Posted on 2008-02-19 11:30
ZelluX 閱讀(455)
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《邊干邊學》上一個簡單的共享內存的例程：
#include?<unistd.h>
#include?<sys/ipc.h>
#include?<sys/shm.h>
#include?<errno.h>
#include?<stdio.h>
#include?<string.h>
#define?KEY?1234
#define?SIZE?1024
int?main()
{
????int?shmid;
????char?*shmaddr;
????struct?shmid_ds?buf;
????shmid?=?shmget(KEY,?SIZE,?IPC_CREAT?|?0600);
????if?(shmid?==?-1)?{
????????printf("Failed?in?creating?shared?memory:?%s\n",?strerror(errno));
????????return?0;
????}
????if?(fork()?==?0)?{
????????shmaddr?=?(char?*)?shmat(shmid,?NULL,?0);
????????if?(shmaddr?==?(void?*)?-1)?{
????????????printf("Failed?in?connecting?to?the?shared?memory:?%s\n",?strerror(errno));
????????????return?0;
????????}
????????
????????strcpy(shmaddr,?"Hello,?this?is?child?process!\n");
????????shmdt(shmaddr);
????????return?0;
????}?else?{
????????sleep(3);
????????shmctl(shmid,?IPC_STAT,?&buf);
????????
????????printf("Size?of?the?shared?memory:?");
????????printf("shm_segsz?=?%d?bytes\n",?buf.shm_segsz);
????????printf("Process?id?of?the?creator:?");
????????printf("shm_cpid?=?%d\n",?buf.shm_cpid);
????????printf("Process?id?of?the?last?operator:?");
????????printf("shm_lpid?=?%d\n",?buf.shm_lpid);
????????shmaddr?=?(char?*)?shmat(shmid,?NULL,?0);
????????if?(shmaddr?==?(void?*)?-1)?{
????????????printf("Failed?in?connecting?the?shared?memory:?%s\n",?strerror(errno));
????????????return?0;
????????}
????????printf("The?content?of?the?shared?memory:?%s\n",?shmaddr);
????????shmdt(shmaddr);
????????shmctl(shmid,?IPC_RMID,?NULL);
????}
}
主要的API：
shmget 創建一塊共享內存
shmat 將一塊已經存在的共享內存映射到一個進程的地址空間
shmdt 取消一個進程的地址空間中的一塊共享塊的映射
shmctl 管理共享內存，和ioctl的風格很像
每一個新創建的共享都由一個shmid_ds{}表示。struct shmid_ds在linux/shm.h中的定義：
/**//*?Obsolete,?used?only?for?backwards?compatibility?and?libc5?compiles?*/
struct?shmid_ds?{
????struct?ipc_perm????????shm_perm;????/**//*?operation?perms?*/
????int????????????shm_segsz;????/**//*?size?of?segment?(bytes)?*/
????__kernel_time_t????????shm_atime;????/**//*?last?attach?time?*/
????__kernel_time_t????????shm_dtime;????/**//*?last?detach?time?*/
????__kernel_time_t????????shm_ctime;????/**//*?last?change?time?*/
????__kernel_ipc_pid_t????shm_cpid;????/**//*?pid?of?creator?*/
????__kernel_ipc_pid_t????shm_lpid;????/**//*?pid?of?last?operator?*/
????unsigned?short????????shm_nattch;????/**//*?no.?of?current?attaches?*/
????unsigned?short?????????shm_unused;????/**//*?compatibility?*/
????void?????????????*shm_unused2;????/**//*?ditto?-?used?by?DIPC?*/
????void????????????*shm_unused3;????/**//*?unused?*/
};其中存放權限信息的ipc_perm{}的定義為：
include/linux/ipc.h
/**//*?Obsolete,?used?only?for?backwards?compatibility?and?libc5?compiles?*/
struct?ipc_perm
{
????__kernel_key_t????key;
????__kernel_uid_t????uid;
????__kernel_gid_t????gid;
????__kernel_uid_t????cuid;
????__kernel_gid_t????cgid;
????__kernel_mode_t????mode;?
????unsigned?short????seq;
};mode為該共享的內存的讀寫權限，和chmod的參數有點像。mode低九位定義了訪問許可，解釋如下：
0400?用戶可讀? 0200用戶可寫? 0040 組成員可讀? 0020 組成員可寫? 0004 其他用戶可讀??0002 其他用戶可寫
沒有執行位 0100 0010 和 0001
?