共享内存的简介

共享内存区是最快的IPC形式。一旦这样的内存映射到共享它的进程的地址空间，这些进程间数据传递不再涉及到内核，换句话说是进程不再通过执行进入内核的系统调用来传递彼此的数据

共享内存示意图

共享内存数据结构

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 */
};

共享内存用到的函数

shmget

功能：用来创建共享内存
原型
int shmget(key_t key, size_t size, int shmflg);
参数
key:这个共享内存段名字
size:共享内存大小
shmflg:由九个权限标志构成，它们的用法和创建文件时使用的mode模式标志是一样的返回值：成功返回一个非负整数，即该共享内存段的标识码；失败返回-1

shmat

功能：将共享内存段连接到进程地址空间
原型
void *shmat(int shmid, const void *shmaddr, int shmflg);参数
shmid: 共享内存标识
shmaddr:指定连接的地址
shmflg:它的两个可能取值是SHM_RND和SHM_RDONLY
返回值：成功返回一个指针，指向共享内存第一个节；失败返回-1

具体的代码

comm.hpp

#ifndef __COMM_HPP__
#define __COMM_HPP__

#include <iostream>
#include <string>
#include <cstdlib>
#include <cstring>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <sys/types.h>
#include <sys/stat.h>

#include "log.hpp"

using namespace std;
string pathname = "/home/zyficl";
int proj_id = 1;
Log log;
#define SIZe 4096
int Gratekey()
{

    int t = ftok(pathname.c_str(), proj_id);
    if (t < 0)
    {
        log(Fatal, "ftok is errno", strerror(errno));
        exit(0);
    }
    return t;
}

int Grate_shame(int flag)
{
    int shmid = shmget(Gratekey(), SIZe, flag);
    if (shmid < 0)
    {
        log(Fatal, "shmget is errno", strerror(errno));
        exit(1);
    }
    return shmid;
}
int Grateshame()
{
    return Grate_shame(IPC_CREAT | IPC_EXCL | 0666);
}
int getshm()
{
    return Grate_shame(IPC_CREAT);
}

#define FIFO_FILE "./myfifo"
#define MODE 0664

enum
{
    FIFO_CREATE_ERR = 1,
    FIFO_DELETE_ERR,
    FIFO_OPEN_ERR
};

class Init
{
public:
    Init()
    {
        // 创建管道
        int n = mkfifo(FIFO_FILE, MODE);
        if (n == -1)
        {
            perror("mkfifo");
            exit(FIFO_CREATE_ERR);
        }
    }
    ~Init()
    {

        int m = unlink(FIFO_FILE);
        if (m == -1)
        {
            perror("unlink");
            exit(FIFO_DELETE_ERR);
        }
    }
};

#endif

log.hpp

#pragma once

#include <iostream>
#include <time.h>
#include <stdarg.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>

#define SIZE 1024

#define Info 0
#define Debug 1
#define Warning 2
#define Error 3
#define Fatal 4

#define Screen 1
#define Onefile 2
#define Classfile 3

#define LogFile "log.txt"

class Log
{
public:
    Log()
    {
        printMethod = Screen;
        path = "./log/";
    }
    void Enable(int method)
    {
        printMethod = method;
    }
    std::string levelToString(int level)
    {
        switch (level)
        {
        case Info:
            return "Info";
        case Debug:
            return "Debug";
        case Warning:
            return "Warning";
        case Error:
            return "Error";
        case Fatal:
            return "Fatal";
        default:
            return "None";
        }
    }

    // void logmessage(int level, const char *format, ...)
    // {
    //     time_t t = time(nullptr);
    //     struct tm *ctime = localtime(&t);
    //     char leftbuffer[SIZE];
    //     snprintf(leftbuffer, sizeof(leftbuffer), "[%s][%d-%d-%d %d:%d:%d]", levelToString(level).c_str(),
    //              ctime->tm_year + 1900, ctime->tm_mon + 1, ctime->tm_mday,
    //              ctime->tm_hour, ctime->tm_min, ctime->tm_sec);

    //     // va_list s;
    //     // va_start(s, format);
    //     char rightbuffer[SIZE];
    //     vsnprintf(rightbuffer, sizeof(rightbuffer), format, s);
    //     // va_end(s);

    //     // 格式：默认部分+自定义部分
    //     char logtxt[SIZE * 2];
    //     snprintf(logtxt, sizeof(logtxt), "%s %s\n", leftbuffer, rightbuffer);

    //     // printf("%s", logtxt); // 暂时打印
    //     printLog(level, logtxt);
    // }
    void printLog(int level, const std::string &logtxt)
    {
        switch (printMethod)
        {
        case Screen:
            std::cout << logtxt << std::endl;
            break;
        case Onefile:
            printOneFile(LogFile, logtxt);
            break;
        case Classfile:
            printClassFile(level, logtxt);
            break;
        default:
            break;
        }
    }
    void printOneFile(const std::string &logname, const std::string &logtxt)
    {
        std::string _logname = path + logname;
        int fd = open(_logname.c_str(), O_WRONLY | O_CREAT | O_APPEND, 0666); // "log.txt"
        if (fd < 0)
            return;
        write(fd, logtxt.c_str(), logtxt.size());
        close(fd);
    }
    void printClassFile(int level, const std::string &logtxt)
    {
        std::string filename = LogFile;
        filename += ".";
        filename += levelToString(level); // "log.txt.Debug/Warning/Fatal"
        printOneFile(filename, logtxt);
    }

    ~Log()
    {
    }
    void operator()(int level, const char *format, ...)
    {
        time_t t = time(nullptr);
        struct tm *ctime = localtime(&t);
        char leftbuffer[SIZE];
        snprintf(leftbuffer, sizeof(leftbuffer), "[%s][%d-%d-%d %d:%d:%d]", levelToString(level).c_str(),
                 ctime->tm_year + 1900, ctime->tm_mon + 1, ctime->tm_mday,
                 ctime->tm_hour, ctime->tm_min, ctime->tm_sec);

        va_list s;
        va_start(s, format);
        char rightbuffer[SIZE];
        vsnprintf(rightbuffer, sizeof(rightbuffer), format, s);
        va_end(s);

        // 格式：默认部分+自定义部分
        char logtxt[SIZE * 2];
        snprintf(logtxt, sizeof(logtxt), "%s %s\n", leftbuffer, rightbuffer);

        // printf("%s", logtxt); // 暂时打印
        printLog(level, logtxt);
    }

private:
    int printMethod;
    std::string path;
};

// int sum(int n, ...)
// {
//     va_list s; // char*
//     va_start(s, n);

//     int sum = 0;
//     while(n)
//     {
//         sum += va_arg(s, int); // printf("hello %d, hello %s, hello %c, hello %d,", 1, "hello", 'c', 123);
//         n--;
//     }

//     va_end(s); //s = NULL
//     return sum;
// }

processa.cc

#include "comm.hpp"
extern Log log;
int main()
{
    Init init;
    int shmid = Grateshame();
    char *shard = (char *)shmat(shmid, nullptr, 0);
    int fd = open(FIFO_FILE, O_RDONLY);
    if (fd == -1)
    {
        log(Fatal, "perror open", strerror(errno));
        exit(2);
    }
    struct shmid_ds shmds;
    while (1)
    {
        char c;
        ssize_t s = read(fd, &c, 1);
        if (s <= 0)
        {
            break;
        }
        cout << "client say@" << shard << endl;
        sleep(1);
        shmctl(shmid, IPC_STAT, &shmds);
        cout << "shm size: " << shmds.shm_segsz << endl;
        cout << "shm nattch: " << shmds.shm_nattch << endl;
        printf("shm key: 0x%x\n", shmds.shm_perm.__key);
        cout << "shm mode: " << shmds.shm_perm.mode << endl;
    }

    shmdt(shard);
    shmctl(shmid, IPC_RMID, nullptr);

    close(fd);
    return 0;
}

processb.cc

#include "comm.hpp"
using namespace std;
int main()
{
    int shmid = getshm();
    if (shmid < 0)
    {
        printf("getshm is error");
        exit(3);
    }
    int fd = open(FIFO_FILE, O_WRONLY);
    if (fd < 0)
    {
        printf("open is error");
        exit(4);
    }
    char *shm = (char *)shmat(shmid, nullptr, 0);
    while (true)
    {
        cout << "please say@";
        fgets(shm, 4096, stdin);
        write(fd, "c", 1); // 通知对方
    }
    shmdt(shm);

    close(fd);
    return 0;
}

最终运行图