进程间通信之： 共享内存

#include <sys/types.h>ys/types.h>

#include <sys/ipc.h>

#include <sys/shm.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#define BUFFER_SIZE 2048

int main()

{

pid_t pid;

int shmid;

char *shm_addr;

char flag[] = "WROTE";

char *buff;

/* 创建共享内存 */

if ((shmid = shmget(IPC_PRIVATE, BUFFER_SIZE, 0666)) < 0)

{

perror("shmget");

exit(1);

}

else

{

printf("Create shared-memory: %d\n",shmid);

}

/* 显示共享内存情况 */

system("ipcs -m");

pid = fork();

if (pid == -1)

{

perror("fork");

exit(1);

}

else if (pid == 0) /* 子进程处理 */

{

/*映射共享内存*/

if ((shm_addr = shmat(shmid, 0, 0)) == (void*)-1)

{

perror("Child: shmat");

exit(1);

}

else

{

printf("Child: Attach shared-memory: %p\n", shm_addr);

}

system("ipcs -m");

/* 通过检查在共享内存的头部是否标志字符串"WROTE"来确认

父进程已经向共享内存写入有效数据 */

while (strncmp(shm_addr, flag, strlen(flag)))

{

printf("Child: Wait for enable data...\n");

sleep(5);

}

/* 获取共享内存的有效数据并显示 */

strcpy(buff, shm_addr + strlen(flag));

printf("Child: Shared-memory :%s\n", buff);

/* 解除共享内存映射 */

if ((shmdt(shm_addr)) < 0)

{

perror("shmdt");

exit(1);

}

else

{

printf("Child: Deattach shared-memory\n");

}

system("ipcs -m");

/* 删除共享内存 */

if (shmctl(shmid, IPC_RMID, NULL) == -1)

{

perror("Child: shmctl(IPC_RMID)\n");

exit(1);

}

else

{

printf("Delete shared-memory\n");

}

system("ipcs -m");

}

else /* 父进程处理 */

{

/*映射共享内存*/

if ((shm_addr = shmat(shmid, 0, 0)) == (void*)-1)

{

perror("Parent: shmat");

exit(1);

}

else

{

printf("Parent: Attach shared-memory: %p\n", shm_addr);

}

sleep(1);

printf("\nInput some string:\n");

fgets(buff, BUFFER_SIZE, stdin);

strncpy(shm_addr + strlen(flag), buff, strlen(buff));

strncpy(shm_addr, flag, strlen(flag));

/* 解除共享内存映射 */

if ((shmdt(shm_addr)) < 0)

{

perror("Parent: shmdt");

exit(1);

}

else

{

printf("Parent: Deattach shared-memory\n");

}

system("ipcs -m");

waitpid(pid, NULL, 0);

printf("Finished\n");

}

exit(0);

}

下面是运行结果。从该结果可以看出，nattch的值随着共享内存状态的变化而变化，共享内存的值根据不同的系统会有所不同。

$ ./shmem

Create shared-memory: 753665

/* 在刚创建共享内存时（尚未有任何地址映射）共享内存的情况 */

------ Shared Memory Segments --------

key shmid owner perms bytes nattch status

0x00000000 753665 david 666 2048 0

Child: Attach shared-memory: 0xb7f59000 /* 共享内存的映射地址 */

Parent: Attach shared-memory: 0xb7f59000

/* 在父子进程中进行共享内存的地址映射之后共享内存的情况*/

------ Shared Memory Segments --------

key shmid owner perms bytes nattch status

0x00000000 753665 david 666 2048 2

Child: Wait for enable data...

Input some string:

Hello /* 用户输入字符串"Hello" */

Parent: Deattach shared-memory

/* 在父进程中解除共享内存的映射关系之后共享内存的情况 */

------ Shared Memory Segments --------

key