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system V IPC —�?�׃�n内存(�?

Marcky — Thu, 20 Aug 2009 06:57:00 GMT

使用�׃�n内存和记录锁实例。本例中�Q�父�q�程创徏一�D�共享内存，然后向其中追加字�W�串"Parent"(在写�?#8220;Parent”的时候，采用一个字�W�一个字�W�的写入�Q�目的是��Z��验证记录锁对父子�q�程同步的正��?�Q�子�q�程向共享内存中�q�加字符�?#8220;Child”�Q�同��h��一个一个的字符写入�Q�。由于记录锁是针�Ҏ��件的�Q�所以得先创��Z��个空文�g作�ؓ记录锁的操作对象�Q�作为共享内存访问的辅助工具�Q�如果一个进�E�对�q�个�I�文件加写锁成功后，��开始访问共享内存，讉K��l�束��对文�g解锁�?br>
代码如下�Q?br>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

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

#include <sys/wait.h>
#include <fcntl.h>

#include <sys/ipc.h>
#include <sys/shm.h>

#define SHM_SIZE 1024            /* shared memory size(bytes) */
#define SHM_MODE 0600            /* user read/write */

#define WRITE_LOCK(fd) regLock((fd), F_SETLKW, F_WRLCK, 0, SEEK_SET, 0)
#define UN_LOCK(fd) regLock((fd), F_SETLK, F_UNLCK, 0, SEEK_SET, 0)
/* 创徏一个文�?nbsp;*/
int createFile()
{
    int fd;

    if ( (fd = open("/tmp/emptyfile4shm", O_RDWR | O_CREAT, 0666)) < 0) {
        fprintf(stderr, "Create a empty file failed!\n");
        exit(EXIT_FAILURE);
    }

    return fd;
}
/* 在文件fd上加锁或解锁*/
int regLock(int fd, int cmd, int type, off_t offset, int whence, off_t len)
{
    struct flock lock;

    lock.l_type = type;         /* lock type: F_RDLCK, F_WRLCK, F_UNLCK */
    lock.l_start = offset;      /* byte offset relative to l_whence */
    lock.l_whence = whence;    /* it's value : SEEK_SET, SEEK_CUR, SEEK_END */
    lock.l_len = len;           /* bytes (0 means to EOF) */

    return fcntl(fd, cmd, &lock);
}

int main(void)
{
    int shmid;
    int filed;
    pid_t pid;
    char *shmptr = NULL;

    filed = createFile();

    if ( (shmid = shmget(IPC_PRIVATE, SHM_SIZE, SHM_MODE)) < 0) { /* Create shared memory */
        fprintf(stderr, "Create shared memory failed!\n");
        exit(EXIT_FAILURE);
    }

    if ( (pid = fork()) < 0) {  /* Create a child process */
        fprintf(stderr, "Create child process failed!\n");
        exit(EXIT_FAILURE);
    }

    if (pid == 0) {             /* In child process */
        if ( (shmptr = shmat(shmid, 0, 0)) == (void *)-1) {
            fprintf(stderr, "Attached shared memory failed!\n");
            exit(EXIT_FAILURE);
        }
        while (1) {
            WRITE_LOCK(filed);  /* add a write lock to filed */
            /* shared memory 剩余的空间不能存�?Child"字符串和�l�束�W?时就退出��@�?nbsp;*/
            if (SHM_SIZE - strlen(shmptr) < strlen("Child") + 1) break;
            strcat(shmptr, "C"); /* �׃��加了锁，每个Child��会�q�箋出现 */
            strcat(shmptr, "h");
            strcat(shmptr, "i");
            strcat(shmptr, "l");
            strcat(shmptr, "d");
            UN_LOCK(filed);     /* release lock */
        }
        printf("child process:\n\t%s\n", shmptr); /* child process print shared memory */

        exit(0);
    }

    /* In parent process */
    if ( (shmptr = shmat(shmid, 0, 0)) == (void *)-1) { /* Attached shared memory */
        fprintf(stderr, "Attached shared memory failed!\n");
        exit(EXIT_FAILURE);
    }

    while (1) {
        WRITE_LOCK(filed);
        /* shared memory 剩余的空间不能存�?Parent"字符串和�l�束�W?时就退出��@�?nbsp;*/
        if (SHM_SIZE - strlen(shmptr) < strlen("Parent") + 1) break;
        strcat(shmptr, "P");/* �׃��加了锁，每个Parent��会�q�箋出现 */
        strcat(shmptr, "a");
        strcat(shmptr, "r");
        strcat(shmptr, "e");
        strcat(shmptr, "n");
        strcat(shmptr, "t");
        UN_LOCK(filed);
    }
    printf("parent process:\n\t%s\n", shmptr); /* parent print shared memory */

    wait(0);
    exit(0);
}

Marcky 2009-08-20 14:57 发表评论

system V IPC —�?�׃�n内存(�?

Marcky — Thu, 20 Aug 2009 06:48:00 GMT

�׃�n内存允许多个�q�程�׃�n一�l�定的存储区。因为数据不需要在两个�q�程之间�q�行copy�Q�所以这是最快的一�U�IPC。��用共享内存技术的时候，需要掌握好的是多个�q�程之间如何同步。信号量和记录锁可以用来实现�׃�n内存的多个进�E�之间的同步�?br>
linux内核定义的shared memory�l�构shmid_ds如下�Q?br>

struct shmid_ds {
    struct ipc_perm  shm_perm;  /*权限*/
    size_t           shm_segsz; /*大小*/
    pid_t            shm_lpid;
    pid_t            shm_cpid;  /*创徏者pid*/
    shmatt_t         shm_nattch;/*�q�接到此�D�内存的�q�程�?/span>*/
    time_t           shm_atime;
    time_t           shm_dtime;
    time_t           shm_ctime;

};

1、创建或使用一�D�共享内存��用shmget函数�Q�此函数��返回共享内存标�C�符�?br>

#include <sys/shm.h>
int shmget(key_t key, size_t size, int flag);

如果key取��gؓIPC_PRIVATE或者key当前为和特定�c�d��的IPC�l�构相结合，�q�且flag指定了IPC_CREAT位，则创��Z��个新的share memory�l�构�?br>size为共享内存段的长度（字节�Q��?br>
2、对一个共享内存段�q�行操作使用shmctl�?br>

#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);

shmid指定需要操作的shared memory
cmd指定需要进行的操作
IPC_STAT取得此段的shmid_ds�l�构攑օ�buf中�?br> IPC_SET用buf的��D��|�此�D�中的：shm_perm.uid�Q�shm_perm.gid�Q�shm_perm.mode�?br> IPC_RMID从系�l�中删除此共享内存段�?br> SHM_LOCK��共享内存锁定到内存中�?br> SHM_UNLOCK解锁�׃�n内存�D�c�?br>
3、将一个共享内存段�q�接到自��q��地址�I�间使用shmat�Q?br>

#include <sys/shm.h>
void *shmat(int shmid, const void *addr, int flag);

推荐addr�?��|��此�D�连接到内核选择的第一个可用的地址上。增加程序的可移植性�?br>
4、对�׃�n内存操作�l�束后，要脱��该�D는�shmdt�Q?br>

#include <sys/shm.h>
int shmdt(void *addr);

addr是shmat的返回倹{�?br>

Marcky 2009-08-20 14:48 发表评论

Linux命��o:(){ :|:& };:分析

Marcky — Tue, 30 Jun 2009 08:46:00 GMT

摘要: 有�h�?(){ :|:& };: 命��o��是一个系�l�炸式V��在linux的shell里执行此命��o��会不停的创建新的进�E�，直到�pȝ��挂掉为止。刚看到�q�个怪七怪八的命令，也是完全不知所云，在水木高手解释后�Q��ȝ��是明白了�?nbsp; 阅读全文

Marcky 2009-06-30 16:46 发表评论