【Linux】System V 共享内存 - ToB企服应用市场:ToB评测及商务社交产业平台

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 */
};

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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

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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，指向共享内存的地址；失败返回-1

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shmaddr 为 NULL，核心自动选择一个地址。
shmaddr 不为 NULL 且 shmflg 无 SHM_RND 标记，则以 shmaddr 为连接地址。
shmaddr 不为 NULL 且 shmflg 设置了 SHM_RND 标记，则连接的地址会自动向下调整为 SHMLBA 的整数倍；
公式：shmaddr - (shmaddr % SHMLBA)。
shmflg = SHM_RDONLY，表示连接操作用来只读共享内存。

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功能：将共享内存段与当前进程脱离
原型
int shmdt(const void* shmaddr);
参数
shmaddr：由 shmat 所返回的指针
返回值：成功返回0；失败返回-1
注意：将共享内存段与当前进程脱离不等于删除共享内存段

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功能：用于控制共享内存
原型
int shmctl(int shmid, int cmd, struct shmid_ds* buf);
参数
shmid：由 shmget 返回的共享内存标识码
cmd：将要采取的动作（有三个可取值）
buf：指向一个保存着共享内存的模式状态和访问权限的数据结构
返回值：成功返回0；失败返回-1

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$ ls
Comm.hpp Fifo.hpp Makefile ShmClient.cc ShmServer.cc
$ cat Makefile
.PHONY:all
all:shm_client shm_server
shm_server:ShmServer.cc
g++ -o $@ $^ -std=c++11
shm_client:ShmClient.cc
g++ -o $@ $^ -std=c++11
.PHONY:clean
clean:
rm -f shm_client shm_server

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#ifndef __COMM_HPP__
#define __COMM_HPP__
#include <iostream>
#include <string>
#include <cerrno>
#include <cstring>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <cassert>
using namespace std;
#define Mode 0666
#define Path "./fifo"
class Fifo
{
public:
Fifo(const string &path = Path)
: _path(path)
{
umask(0);
int n = mkfifo(_path.c_str(), Mode);
if (n == 0)
{
cout << "mkfifo success" << endl;
}
else
{
cerr << "mkfifo failed, errno: " << errno << ", errstring: " << strerror(errno) << endl;
}
}
~Fifo()
{
int n = unlink(_path.c_str());
if (n == 0)
{
cout << "remove fifo file " << _path << " success" << endl;
}
else
{
cerr << "remove failed, errno: " << errno << ", errstring: " << strerror(errno) << endl;
}
}
private:
string _path; // 文件路径 + 文件名
};
class Sync
{
public:
Sync()
: rfd(-1), wfd(-1)
{
}
void OpenReadOrDie()
{
rfd = open(Path, O_RDONLY);
if (rfd < 0)
exit(1);
}
void OpenWriteOrDie()
{
wfd = open(Path, O_WRONLY);
if (wfd < 0)
exit(1);
}
bool Wait()
{
bool ret = true;
uint32_t c = 0;
ssize_t n = read(rfd, &c, sizeof(uint32_t));
if (n == sizeof(uint32_t))
{
std::cout << "server wakeup, begin read shm..." << std::endl;
}
else if (n == 0)
{
ret = false;
}
else
{
return false;
}
return ret;
}
void Wakeup()
{
uint32_t c = 0;
ssize_t n = write(wfd, &c, sizeof(c));
assert(n == sizeof(uint32_t));
std::cout << "wakeup server..." << std::endl;
}
~Sync()
{
}
private:
int rfd;
int wfd;
};
#endif

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#pragma once
#include <iostream>
#include <cerrno>
#include <cstring>
#include <cstdlib>
#include <string>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
using namespace std;
const char *pathname = "/home/ubuntu";
const int proj_id = 0x66;
// 在内核中，共享内存的大小是以4KB为基本单位的，你只能用你申请的大小。建议申请大小是N*4KB
const int defaultsize = 4096; // 单位是字节
std::string ToHex(key_t k)
{
char buffer[1024];
snprintf(buffer, sizeof(buffer), "0x%x", k);
return buffer;
}
key_t GetShmKeyOrDie()
{
key_t k = ftok(pathname, proj_id);
if (k < 0)
{
std::cerr << "ftok error, errno: " << errno << ", error string: " << strerror(errno) << std::endl;
exit(1);
}
return k;
}
int CreateShmOrDie(key_t key, int size, int flag)
{
int shmid = shmget(key, size, flag);
if (shmid < 0)
{
std::cerr << "shmget error, errno: " << errno << ", error string: " << strerror(errno) << std::endl;
exit(2);
}
return shmid;
}
int CreateShm(key_t key, int size)
{
// IPC_CREAT: 不存在就创建，存在就获取
// IPC_EXCL: 没有意义
// IPC_CREAT | IPC_EXCL: 不存在就创建，存在就出错返回
return CreateShmOrDie(key, size, IPC_CREAT | 0666);
}
int GetShm(key_t key, int size)
{
return CreateShmOrDie(key, size, IPC_CREAT);
}
void DeleteShm(int shmid)
{
int n = shmctl(shmid, IPC_RMID, nullptr);
if (n < 0)
{
std::cerr << "shmctl error" << std::endl;
}
else
{
std::cout << "shmctl delete shm success, shmid: " << shmid << std::endl;
}
}
void ShmDebug(int shmid)
{
struct shmid_ds shmds;
int n = shmctl(shmid, IPC_STAT, &shmds);
if (n < 0)
{
std::cerr << "shmctl error" << std::endl;
return;
}
std::cout << "shmds.shm_segez: " << shmds.shm_segsz << std::endl;
std::cout << "shmds.shm_nattch: " << shmds.shm_nattch << std::endl;
std::cout << "shmds.shm_ctime: " << shmds.shm_ctime << std::endl;
std::cout << "shmds.shm_perm.__key: " << ToHex(shmds.shm_perm.__key) << std::endl;
}
void *ShmAttach(int shmid)
{
// 核心自动选择一个地址
void *addr = shmat(shmid, nullptr, 0);
if ((long long int)addr == -1)
{
std::cerr << "shmat error" << std::endl;
return nullptr;
}
return addr;
}
void ShmDetach(void *addr)
{
int n = shmdt(addr);
if (n < 0)
{
std::cerr << "shmdt error" << std::endl;
}
}

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#include "Comm.hpp"
#include "Fifo.hpp"
int main()
{
// 1. 获取key
key_t key = GetShmKeyOrDie();
std::cout << "key: " << ToHex(key) << std::endl;
// 2. 创建共享内存
int shmid = CreateShm(key, defaultsize);
std::cout << "shmid: " << shmid << std::endl;
// 4. 将共享内存和进程进行挂接（关联）
char *addr = (char *)ShmAttach(shmid);
std::cout << "Attach shm success, addr: " << ToHex((uint64_t)addr) << std::endl;
// 0. 先引入管道
Fifo fifo;
Sync syn;
syn.OpenReadOrDie();
// 可以进行通信了
for (;;)
{
if (!syn.Wait())
break;
std::cout << "shm content: " << addr << std::endl;
}
ShmDetach(addr);
std::cout << "Detach shm success, addr: " << ToHex((uint64_t)addr) << std::endl;
// 3. 删除共享内存
DeleteShm(shmid);
return 0;
}

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#include "Comm.hpp"
#include "Fifo.hpp"
int main()
{
key_t key = GetShmKeyOrDie();
std::cout << "key: " << ToHex(key) << std::endl;
int shmid = GetShm(key, defaultsize);
std::cout << "shmid: " << shmid << std::endl;
char *addr = (char *)ShmAttach(shmid);
std::cout << "Attach shm success, addr: " << ToHex((uint64_t)addr) << std::endl;
memset(addr, 0, defaultsize);
Sync syn;
syn.OpenWriteOrDie();
// 可以进行通信了
for (char c = 'A'; c <= 'Z'; c++) // shm没有使用系统调用
{
addr[c - 'A'] = c;
syn.Wakeup();
sleep(1);
}
ShmDetach(addr);
std::cout << "Detach shm success, addr: " << ToHex((uint64_t)addr) << std::endl;
sleep(5);
return 0;
}

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# 查看共享内存
ipcs -m
# 删除指定共享内存
ipcrm -m [shmid]

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# 实机演示
$ ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 0 root 644 80 2
0x00000000 1 root 644 16384 2
0x00000000 2 root 644 280 2
0x6602fc97 10 ubuntu 666 4096 1
$ ipcrm -m 10 # 指定shmid即可删除该共享内存资源

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