Unix IPC之基于共享内存的计数器

目的

本文主要实现一个基于共享内存的计数器，通过父子进程对其访问。

本文程序需基于<<Unix网络编程-卷2>>的环境才能运行。程序中大写开头的函数为其小写同名函数的包裹函数，增加了错误处理信息。

1 函数介绍

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#include <sys/mman>   /**  * Map addresses starting near ADDR and extending for LEN bytes.  * from OFFSET into the file FD describes according to PROT and FLAGS.  * If ADDR is nonzero, it is the desired mapping address.  * If the MAP_FIXED bit is set in FLAGS, the mapping will be at ADDR exactly (which must be  * page-aligned); otherwise the system chooses a convenient nearby address.  * The return value is the actual mapping address chosen or MAP_FAILED  * for errors (in which case `errno' is set).  A successful `mmap' call  * deallocates any previous mapping for the affected region.  */ void *mmap (void *__addr, size_t __len, int __prot,             int __flags, int __fd, __off_t __offset);   /* Deallocate any mapping for the region starting at ADDR and extending LEN    bytes.  Returns 0 if successful, -1 for errors (and sets errno).  */ extern int munmap (void *__addr, size_t __len) __THROW;   /* Synchronize the region starting at ADDR and extending LEN bytes with the    file it maps.  Filesystem operations on a file being mapped are    unpredictable before this is done.  Flags are from the MS_* set.      This function is a cancellation point and therefore not marked with    __THROW.  */ extern int msync (void *__addr, size_t __len, int __flags);

 

2 计数器——非共享内存

默认情况下，通过fork派生的子进程并不与其父进程共享内存区。

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#include    "unpipc.h"   #define SEM_NAME    "mysem" int     count = 0; // 计数器   int main(int argc, char **argv) {     int     i, nloop;     sem_t   *mutex;       if (argc != 2)         err_quit("usage: incr1 <#loops>");     nloop = atoi(argv[1]);       /* 4create, initialize, and unlink semaphore */     mutex = Sem_open(Px_ipc_name(SEM_NAME), O_CREAT | O_EXCL, FILE_MODE, 1);     /**      * sem_unlink()       * removes the named semaphore referred to by name.       * The semaphore name is removed immediately.       * The semaphore is destroyed once all other processes that have the semaphore open close it.      */     Sem_unlink(Px_ipc_name(SEM_NAME));       // 非缓冲模式，防止两个线程的输出交叉     // 父子线程共同访问一个信号量     // 由于mutex是2值信号量，相当于同步父子线程     // 所以实际上这里并不会发生交叉     setbuf(stdout, NULL);   /* stdout is unbuffered */     if (Fork() == 0)        /* child */     {         for (i = 0; i < nloop; i++)         {             Sem_wait(mutex);             printf("child: %d\n", count++);             Sem_post(mutex);         }         exit(0);     }       /* 4parent */     for (i = 0; i < nloop; i++)     {         Sem_wait(mutex);         printf("parent: %d\n", count++);         Sem_post(mutex);     }     exit(0); }

由于子进程为父进程的拷贝，所以子进程自己有一个count的副本，所以父子进程操作自己的count。这里使用一个有名信号量来同步父子进程。

 

3 计数器——共享内存

程序说明：

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#include    "unpipc.h"   #define SEM_NAME    "mysem" // 有名信号量用于同步父子进程(加锁)   int main(int argc, char **argv) {     int     fd, i, nloop, zero = 0;     int     *ptr; // 访问共享内存的指针     sem_t   *mutex;       if (argc != 3)         err_quit("usage: incr2 <pathname> <#loops>");     nloop = atoi(argv[2]);       /* 4open file, initialize to 0, map into memory */     fd = Open(argv[1], O_RDWR | O_CREAT, FILE_MODE);     Write(fd, &zero, sizeof(int)); // 向文件中写入一个int型的0     // ptr返回共享内存起始位置     ptr = Mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); // 映射一个int型大小的共享内存     Close(fd);       /* 4create, initialize, and unlink semaphore */     mutex = Sem_open(Px_ipc_name(SEM_NAME), O_CREAT | O_EXCL, FILE_MODE, 1); // 信号量     Sem_unlink(Px_ipc_name(SEM_NAME));       setbuf(stdout, NULL);   /* stdout is unbuffered */     if (Fork() == 0)        /* child */     {         for (i = 0; i < nloop; i++)         {             Sem_wait(mutex);             printf("child: %d\n", (*ptr)++);             Sem_post(mutex);         }         exit(0);     }       /* 4parent */     for (i = 0; i < nloop; i++)     {         Sem_wait(mutex);         printf("parent: %d\n", (*ptr)++);         Sem_post(mutex);     }     exit(0); }

运行结果：

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[dell@localhost shm]$ ./incr2 ./count.data 100 parent: 0 parent: 1 parent: 2 parent: 3 .... child: 197 child: 198 child: 199 [dell@localhost shm]$ ll count.data -rw-r--r--. 1 dell dell 4 8月  18 14:28 count.data [dell@localhost shm]$ file count.data count.data: data [dell@localhost shm]$ hexdump -d count.data 0000000   00200   00000                                                0000004 [dell@localhost shm]$

注意：这里的count.data文件类型data类型，需用od、xxd、hexdump等命令才能查看。

 

4 计数器——共享内存(2)

上面3中的信号量为有名信号量，其具体实现由Posix决定，但是至少是内核持续性的。这里将其改为基于内存的信号量，并将其放置在共享内存中。

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#include    "unpipc.h"   struct shared {     sem_t mutex;      /* the mutex: a Posix memory-based semaphore */ // 匿名信号量     int   count;      /* and the counter */ } shared;   int main(int argc, char **argv) {     int     fd, i, nloop;     struct shared   *ptr;       if (argc != 3)         err_quit("usage: incr3 <pathname> <#loops>");     nloop = atoi(argv[2]);       /* 4open file, initialize to 0, map into memory */     fd = Open(argv[1], O_RDWR | O_CREAT, FILE_MODE);     Write(fd, &shared, sizeof(struct shared));     ptr = Mmap(NULL, sizeof(struct shared), PROT_READ | PROT_WRITE,                MAP_SHARED, fd, 0);     Close(fd);       /* 4initialize semaphore that is shared between processes */     Sem_init(&ptr->mutex, 1, 1); // 初始化匿名信号量，设置为进程共享，初始值为1       setbuf(stdout, NULL);   /* stdout is unbuffered */     if (Fork() == 0)        /* child */     {         for (i = 0; i < nloop; i++)         {             Sem_wait(&ptr->mutex);             printf("child: %d\n", ptr->count++);             Sem_post(&ptr->mutex);         }         exit(0);     }       /* 4parent */     for (i = 0; i < nloop; i++)     {         Sem_wait(&ptr->mutex);         printf("parent: %d\n", ptr->count++);         Sem_post(&ptr->mutex);     }     exit(0); }

5 计数器——共享内存(无亲缘关系进程)

 服务器端：创建有名信号量及共享内存

#include    "unpipc.h"

struct shmstruct    /* struct stored in shared memory */
{
    int   count;
};
sem_t   *mutex;     /* pointer to named semaphore */ // 有名信号量

int
main(int argc, char **argv)
{
    int     fd;
    struct shmstruct    *ptr;

    if (argc != 3)
        err_quit("usage: server1 <shmname> <semname>");

    shm_unlink(Px_ipc_name(argv[1]));       /* OK if this fails */
    /* 4create shm, set its size, map it, close descriptor */
    fd = Shm_open(Px_ipc_name(argv[1]), O_RDWR | O_CREAT | O_EXCL, FILE_MODE);
    Ftruncate(fd, sizeof(struct shmstruct)); // 设置共享内存大小
    ptr = Mmap(NULL, sizeof(struct shmstruct), PROT_READ | PROT_WRITE,
               MAP_SHARED, fd, 0);
    Close(fd);

    sem_unlink(Px_ipc_name(argv[2]));       /* OK if this fails */
    mutex = Sem_open(Px_ipc_name(argv[2]), O_CREAT | O_EXCL, FILE_MODE, 1);
    Sem_close(mutex);

    exit(0);
}

 

客户端：使用有名信号量；映射共享内存空间，并给共享内存空间内的计数器+1

#include    "unpipc.h"

struct shmstruct    /* struct stored in shared memory */
{
    int   count;
};
sem_t   *mutex;     /* pointer to named semaphore */

int
main(int argc, char **argv)
{
    int     fd, i, nloop;
    pid_t   pid;
    struct shmstruct    *ptr; // 注意这里的指针类型

    if (argc != 4)
        err_quit("usage: client1 <shmname> <semname> <#loops>");
    nloop = atoi(argv[3]);

    fd = Shm_open(Px_ipc_name(argv[1]), O_RDWR, FILE_MODE);
    ptr = Mmap(NULL, sizeof(struct shmstruct), PROT_READ | PROT_WRITE,
               MAP_SHARED, fd, 0);
    Close(fd);

    mutex = Sem_open(Px_ipc_name(argv[2]), 0);

    pid = getpid();
    for (i = 0; i < nloop; i++)
    {
        Sem_wait(mutex);
        printf("pid %ld: %d\n", (long) pid, ptr->count++);
        Sem_post(mutex);
    }
    exit(0);
}

说明：服务器运行一个实例；客户端运行多个实例，实例之间互斥对计数器进行+1操作；

来自为知笔记(Wiz)