内存共享允许两个或多个不相关的进程，访问同一个逻辑内存，共享内存的具体实现，由不同进程之间共享的内存安排为同一物理内存。

过个进程就像通过malloc获取的内存一样去使用，但是需要额外的小消息来同队内存的访问。

可以通过信号量，传递消息(消息队列)，生成信号来同步对内存的访问。

1 shmget

#include

#include

int shmget(key_t key, size_t size, int shmflg);

shmget() returns the identifier of the System V shared memory segment associated with the value of the argument key. A new shared memory segment, with size equal to the value of size rounded up to a multiple of PAGE_SIZE, is created if key has the value IPC_PRI‐VATE or key isn’t IPC_PRIVATE, no shared memory segment corresponding to key exists, and IPC_CREAT is specified in shmflg.

2 shmat

把创建的内存地址空间连接到进程的地址空间。

#include

#include

void *shmat(int shmid, const void *shmaddr, int shmflg);

int shmdt(const void *shmaddr);

3 shmdt

#include

#include

void *shmat(int shmid, const void *shmaddr, int shmflg);

int shmdt(const void *shmaddr);

把共享的内存地址从进程中分离。

4 shmctl

控制共享内存

#include

#include

int shmctl(int shmid, int cmd, struct shmid_ds *buf);

控制共享内存；

5　使用共享内存

/* Our first program is a consumer. After the headers the shared memory segment

(the size of our shared memory structure) is created with a call to shmget,

with the IPC_CREAT bit specified. */

#include

#include

#include

#include

#include

#include "shm_com.h"

int main()

{

int running = 1;

void *shared_memory = (void *)0;

struct shared_use_st *shared_stuff;

int shmid;

srand((unsigned int)getpid());

shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT);

if (shmid == -1) {

fprintf(stderr, "shmget failed\n");

exit(EXIT_FAILURE);

}

/* We now make the shared memory accessible to the program. */

shared_memory = shmat(shmid, (void *)0, 0);

if (shared_memory == (void *)-1) {

fprintf(stderr, "shmat failed\n");

exit(EXIT_FAILURE);

}

printf("Memory attached at %X\n", (int)shared_memory);

/* The next portion of the program assigns the shared_memory segment to shared_stuff,

which then prints out any text in written_by_you. The loop continues until end is found

in written_by_you. The call to sleep forces the consumer to sit in its critical section,

which makes the producer wait. */

shared_stuff = (struct shared_use_st *)shared_memory;

shared_stuff->written_by_you = 0;

while(running) {

if (shared_stuff->written_by_you) {

printf("You wrote: %s", shared_stuff->some_text);

sleep( rand() % 4 ); /* make the other process wait for us ! */

shared_stuff->written_by_you = 0;

if (strncmp(shared_stuff->some_text, "end", 3) == 0) {

running = 0;

}

}

}

/* Lastly, the shared memory is detached and then deleted. */

if (shmdt(shared_memory) == -1) {

fprintf(stderr, "shmdt failed\n");

exit(EXIT_FAILURE);

}

if (shmctl(shmid, IPC_RMID, 0) == -1) {

fprintf(stderr, "shmctl(IPC_RMID) failed\n");

exit(EXIT_FAILURE);

}

exit(EXIT_SUCCESS);

}

/* The second program is the producer and allows us to enter data for consumers. It's very similar to shm1.c and looks like this. */

#include

#include

#include

#include

#include

#include "shm_com.h"

int main()

{

int running = 1;

void *shared_memory = (void *)0;

struct shared_use_st *shared_stuff;

char buffer[BUFSIZ];

int shmid;

shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT);

if (shmid == -1) {

fprintf(stderr, "shmget failed\n");

exit(EXIT_FAILURE);

}

shared_memory = shmat(shmid, (void *)0, 0);

if (shared_memory == (void *)-1) {

fprintf(stderr, "shmat failed\n");

exit(EXIT_FAILURE);

}

printf("Memory attached at %X\n", (int)shared_memory);

shared_stuff = (struct shared_use_st *)shared_memory;

while(running) {

while(shared_stuff->written_by_you == 1) {

sleep(1);

printf("waiting for client...\n");

}

printf("Enter some text: ");

fgets(buffer, BUFSIZ, stdin);

strncpy(shared_stuff->some_text, buffer, TEXT_SZ);

shared_stuff->written_by_you = 1;

if (strncmp(buffer, "end", 3) == 0) {

running = 0;

}

}

if (shmdt(shared_memory) == -1) {

fprintf(stderr, "shmdt failed\n");

exit(EXIT_FAILURE);

}

exit(EXIT_SUCCESS);

}