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使用Posix共享内存区实现进程间通信

使用Posix共享内存区通常涉以下步骤:

• 进程A 调用shm_open 创建共享内存区
• 进程A调用ftruncate修改共享内存区大小
• 进程A 调用mmap将共享内存区映射到进程地址空间ptrA
• 进程A 使用ptrA对共享内存区进程更改
• 进程B 使用shm_open打开已有共享内存区
• 进程B 调用fstat获取共享内存区大小
• 进程B 调用mmap将共享内存区映射到进程地址空间ptrB
• 进程B 使用ptrB对共享内存区进行更改
• 最后由进程A/B调用shm_unlink删除共享内存区

1、shm_open 创建/打开共享内存区

shm_open调用成功后返回共享内存区的文件描述符, 并在/dev/shm目录下生成相应的文件。

shm_open函数声明包含在文件 sys/mman.h 中;

#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */int shm_open(const char *name, int oflag, mode_t mode);

oflag的值包含在文件 fcntl.h中

#define O_RDONLY     00   //只读
#define O_WRONLY     01   //只写
#define O_RDWR       02   //可读可写
#define O_CREAT      0100 //创建
#define O_EXCL       0200 //如果共享内存区已存在则返错误

mode的值包含在文件sys/stat.h中

#define	S_IRUSR	__S_IREAD	/* Read by owner.  */
#define	S_IWUSR	__S_IWRITE	/* Write by owner.  */
#define	S_IXUSR	__S_IEXEC	/* Execute by owner.  */#define	S_IRGRP	(S_IRUSR >> 3)	/* Read by group.  */
#define	S_IWGRP	(S_IWUSR >> 3)	/* Write by group.  */
#define	S_IXGRP	(S_IXUSR >> 3)	/* Execute by group.  */#define	S_IROTH	(S_IRGRP >> 3)	/* Read by others.  */
#define	S_IWOTH	(S_IWGRP >> 3)	/* Write by others.  */
#define	S_IXOTH	(S_IXGRP >> 3)	/* Execute by others.  */

2、ftruncate修改内存区大小

ftruncate包含在头文件unistd.h中

/* Truncate the file FD is open on to LENGTH bytes.  */int ftruncate (int __fd, __off_t __length)

fd:shm_ope返回的描述符
length:共享内存区的大小

3、fstat获取文件大小

fstat包含在头文件sys/stat.h中

在打开已存在的共享内存区时，可以用fstat获取共享内存区的大小。

/* Get file attributes for the file, device, pipe, or socketthat file descriptor FD is open on and put them in BUF.  */
int fstat (int __fd, struct stat *__buf)

fd:shm_open返回的描述符
stat.st_size是共享内存区的大小

4、mmap将共享内存区映射到进程地址空间

mmap包含在头文件mman.h中

void *mmap (void *__addr, size_t __len, int __prot,int __flags, int __fd, __off_t __offset)

参数

__addr: 映射到进程的地址,直接传入NULL就行。
__len: 共享内存区的大小。__prot: 权限
PROT_READ   可读
PROT_WRITE  可写
PROT_EXEC   可执行__flags:映射方式
MAP_SHARED: 当前进程所做的修改其他进程可见 
MAP_PRIVATE:当前进程所做的修改其他进程不可见__fd: shm_open返回的描述符 
__offset:偏移量,一般都传入0

5、shm_unlink删除共享内存区

shm_unlink和shm_open在同一文件中,用于删除共享内存区。

/* Remove shared memory segment.  */
int shm_unlink (const char *__name);

6、练习

• 进程shmserver创建共享内存区shmtest,并创建子进程等待其他进程向shmtest中写入数据。
• 进程shmcli向共享内存区shmtest写入数据,并给进程shmserver发送信号。
• 进程shmserver取出共享内存区shmtest中的数据并输出到窗口。

cond_h.h

#define COUNT 10typedef struct shm_block
{pthread_mutex_t mutex;pthread_cond_t  cond;int arr[COUNT];int nput;int nread;int ncount;
}shm_block;void shm_block_init(shm_block* shm)
{pthread_mutexattr_t attr;            pthread_mutexattr_init(&attr);pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);  pthread_mutex_init(&shm->mutex, &attr);pthread_mutexattr_destroy(&attr);pthread_condattr_t cond_attr;pthread_condattr_init(&cond_attr);pthread_condattr_setpshared(&cond_attr, PTHREAD_PROCESS_SHARED);pthread_cond_init(&shm->cond, &cond_attr);pthread_condattr_destroy(&cond_attr);bzero(shm->arr, sizeof(shm->arr));shm->ncount  = 0;shm->nput    = 0;shm->nread   = 0;
}

shm_server.c

#include<unistd.h>
#include<pthread.h>
#include<mqueue.h>
#include<sys/mman.h>
#include<fcntl.h>
#include<sys/stat.h>
#include<stdio.h>
#include<string.h>
#include<errno.h>
#include<strings.h>
#include<wait.h>
#include"cond_h.h"int main(int argc, char* argv[])
{   if (argc != 2){printf("shmcreate [shmname]\n");return 0;}shm_block shm;shm_block_init(&shm);int shmfd = shm_open(argv[1], O_CREAT|O_EXCL|O_RDWR, S_IRUSR|S_IWUSR);if (shmfd == -1){printf("%d : %s\n",__LINE__, strerror(errno));return -1;}printf("create shared memory %s. \n", argv[1]);if (ftruncate(shmfd, sizeof(shm_block)) < 0){printf("%d : %s\n",__LINE__, strerror(errno));return -1;}shm_block* pBlock =  (shm_block*)mmap(NULL, sizeof(shm_block), PROT_WRITE|PROT_READ, MAP_SHARED, shmfd, 0);if (NULL == pBlock){printf("%d : %s\n",__LINE__, strerror(errno));return -1;}close(shmfd);memcpy(pBlock, &shm, sizeof(shm));int pid = fork();if (pid == 0){int fd = shm_open(argv[1], O_RDWR, S_IRUSR|S_IWUSR);if (fd < 0){printf("%d : %s\n",__LINE__, strerror(errno));return -1;}struct stat st;fstat(fd, &st);printf("%d: st_size = %d\n", __LINE__, st.st_size);shm_block *p = (shm_block*)mmap(NULL, st.st_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);if (NULL == p){printf("%d : %s\n",__LINE__, strerror(errno));return -1;}while (1){pthread_mutex_lock(&p->mutex);while (p->ncount <= 0)pthread_cond_wait(&p->cond, &p->mutex);int value = p->arr[p->nread];p->ncount--;p->nread++;p->nread %= COUNT; pthread_mutex_unlock(&p->mutex);printf("%d: p->nread:%d, arr[nread] : %d\n",__LINE__, p->nread, value);}munmap(p, st.st_size);close(fd);return 0;}int stat_loc = 0;waitpid(pid, &stat_loc, 0);munmap(pBlock, sizeof(shm));if (WIFEXITED(stat_loc)) printf("Child process exited with status: %d\n", WEXITSTATUS(stat_loc));return 0;
}

shm_cli.c

#include<sys/mman.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>
#include<pthread.h>
#include<stdlib.h>
#include<stdio.h>
#include<string.h>
#include<errno.h>
#include"cond_h.h"int main(int argc, char *argv[])
{if (argc != 3){printf("cond_put <shmname> <value>");return -1;}char *shmname = argv[1];int   value   = atoi(argv[2]);int   shmfd   = shm_open(shmname, O_RDWR, 0);if (-1 == shmfd){printf("%d: %s\n", __LINE__, strerror(errno));return -1;}shm_block* p =  (shm_block*)mmap(NULL, sizeof(shm_block), PROT_READ|PROT_WRITE, MAP_SHARED, shmfd, 0);if (NULL == p){printf("%d: %s\n", __LINE__, strerror(errno));return -1;}close(shmfd);pthread_mutex_lock(&p->mutex);do{if (p->ncount == COUNT)break;p->arr[p->nput] = value;printf("count: %d,  nput: %d, nread: %d,arr[nput]:%d\n", p->ncount, p->nput, p->nread,value);p->nput         = (p->nput + 1) % COUNT;p->ncount++;if (p->ncount == 1)pthread_cond_signal(&p->cond);} while (0);pthread_mutex_unlock(&p->mutex);close(shmfd);return 0;
}

使用方法：

unlink /dev/shm/shmtest
gcc shm_server.c -lrt -lpthread -o shmserver
./shmserver shmtest

gcc shm_cli.c -lrt -lpthread -o shmcli
./shmcli  shmtest 33
./shmcli  shmtest 34
./shmcli  shmtest 35

shmserver输出:

create shared memory shmtest. 
60: st_size = 144
83: p->nread:1, arr[nread] : 33
83: p->nread:2, arr[nread] : 34
83: p->nread:3, arr[nread] : 35