先給自己打個廣告,本人的微信公衆號正式上線了,搜索:張笑生的地盤,主要關注嵌入式軟件開發,股票基金定投,足球等等,希望大家多多關注,有問題可以直接留言給我,一定盡心盡力回答大家的問題
一 what
在《linux進程間通信(四)----IPC篇----共享內存初識篇》文章中,我們知道了共享內存是什麼,通過幾個常用的函數shmget、ftok、shmat、shmdt、shmctl,瞭解瞭如何創建共享內存,但是創建好的共享內存之間,如何實現進程間通信呢?
共享內存對象是存在於內核中的,兩個進程間如何訪問這個共享內存對象呢?這篇文章介紹兩種訪問方式,分別是有親緣關係的父子進程訪問同一個共享內存,以及非親緣關係之間訪問同一個共享內存。
共享內存使用步驟
1. 創建/打開共享內存
2. 映射共享內存,即把指定的共享內存映射到進程的地址空間用於訪問
3. 讀寫共享內存
4. 撤銷共享內存映射
5. 刪除共享內存對象
使用共享內存時的一些注意點或是限制條件
6. 共享內存的數量是有限制的,通過ipcs -l命令查看,當然如果我們具有管理員權限,可以通過 cat /proc/sys/kernel/shmmax來查看
7. 共享內存刪除的時間點,shmctl添加刪除標記,只有當所有進車都取消共享內存映射時(即所有進程調用shmdt之後),纔會刪除共享內存。
二 how
2.1 親緣關係的進程訪問同一個共享內存
要想讓共享內存能夠在親緣關係的進程中訪問,fork函數必須在shmget函數之後
#include "stdio.h"
#include "stdlib.h"
#include <unistd.h>
#include "sys/types.h"
#include "sys/shm.h"
#include "signal.h"
#include "string.h"
void myfun(int signum)
{
return;
}
int main(int argc, char *argv[])
{
int shmid;
int key;
char *p;
int pid;
// shmget函數必須在fork函數之前,先創建一個共享內存
shmid = shmget(IPC_PRIVATE, 128, IPC_CREAT | 0777);
if (shmid < 0) {
printf("create shared memory fail\n");
return -1;
}
printf("create shared memory sucess, shmid = %d\n", shmid);
pid = fork();
if (pid > 0) { // parent process
signal(SIGUSR2, myfun);
p = (char *)shmat(shmid, NULL, 0);
if (p == NULL) {
printf("shmat fail\n");
return -1;
}
printf("parent process shmat sucess\n");
while (1) {
//write share memory
printf("parent process begin to write memory data\n");
fgets(p, 128, stdin);
kill(pid, SIGUSR1); // tell child process to read data
pause(); // wait child process read
}
}
if (pid == 0) { // child process to read
signal(SIGUSR1, myfun);
p = (char *)shmat(shmid, NULL, 0);
if (p == NULL) {
printf("shmat fail\n");
return -1;
}
printf("child process shmat sucess\n");
while (1) {
pause(); // wait parent process write
//start read share memory
printf("child process read share memory data:%s\n", p);
kill(getppid(), SIGUSR2);
}
}
//在用戶空間刪除共享內存的地址
shmdt(p);
//memcpy(p, "abcd", 4); //執行這個語句會出現segment fault
shmctl(shmid, IPC_RMID, NULL);
system("ipcs -m");
return 0;
}
2.2 非親緣關係的進程訪問同一個共享內存
進程server
#include "stdio.h"
#include "stdlib.h"
#include <unistd.h>
#include "sys/types.h"
#include "sys/shm.h"
#include "signal.h"
#include "string.h"
struct mybuf
{
int pid;
char buf[124];
};
void myfun(int signum)
{
return;
}
int main(int argc, char *argv[])
{
int shmid;
int key;
struct mybuf *p;
int pid;
key = ftok("./a.c", 'a');
if (key , 0) {
printf("create key fail\n");
return -1;
}
printf("create key sucess\n");
shmid = shmget(key, 128, IPC_CREAT | 0777);
if (shmid < 0) {
printf("create shared memory fail\n");
return -1;
}
printf("create shared memory sucess, shmid = %d\n", shmid);
signal(SIGUSR2, myfun);
p = (struct mybuf *)shmat(shmid, NULL, 0);
if (p == NULL) {
printf("shmat fail\n");
return -1;
}
printf("parent process shmat sucess\n");
// get client pid
p->pid = getpid(); //write server pid to share memory
pause(); // wait client to read server pid
pid=p->pid;
while (1) {
//write share memory
printf("parent process begin to write memory data\n");
fgets(p->buf, 124, stdin);
kill(pid, SIGUSR1); // tell client process to read data
pause(); // wait client process read
}
//在用戶空間刪除共享內存的地址
shmdt(p);
//memcpy(p, "abcd", 4); //執行這個語句會出現segment fault
shmctl(shmid, IPC_RMID, NULL);
system("ipcs -m");
return 0;
}
進程client
#include "stdio.h"
#include "stdlib.h"
#include <unistd.h>
#include "sys/types.h"
#include "sys/shm.h"
#include "signal.h"
#include "string.h"
struct mybuf
{
int pid;
char buf[124];
};
void myfun(int signum)
{
return;
}
int main(int argc, char *argv[])
{
int shmid;
int key;
struct mybuf *p;
int pid;
key = ftok("./a.c", 'a');
if (key , 0) {
printf("create key fail\n");
return -1;
}
printf("create key sucess\n");
shmid = shmget(key, 128, IPC_CREAT | 0777);
if (shmid < 0) {
printf("create shared memory fail\n");
return -1;
}
printf("create shared memory sucess, shmid = %d\n", shmid);
signal(SIGUSR1, myfun);
p = (struct mybuf *)shmat(shmid, NULL, 0);
if (p == NULL) {
printf("shmat fail\n");
return -1;
}
printf("client process shmat sucess\n");
// get server pid
//read share memory
pid = p->pid;
// write client pid to share memory
p->pid = getpid();
kill(pid, SIGUSR2); // tell client process to read data
//client start to read share memory
while (1) {
pause(); // wait server process write share memory
printf("client process read data:%s\n", p->buf); // read data
kill(pid, SIGUSR2); // server can write share memory
}
//在用戶空間刪除共享內存的地址
shmdt(p);
//memcpy(p, "abcd", 4); //執行這個語句會出現segment fault
shmctl(shmid, IPC_RMID, NULL);
system("ipcs -m");
return 0;
}