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);
}