基于system V共享内存学习

共享内存函数主要有以下几个,shmget用来创建新的共享内存区，或者访问已经存在的内存区。shmat函数则用来把shmget创建或者打开的共享内存区附接到调用进程的地址空间。shmdt则用来断开这个地址，shmctl提供对共享内存的多种操作，需要注意的是shmdt函数不能删除所在共享区，删除工作需要交给shmctl来操作。这几个函数原型如下：

#include<sys/shm.h>

int shmget (key_t key,size_t,size ,int flag):

size以字节为单位制定内存区域大小，flag是各种权限。

#include<sys/shm.h>

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

shmid为shmget 返回值，shmaddr为共享内存地址。

#include<sys/shm.h>

int shmdt(const void *shmaddr);

参数shmaddr同上

#include<sys/shm.h>

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

该函数主要提供三个命令，IPC_RMID为从系统中删除shmid标识符的共享内存区域。

IPC_SET设置最后一个参数结构体里面的三个成员函数：shm_perm.uid,shm_perm.gid,shm_perm.mode.IPC_STAT则是返回最后一个参数的结构体信息。

具体用法距离如下：首选创建一个头文件"shm_com.h"

#define TEXT_SZ 2048
struct shared_use_st {
int written_by_you;
char some_text[TEXT_SZ];
};

之后是shm1和shm2函数来实现内存共享。shm1.c如下
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <sys/shm.h>

#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);
}

shm2.c代码如下：
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <sys/shm.h>

#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);
}
测试时候输入end字符串时候程序结束。两者为典型的生产者和消费者的关系。