 0;
}