进程数据共享， mmap 和 pthread_mutex

#include<sys/mman.h>
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<errno.h>
#include<string.h>
#include<fcntl.h>
#include<pthread.h>
#include<assert.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<signal.h>

struct sh_mem{
    pthread_mutex_t mutex;
    int val;
};

int main(int argc, char *argv[])
{
    int        pid;
    int        nloop;
    struct sh_mem *ptr;
    int        fd;
    int        ret;

    #ifndef _POSIX_THREAD_PROCESS_SHARED
        fprintf(stderr," Not define _POSIX_THREAD_PROCESS_SHARED");
        exit(-1);
    #endif

    if(argc != 3){
        fprintf(stderr, "Usage: you need three paremter\n");
        exit(EXIT_FAILURE);
    }

    nloop = atoi(argv[2]);

    fd = open(argv[1], O_RDWR | O_CREAT, 0644);
    ret = ftruncate(fd,sizeof(int));
    if(fd == -1){
        fprintf(stderr, "open error: %s\n", strerror(errno));
        exit(-1);
    }

    ptr = mmap(NULL, sizeof(struct sh_mem), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if(ptr == MAP_FAILED){
        fprintf(stderr, " mmap failed %s \n", strerror(errno));
        exit(EXIT_FAILURE);
    }
    close(fd);
    ptr->val = 0;


    int pshared = PTHREAD_PROCESS_SHARED;
    pthread_mutexattr_t attr;

    ret = pthread_mutexattr_init( &attr );
    if(ret != 0){
        fprintf(stderr, "pthread_mutexattr_init() error: %s\n" ,strerror(errno));
        exit(-1);
    }

    ret = pthread_mutexattr_setpshared( &attr, pshared);
    if(ret != 0){
        fprintf(stderr, "pthread_mutexattr_setpshared() error: %s\n" ,strerror(errno));
        exit(-1);
    }

    ret = pthread_mutexattr_getpshared( &attr, &pshared);
    if(ret != 0){
        fprintf(stderr, "pthread_mutexattr_getpshared() error: %s\n" ,strerror(errno));
        exit(-1);
    }
    assert(pshared == PTHREAD_PROCESS_SHARED);

    ret = pthread_mutex_init( &(ptr->mutex), &attr);
    assert(ret == 0);

    ret = pthread_mutexattr_destroy(&attr);
    assert(ret == 0);

    int i;
    signal(SIGCHLD, SIG_IGN);
    pid = fork();
    assert(pid >=0);

    if(pid == 0){
        for(i=0; i<nloop; i++){
            pthread_mutex_lock(&ptr->mutex);
            ptr->val++;
            printf("Child process: %d\n", ptr->val);
            pthread_mutex_unlock(&ptr->mutex);
        }
        return 0 ;
    }
    for(i=0; i<nloop; i++){
        pthread_mutex_lock(&ptr->mutex);
        ptr->val++;
        printf("Main process: %d\n", ptr->val);
        pthread_mutex_unlock(&ptr->mutex);
    }
    return 0;
}

这个代码是一个测试性的代码，有几点需要注意的地方：

1、采用的是pthread_mutex_t mutex 和mmap实现的数据同步和共享
2、mutex属性必须设置为进程共享的
3、open一个文件作为内存映射过后，必须对该问价设置大小，或者是通过fd向该文件写入一些数据，再做mmap映射，不然会出现总线错误。原因应该是open创建的文件大小是0，映射到地址空间过后不可用