Unix IPC之基于共享内存的计数器

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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);

#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);
}

#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);
}

[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]$

#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);
}