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本文關鍵字： 內存共享
1．實(shi)驗目的
通(tong)過(guo)編寫共(gong)享(xiang)(xiang)內(nei)存實驗(yan)，進一步(bu)了解使用(yong)(yong)共(gong)享(xiang)(xiang)內(nei)存的(de)具體步(bu)驟，同(tong)(tong)時(shi)加深對共(gong)享(xiang)(xiang)內(nei)存的(de)理解。在(zai)本實驗(yan)中，采用(yong)(yong)信(xin)號(hao)量作為同(tong)(tong)步(bu)機制(zhi)(zhi)完善兩個(ge)進程(cheng)（“生(sheng)產者(zhe)(zhe)”和“消費者(zhe)(zhe)”）之間的(de)通(tong)信(xin)，其功能類似(si)于4.6節中的(de)實例。在(zai)實例中使用(yong)(yong)信(xin)號(hao)量同(tong)(tong)步(bu)機制(zhi)(zhi)。
2．實驗內容
該實(shi)現要求利用共(gong)享內存(cun)實(shi)現文件(jian)的打開和讀寫操作。
3．實驗步驟
（1）畫出流程(cheng)圖(tu)。該實驗流程(cheng)圖(tu)如圖(tu)1所(suo)示。
圖1 實驗流程圖
; （2）編寫代碼。下面是共享內存緩沖區的數據結構(gou)的定義：
/* shm_com.h */
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SHM_BUFF_SZ 2048
struct shm_buff
{
int pid;
char buffer[SHM_BUFF_SZ];
};
以下是“生產者”程序部分：
/* sem_com.h 和 sem_com.c 與4.4節示例中的同名程序相同 */
/* producer.c */
#include "shm_com.h"
#include "sem_com.h"
#include <signal.h>
int ignore_signal(void)
{ /* 忽略一些信號，以免非法退出程序 */
signal(SIGINT, SIG_IGN);
signal(SIGSTOP, SIG_IGN);
signal(SIGQUIT, SIG_IGN);
return 0;
}
int main()
{
void *shared_memory = NULL;
struct shm_buff *shm_buff_inst;
char buffer[BUFSIZ];
int shmid, semid;
/* 定義信號量，用于實現訪問共享內存的進程間的互斥 */
ignore_signal(); /* 防止程序非正常退出 */
semid = semget(ftok(".", 'a'), 1, 0666|IPC_CREAT); /* 創建一個信號量 */
init_sem(semid);/* 初始值為1 */
/* 創建共享內存 */
shmid = shmget(ftok(".", 'b'), sizeof(struct shm_buff), 0666|IPC_CREAT);
if (shmid == -1)
{
perror("shmget failed");
del_sem(semid);
exit(1);
}
/* 將共享內存地址映射到當前進程地址空間 */
shared_memory = shmat(shmid, (void*)0, 0);
if (shared_memory == (void*)-1)
{
perror("shmat");
del_sem(semid);
exit(1);
}
printf("Memory attached at %X\n", (int)shared_memory);
/* 獲得共享內存的映射地址 */
shm_buff_inst = (struct shared_use_st *)shared_memory;
do
{
sem_p(semid);
printf("Enter some text to the shared memory(enter 'quit' to exit):");
/* 向共享內存寫入數據 */
if (fgets(shm_buff_inst->buffer, SHM_BUFF_SZ, stdin) == NULL)
{
perror("fgets");
sem_v(semid);
break;
}
shm_buff_inst->pid = getpid();
sem_v(semid);
} while(strncmp(shm_buff_inst->buffer, "quit", 4) != 0);
/* 刪除信號量 */
del_sem(semid);
/* 刪除共享內存到當前進程地址空間中的映射 */
if (shmdt(shared_memory) == 1)
{
perror("shmdt");
exit(1);
}
exit(0);
}
以下是“消(xiao)費者”程序部分(fen)：
/* customer.c */
#include "shm_com.h"
#include "sem_com.h"
int main()
{
void *shared_memory = NULL;
struct shm_buff *shm_buff_inst;
int shmid, semid;
/* 獲得信號量 */
semid = semget(ftok(".", 'a'), 1, 0666);
if (semid == -1)
{
perror("Producer is'nt exist");
exit(1);
}
/* 獲得共享內存 */
shmid = shmget(ftok(".", 'b'), sizeof(struct shm_buff), 0666|IPC_CREAT);
if (shmid == -1)
{
perror("shmget");
exit(1);
}
/* 將共享內存地址映射到當前進程地址空間 */
shared_memory = shmat(shmid, (void*)0, 0);
if (shared_memory == (void*)-1)
{
perror("shmat");
exit(1);
}
printf("Memory attached at %X\n", (int)shared_memory);
/* 獲得共享內存的映射地址 */
shm_buff_inst = (struct shm_buff *)shared_memory;
do
{
sem_p(semid); printf("Shared memory was written by process %d :%s",
shm_buff_inst->pid, shm_buff_inst->buffer);
if (strncmp(shm_buff_inst->buffer, "quit", 4) == 0)
{
break;
}
shm_buff_inst->pid = 0;
memset(shm_buff_inst->buffer, 0, SHM_BUFF_SZ);
sem_v(semid);
} while(1);
/* 刪除共享內存到當前進程地址空間中的映射 */
if (shmdt(shared_memory) == -1)
{
perror("shmdt");
exit(1);
}
/* 刪除共享內存 */
if (shmctl(shmid, IPC_RMID, NULL) == -1)
{
perror("shmctl(IPC_RMID)");
exit(1);
}
exit(0);
}
4．實驗結果
實(shi)驗運行結(jie)果如下(xia)：
$./producer
Memory attached at B7F90000
Enter some text to the shared memory(enter 'quit' to exit):First message
Enter some text to the shared memory(enter 'quit' to exit):Second message
Enter some text to the shared memory(enter 'quit' to exit):quit
$./customer
Memory attached at B7FAF000
Shared memory was written by process 3815 :First message
Shared memory was written by process 3815 :Second message
; Shared memory was written by process 3815 :quit
本文選自華清遠見嵌入式培訓教材《從實踐中學嵌入式Linux應用程序開發》
熱點鏈接：
1、linux 共享內存
2、linux有名管道通信實驗
3、linux 消息隊列
4、linux下的信號量
5、linux下的信號處理實例
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