linux CentOS下的C语言异步访问共享内存问题，高手请进！

[luvi88]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <semaphore.h>

#define USE_SEMAPHORE 1
#define OAM_FTOK_ID 8
#define N 10
#define NUM_PROCESS 2

typedef struct {
    char name[8];
    char age;
    char sex;
    short id;
} student_t;

union semun {
    int val;
    struct semid_ds *buf;
    unsigned short *array;
};

/*         global variable            */
//sem_t mutex;
int sem_id;          // semaphore

void display(const student_t const *buf, const int n);
int child_process(const void *buf, const int len);
//int child_process(void *buf, int len, sem_t *pmutex);
static int set_semvalue(void);
static void del_semvalue(void);
static int semaphore_p(void);
static int semaphore_v(void);
void modify(student_t *buf, const int start, const int n);
int main(int argc, char **argv)
{
    int id;
    int i;
    int pid;
    int fd;
    student_t *buf;
//  student_t *p;

    id = ftok(*argv, OAM_FTOK_ID);
    if (-1 == id)
    {
        perror("ftok");
        return -1;
    }

    /* initialize semaphore */
/*  if (-1 == sem_init(&mutex, 0, 1))
    {
        perror("sem_init");
        exit(-3);
    }
*/
    sem_id = semget(id, 1, 0600|IPC_CREAT);
    if (-1 == sem_id)
    {
        perror("semget");
        exit(-4);
    }
    if (set_semvalue())
    {
        fprintf(stderr, "%s: semctl", __FUNCTION__);
        exit(-5);
    }

//  p = (student_t *)malloc(N * sizeof(student_t));
    fd = open("ccccccccccccc.txt", O_CREAT | O_RDWR | O_TRUNC, 00666);
    ftruncate(fd, N * sizeof(student_t));
    buf = (student_t *)mmap(NULL, N*sizeof(student_t), \
        PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    printf("buf = %p\n", buf);
    if (NULL == buf)
    {
        perror("mmap");
        return -1;
    }

    /* init */
    puts("Init:");
    for (i=0; i<N; i++)
    {
        buf[i].name[0] = 'a'+i;
        buf[i].name[1] = '\0';
        buf[i].age = 20+i;
        buf[i].sex = i % 2;
        buf[i].id = (short) i;
    }

    display(buf, N);

    for (i=0; i<NUM_PROCESS; i++)
    {

        pid = fork();
        if (pid < 0)
        {
            perror("fork");
            exit(-2);
        }
        else if (0 == pid)
        {  // child, modify or display result
            student_t *pbuf;

            /* remap share memory */
            fd = open("ccccccccccccc.txt", O_CREAT | O_RDWR | O_TRUNC, 00666);
            ftruncate(fd, N * sizeof(student_t));
            pbuf = (student_t *)mmap(NULL, N*sizeof(student_t), \
                PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
            if (NULL == pbuf)
            {
                perror("mmap");
                return -1;
            }
            printf("Child: pbuf = %p\n", pbuf);

            if (1 == i)
            {  // show result
                sleep(3);
                display(pbuf, N);
            }
            else
            {  // update data in share memory
                modify(pbuf, 'A', N);
            }
//          child_process(pbuf, N);
//          child_process(pbuf, N, &mutex);
            munmap(pbuf, N*sizeof(student_t));
//          free(p);
            close(fd);
            exit(1);
        }
        else
        {  // Father, waiting for children
/*
            sleep(1);
//          sem_wait(&mutex);
            for (i=0; i<N; i++)
            {
                buf[i].name[0] = 'A'+i;
                buf[i].name[1] = '\0';
                buf[i].age = 20+i;
                buf[i].sex = i % 2;
                buf[i].id = (short) i;
            }
            puts("Father 2:");
//          sem_post(&mutex);
            display(buf, N);
            wait(NULL);
*/
            int status;
            pid = waitpid(pid, &status, 0);
            fprintf(stderr, "[pid: %d], status: %d\n", pid, status);
        }
    }  // end for

    puts("Father(After ending children):");
    printf("&buf == %p\n", buf);
    display(buf, N);
    del_semvalue();
    munmap((void *)buf, N * sizeof(student_t));
    close(fd);
//  sem_destroy(&mutex);
//  free(p);
    return 0;
}

void display(const student_t const *buf, const int n)
{
    int i;

    semaphore_p();
    printf("[%d] Result:\n", getpid());
    for (i=0; i<n; i++)
    {
        printf("{(%d)\tid: %d, name: %s, age: %d, sex: %d}\n", \
            getpid(), buf[i].id, buf[i].name, buf[i].age, buf[i].sex);
        usleep(50000);
    }
    printf("&buf.name = %p\n", buf[0].name);
    semaphore_v();
}

void modify(student_t *buf, const int start, const int n)
{
    int i;

    /* init */
    semaphore_p();
    printf("[%d] Updating:\n", getpid());
    for (i=start; i<start+n; i++)
    {
        buf[i].name[0] = i;
        buf[i].name[1] = '\0';
        buf[i].age = 20+i;
        buf[i].sex = i & 1;
        buf[i].id = (short) i;
        printf("[%p]  '%s'\n", buf[i].name, buf[i].name);
        usleep(50000);
    }
    semaphore_v();
    printf("[%d]: name: %s\n", getpid(), buf[0].name);
    display(buf, n);
}

int child_process(const void *buf, const int len)
//int child_process(void *buf, int len, sem_t *pmutex)
{
//  sem_wait(&mutex);
    puts("child 1:");
    display(buf, len);
//  sem_post(&mutex);
    sleep(2);
//  sem_wait(&mutex);
    puts("child 2:");
    display(buf, len);
//  sem_post(&mutex);

    return 0;
}

static int set_semvalue(void)
{
    union semun sem_union;

    sem_union.val = 1;
    if (-1 == semctl(sem_id, 0, SETVAL, sem_union))
    {
        return -1;
    }

    return 0;
}

static void del_semvalue(void)
{
    union semun sem_union;

    if (-1 == semctl(sem_id, 0, IPC_RMID, sem_union))
    {
        perror("semctl");
    }
}

static int semaphore_p(void)
{
    struct sembuf sem_b;
    sem_b.sem_num = 0;
    sem_b.sem_op = -1;
    sem_b.sem_flg = SEM_UNDO;
    if (-1 == semop(sem_id, &sem_b, 1))
    {
        perror("semop");
        return -1;
    }

    return 0;
}

static int semaphore_v(void)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;
    sem_b.sem_op = 1;
    sem_b.sem_flg = SEM_UNDO;
    if (-1 == semop(sem_id, &sem_b, 1))
    {
        perror("semop");
        return -1;
    }

    return 0;
}

#if 0
在CentOS 4.8 平台上，运行结果如下:

/************************* 运行结果 ***************************/
[root@reduce aa]# ./a.out
buf = 0xb7fff000
Init:
[3810] Result:
{(3810) id: 0, name: a, age: 20, sex: 0}
{(3810) id: 1, name: b, age: 21, sex: 1}
{(3810) id: 2, name: c, age: 22, sex: 0}
{(3810) id: 3, name: d, age: 23, sex: 1}
{(3810) id: 4, name: e, age: 24, sex: 0}
{(3810) id: 5, name: f, age: 25, sex: 1}
{(3810) id: 6, name: g, age: 26, sex: 0}
{(3810) id: 7, name: h, age: 27, sex: 1}
{(3810) id: 8, name: i, age: 28, sex: 0}
{(3810) id: 9, name: j, age: 29, sex: 1}
&buf.name = 0xb7fff000
Child: pbuf = 0xb7ffd000
[3811] Updating:
[0xb7ffd30c] 'A'
[0xb7ffd318] 'B'
[0xb7ffd324] 'C'
[0xb7ffd330] 'D'
[0xb7ffd33c] 'E'
[0xb7ffd348] 'F'
[0xb7ffd354] 'G'
[0xb7ffd360] 'H'
[0xb7ffd36c] 'I'
[0xb7ffd378] 'J'
[3811]: name:
[3811] Result:
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
{(3811) id: 0, name: , age: 0, sex: 0}
&buf.name = 0xb7ffd000
[pid: 3811], status: 256
Child: pbuf = 0xb7ffd000
[3812] Result:
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
{(3812) id: 0, name: , age: 0, sex: 0}
&buf.name = 0xb7ffd000
[pid: 3812], status: 256
Father(After ending children):
&buf == 0xb7fff000
[3810] Result:
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
{(3810) id: 0, name: , age: 0, sex: 0}
&buf.name = 0xb7fff000
/********************************* 运行结果结束 *******************************/

结果分析：
也就是说，父进程初始化结束，对共享内存的内容产生了影响。
在子进程中的修改结果只在for修改语句中有效，出了这个for语句之后，
子进程对自身重映射的共享内存的修改结果就无效了。
并且在子进程中，也影响了父进程共享内存的内容，导致其内容丢失。
请高手分析和指出错误并加以指正，泪奔式谢谢广大童鞋了！

#endif

[luvi88]

附注：
当只有父亲和一个孩子的时候，结果是正确的。两个孩子的时候，这两个孩子相互没有父子性亲缘关系了，问题就出来了。。。

[zecard_fu]

实在下不起决心看完! 友情帮顶下好了~

[luvi88]

谢谢了，因为只是一小测试，没有写注释，发贴的时候，应该把注释加上的。。。。
最下边4个接口分别是信号量操作的。
static int set_semvalue(void); /* 设置信号量的值 */
static void del_semvalue(void); /* 删除信号量 */
static int semaphore_p(void); /* 信号量p操作 */
static int semaphore_v(void); /* v操作 */
int child_process(const void *buf, const int len); /* 此接口无用，是单子进程时用的，双子进程时，使用下边两个接口访问共享内存： */
void display(const student_t const *buf, const int n); /* 互斥显示共享内存内容 */
void modify(student_t *buf, const int start, const int n); /* 互斥修改共享内存内容 */

[luvi88]

没有人有耐心吗？