进程间通信——共享内存区对象

Posix共享内存区对象使用涉及两个步骤要求：

指定一个名字参数调用shm_open，以创建一个新的共享内存区对象或打开一个已存在的共享内存区对象；

调用mmap把这个共享内存区映射到调用进程的地址空间。

#include <sys/mman.h>
int shm_open(const char *name, int oflag, mode_t mode); //return fd/-1
int shm_unlink(const char *name); //return 0/-1

oflag参数必须或者含有O_RDONLY，或者含有O_RDWR；若O_RDWR指定O_TRUNC标志，且所需共享内存区已存在，那么它将被截断为0；mode参数指定权限位，它在指定了O_CREAT的前提下使用，若无用可指定为0。
所指定的共享内存区对象在父子进程中被内存映射到不同的起始地址。

例.多个进程给一个共享的计数器持续加1

// create one posix shared mem and one semaphore

#include	"unpipc.h"

struct shmstruct {	/* struct stored in shared memory */
  int	count;
};
sem_t	*mutex;		/* pointer to named semaphore */

int main(int argc, char **argv)
{
	int		fd;
	struct shmstruct	*ptr;

	if (argc != 3)
		err_quit("usage: server1 <shmname> <semname>");

	shm_unlink(Px_ipc_name(argv[1]));		/* OK if this fails */
		/* 4create shm, set its size, map it, close descriptor */
	fd = Shm_open(Px_ipc_name(argv[1]), O_RDWR | O_CREAT | O_EXCL, FILE_MODE);
	Ftruncate(fd, sizeof(struct shmstruct));
	ptr = Mmap(NULL, sizeof(struct shmstruct), PROT_READ | PROT_WRITE,
			   MAP_SHARED, fd, 0);
	Close(fd);

	sem_unlink(Px_ipc_name(argv[2]));		/* OK if this fails */
	mutex = Sem_open(Px_ipc_name(argv[2]), O_CREAT | O_EXCL, FILE_MODE, 1);
	Sem_close(mutex);

	exit(0);
}

// u can run many instance, all the instance can incr the num in shm

#include	"unpipc.h"

struct shmstruct {	/* struct stored in shared memory */
  int	count;
};
sem_t	*mutex;		/* pointer to named semaphore */

int main(int argc, char **argv)
{
	int		fd, i, nloop;
	pid_t	pid;
	struct shmstruct	*ptr;

	if (argc != 4)
		err_quit("usage: client1 <shmname> <semname> <#loops>");
	nloop = atoi(argv[3]);

	fd = Shm_open(Px_ipc_name(argv[1]), O_RDWR, FILE_MODE);
	ptr = Mmap(NULL, sizeof(struct shmstruct), PROT_READ | PROT_WRITE,
			   MAP_SHARED, fd, 0);
	Close(fd);

	mutex = Sem_open(Px_ipc_name(argv[2]), 0);

	pid = getpid();
	for (i = 0; i < nloop; i++) {
		Sem_wait(mutex);
		printf("pid %ld: %d\n", (long) pid, ptr->count++);
		Sem_post(mutex);
	}
	exit(0);
}