共享内存通信方式效率最高,毕竟是直接操作内存,但是要保证多个进程对同一块内存访问的同步互斥比较麻烦,借助信号量实现
对每个共享存储段,内核维护一个shmid_ds类型的结构体,定义在<sys/shm.h>文件中
- struct shmid_ds
- {
- struct ipc_perm shm_perm; //共享内存的ipc_perm结构
- size_t shm_segsz; //共享内存区域大小,字节表示
- pid_t shm_lpid; //最后一次调用shmop函数的进程ID
- pid_t shm_cpid; //创建此共享内存的进程ID
- unsigned short shm_lkcnt; //共享内存被锁定的时间数
- unsigned long shm_nattch; //当前使用共享内存的进程数
- time_t shm_atime; //最后一次附加操作时间
- time_t shm_dtime; //最后一次分离操作时间
- time_t shm_ctime; //最后一次修改时间
- }
共享内存基本操作
1,创建或打开一个共享内存(shmget)
- //create_shm.c
- #include<sys/types.h>
- #include<sys/ipc.h>
- #include<sys/shm.h>
- #include<stdio.h>
-
- #define BUFSZ 1024
-
- int main()
- {
- int shm_id; //共享内存ID
- shm_id = shmget(IPC_PRIVATE,BUFSZ,0666);
- if(shm_id < 0){
- printf("shmget failed\n");
- return -1;
- }
- printf("create shared memory succeed: %d\n",shm_id);
- system("ipcs -m"); //查看共享内存ID
- return 0;
- }
2,附加共享内存到进程空间(shmat/shmdt)
- //attach_shm.c
- #include<sys/types.h>
- #include<sys/ipc.h>
- #include<sys/shm.h>
- #include<stdio.h>
-
- #define BUFSZ 1024
-
- int main()
- {
- int *shm;
- shm = shmat(104529925,NULL,0);
- if(*shm == -1){
- printf("shmat failed\n");
- return -1;
- }
- printf("attach shared memory succeed: %d\n",*shm);
- system("ipcs -m"); //查看共享内存调用状态
- if(shmdt(shm) == -1){
- printf("shmdt failed\n");
- return -1;
- }
- system("ipcs -m"); //查看共享内存调用状态
- return 0;
- }
3,共享内存控制函数(shmctl)
下面写个简单的例子
共享内存写端(write_shm.c)
- //write_shm.c
- #include<sys/types.h>
- #include<sys/ipc.h>
- #include<sys/shm.h>
- #include<stdio.h>
-
- typedef struct
- {
- char name[4];
- int age;
- }people;
-
- int main()
- {
- int i;
- char *t = 'a';
- people *p_shm = NULL;
- p_shm = shmat(104529925,NULL,0);
- if(p_shm == NULL){
- printf("shmat failed\n");
- return -1;
- }
- for(i=0;i<5;i++) {
- t += 1;
- memcpy((*(p_shm+i)).name,&t,1);
- (*(p_shm+i)).age = 20+i;
- }
- if(shmdt(p_shm) == -1){
- printf("shmdt failed\n");
- return -1;
- }
- return 0;
- }
共享内存读端(read_shm.c)
- //read_shm.c
- #include<sys/types.h>
- #include<sys/ipc.h>
- #include<sys/shm.h>
- #include<stdio.h>
-
- typedef struct
- {
- char name[4];
- int age;
- }people;
-
- int main()
- {
- int i;
- char *t = 'a';
- people *p_shm = NULL;
- p_shm = shmat(104529925,NULL,0);
- if(p_shm == NULL){
- printf("shmat failed\n");
- return -1;
- }
- for(i=0;i<5;i++) {
- printf("name:%s age:%d\n",(*(p_shm+i)).name,(*(p_shm+i)).age);
- }
- if(shmdt(p_shm) == -1){
- printf("shmdt failed\n");
- return -1;
- }
- return 0;
- }
先后编译执行"写代码"与"读代码",结果如下
- root$ ./write_shm.out
- root$ ./read_shm.out
- name:b age:20
- name:c age:21
- name:d age:22
- name:e age:23
- name:f age:24
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