Linux进程间通信之System V IPC——共享内存原理

int *shared_counter = (int*)shmat(shm_id, NULL, 0);
*shared_counter += 1; // 修改对其他进程立即可见
 

// 进程视角：共享内存如同本地变量
char *shm_ptr = shmat(shm_id, NULL, 0);  // 映射共享内存到虚拟地址空间
strcpy(shm_ptr, "Hello from Process A");  // 直接写入

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 */
};

struct shmid_ds {
    struct ipc_perm shm_perm;   // 权限控制（UID/GID/模式）
    size_t shm_segsz;           // 段大小（字节）
    time_t shm_atime;           // 最后一次映射时间
    time_t shm_dtime;           // 最后一次解除映射时间
    time_t shm_ctime;           // 最后一次修改时间
    pid_t shm_cpid;             // 创建者PID
    pid_t shm_lpid;             // 最后一次操作者PID
    unsigned short shm_nattch;  // 当前映射进程数
    // ... 兼容性保留字段
};

struct shmid_kernel {
    struct kern_ipc_perm shm_perm;  // IPC权限控制块
    struct file *shm_file;          // 关联的shm文件对象
    unsigned long shm_nattch;       // 映射计数
    size_t shm_segsz;               // 段大小
    struct pid *shm_cprid;          // 创建者PID（内核态）
    struct pid *shm_lprid;          // 最后操作者PID
    // ... 其他内核级字段
};

int shmget(key_t key, size_t size, int shmflg) {
    // 1. 根据key查找或新建shmid_kernel
    // 2. 在shm文件系统中创建匿名文件
    struct file *file = shmem_file_setup("SYSV<key>", size, flags);
    // 3. 初始化shmid_kernel：绑定file，设置size/权限等
    // 4. 将shmid_kernel插入全局红黑树
}

void *shmat(int shmid, void *addr, int flag) {
    // 1. 根据shmid找到shmid_kernel
    struct shmid_kernel *shp = find_shm(shmid);
    // 2. 在进程地址空间创建VMA区域
    vma = vm_area_alloc(current->mm);
    vma->vm_file = shp->shm_file;  // 关联shm文件
    vma->vm_ops = &shmem_vm_ops;   // 设置内存操作函数
    // 3. 更新shm_nattch引用计数
    shp->shm_nattch++;
}

#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);

struct shmid_ds {
    struct ipc_perm shm_perm;   // 权限控制块
    size_t shm_segsz;           // 段大小（=size参数）
    time_t shm_atime;           // 最后一次attach时间
    time_t shm_dtime;           // 最后一次detach时间
    time_t shm_ctime;           // 最后一次修改时间
    pid_t shm_cpid;             // 创建者PID
    pid_t shm_lpid;             // 最后操作者PID
    unsigned short shm_nattch;  // 当前附加进程数
};

#include <sys/ipc.h>
key_t ftok(const char *pathname, int proj_id);

// 服务端创建共享内存
key_t config_key = ftok("/etc/app_config", 123);  // 基于配置文件生成key
int shmid = shmget(config_key, 4096, IPC_CREAT | 0666);

// 客户端访问同一内存
key_t client_key = ftok("/etc/app_config", 123);   // 相同参数生成相同key
int client_shmid = shmget(client_key, 0, 0);       // size=0表示获取已有段

#define APP_SHM_KEY 0x1234  // 预定义全局常量

// 进程A
int shmid_A = shmget(APP_SHM_KEY, 1024, IPC_CREAT | 0600);

// 进程B
int shmid_B = shmget(APP_SHM_KEY, 0, 0);  // 通过相同key访问

int shmid = shmget(IPC_PRIVATE, 4096, IPC_CREAT | 0600);

void *shmat(int shmid, const void *shmaddr, int shmflg);

int shmdt(const void *shmaddr);

int shmctl(int shmid, int cmd, struct shmid_ds *buf);

ipcrm -m <SHMID>  # 删除指定SHMID的共享内存

# 删除用户alice创建的所有共享内存
ipcs -m | awk '/alice/{print $2}' | xargs -n1 ipcrm -m

# 删除键值为0x12345的共享内存
ipcs -m | awk '/0x12345/{system("ipcrm -m "$2)}'

pmap -x <PID>  # 查看指定进程的内存映射

Address    Kbytes   RSS   Mode  Mapping
7f2a1a000000 1024    rw-s  /SYSV00000000  # 共享内存标识（KEY为0x00000000）

gdb -p <PID> -ex "call shmat(<SHMID>, NULL, 0)" --batch

#define ERR_EXIT(m)         \
    do                      \
    {                       \
        perror(m);          \
        exit(EXIT_FAILURE); \
    } while (0)

#pragma once

#include <iostream>
#include <cstdio>
#include <string>
#include <sys/ipc.h>
#include <sys/shm.h>

#define ERR_EXIT(m)         \
    do                      \
    {                       \
        perror(m);          \
        exit(EXIT_FAILURE); \
    } while (0)

const int gdefaultid = -1;
const int gsize = 4096;
const std::string pathname = ".";
const int proj_id = 0x66;

#define CREATER "creater"
#define USER "user"

class Shm
{
private:
    int _shmid;
    int _size;
    key_t _key;
    std::string _usertype;

    void CreatShm(int flg)
    {
        _shmid = shmget(_key, _size, flg);
        if(_shmid < 0)
        {
            ERR_EXIT("shmget");
        }
        printf("shmid: %d\n", _shmid);
    }

    void Creat()
    {
        CreatShm(IPC_CREAT | IPC_EXCL | 0666);
    }

    void Get()
    {
        CreatShm(IPC_CREAT);
    }

public:
    Shm(const std::string& pathneme, int projid, const std::string& usertype)
        : _shmid(gdefaultid), _size(gsize), _usertype(usertype)
    {
        _key = ftok(pathname.c_str(), projid);
        if (_key < 0)
        {
            ERR_EXIT("ftok");
        }
        printf("key: 0x%x\n", _key);

        if(_usertype == CREATER)
        {
            // 用户是服务端则创建共享内存段
            Creat();
        }
        else if(_usertype == USER)
        {
            // 用户是客户端则获取共享内存段
            Get();
        }

    }
    ~Shm() {}
};

    // 新增一个成员变量
    void* _start_mem;// 共享内存首地址指针

    void Attach()
    {
        _start_mem = shmat(_shmid, NULL, 0);
        if((long long)_start_mem < 0)
        {
            ERR_EXIT("shmat");
        }
        printf("attach success\n");
    }

public:
    Shm(const std::string& pathneme, int projid, const std::string& usertype)
        : _shmid(gdefaultid), _size(gsize), _start_mem(nullptr), _usertype(usertype)
    {
        _key = ftok(pathname.c_str(), projid);
        if (_key < 0)
        {
            ERR_EXIT("ftok");
        }
        printf("key: 0x%x\n", _key);

        if(_usertype == CREATER)
        {
            // 用户是服务端则创建共享内存段
            Creat();
        }
        else if(_usertype == USER)
        {
            // 用户是客户端则获取共享内存段
            Get();
        }

        Attach();
    }

    void* VirtualAddr()
    {
        printf("VirtualAddr: %p\n", _start_mem);
        return _start_mem;
    }

    int Size()
    {
        return _size;
    }

    void Detach()
    {
        int n = shmdt(_start_mem);
        if(n < 0)
        {
            ERR_EXIT("shmdt");
        }
        printf("Detach success\n");
    }

    void Destroy()
    {
        Detach();
        if(_shmid == gdefaultid)
            return;
        if(_usertype == CREATER)
        {
            int n = shmctl(_shmid, IPC_RMID, NULL);
            if(n < 0)
            {
                ERR_EXIT("shmctl");
            }
            printf("shmctl delete shm: %d success!\n", _shmid);
        }
    }

#include "Shm.hpp"

int main()
{
    Shm shm(pathname, proj_id, CREATER);
    char* mem = (char*)shm.VirtualAddr();
    while(true)
    {
        printf("%s\n", mem);
        sleep(1);
    }
    return 0;
}

#include "Shm.hpp"

int main()
{
    Shm shm(pathname, proj_id, USER);
    char* mem = (char*)shm.VirtualAddr();
    for(char c = 'A'; c <= 'Z'; c++)
    {
        mem[c - 'A'] = c;
        sleep(1);
    }
    return 0;
}

#pragma once

#include <iostream>
#include <cstdio>
#include <string>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>

#define ERR_EXIT(m)         \
    do                      \
    {                       \
        perror(m);          \
        exit(EXIT_FAILURE); \
    } while (0)

const int gdefaultid = -1;
const int gsize = 4096;
const std::string pathname = ".";
const int proj_id = 0x66;

#define CREATER "creater"
#define USER "user"

class Shm
{
private:
    int _shmid;
    int _size;
    key_t _key;
    std::string _usertype;
    void* _start_mem;// 共享内存首地址指针(挂接后的进程虚拟地址)

    void CreatShm(int flg)
    {
        _shmid = shmget(_key, _size, flg);
        if(_shmid < 0)
        {
            ERR_EXIT("shmget");
        }
        printf("shmid: %d\n", _shmid);
    }

    void Creat()
    {
        CreatShm(IPC_CREAT | IPC_EXCL | 0666);
    }

    void Get()
    {
        CreatShm(IPC_CREAT);
    }

    void Attach()
    {
        _start_mem = shmat(_shmid, nullptr, 0);
        if((long long)_start_mem < 0)
        {
            ERR_EXIT("shmat");
        }
        printf("attach success\n");
    }

    void Detach()
    {
        int n = shmdt(_start_mem);
        if(n < 0)
        {
            ERR_EXIT("shmdt");
        }
        printf("Detach success\n");
    }

    void Destroy()
    {
        Detach();
        if(_shmid == gdefaultid)
            return;
        if(_usertype == CREATER)
        {
            int n = shmctl(_shmid, IPC_RMID, NULL);
            if(n < 0)
            {
                ERR_EXIT("shmctl");
            }
            printf("shmctl delete shm: %d success!\n", _shmid);
        }
    }

public:
    Shm(const std::string& pathneme, int projid, const std::string& usertype)
        : _shmid(gdefaultid), _size(gsize), _start_mem(nullptr), _usertype(usertype)
    {
        _key = ftok(pathname.c_str(), projid);
        if (_key < 0)
        {
            ERR_EXIT("ftok");
        }
        printf("key: 0x%x\n", _key);

        if(_usertype == CREATER)
        {
            // 用户是服务端则创建共享内存段
            Creat();
        }
        else if(_usertype == USER)
        {
            // 用户是客户端则获取共享内存段
            Get();
        }

        Attach();
    }

    void* VirtualAddr()
    {
        printf("VirtualAddr: %p\n", _start_mem);
        return _start_mem;
    }

    int Size()
    {
        return _size;
    }

    ~Shm() 
    {
        Destroy();
    }
};