一起使用shm_open和mmap有什么用？

问题描述

阅读手册页后，我了解到shm_open和shm_unlink基本上类似于mmap和munmap，不同之处在于shm用于系统V和mmap用于POSIX。由于两者都可以用于共享内存，因此一起使用是否有优势？例如：

此代码

int main( ) {
    
    int size = 1024;
    char* name   = "test";

    int fd  = shm_open(name,O_RDWR|O_CREAT|O_TRUNC,0600 );
    ftruncate(fd,size);
    char *ptr = (char *) mmap(NULL,size,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);

    if (fork()) { // parent
        ftruncate(fd,size); // cut the file to a specific length
        char* msg = (char*)malloc(50*sizeof(char));
        sprintf(msg,"parent process with id %d wrote this in shared memory",getpid()); // copy this in msg
        strcpy((char*)ptr,msg); // copy msg in ptr (the mmap)
        munmap(ptr,size); // parent process no longer has access to this memory space
        shm_unlink(name); // parent process is no longer linked to the space named "test"
    } else {
        sleep(0.00001);
        printf("Inside child process %d :  %s\n",getpid(),ptr);
        munmap(ptr,size);
        exit(0);
    }
}

将输出

Inside child process 5149 :  parent process with id 5148 wrote this in shared memory

如果我删除fd并将其替换为-1并添加标志MAP_ANONYMOUS，

int main( ) {
    
    int size = 1024;
    char* name   = "test";

    char *ptr = (char *) mmap(NULL,MAP_SHARED | MAP_ANONYMOUS,-1,0);

    if (fork() != 0) { // parent
        char* msg = (char*)malloc(50*sizeof(char));
        sprintf(msg,size); // parent process no longer has access to this memory space
    } else {
        sleep(0.00001);
        printf("Inside child process %d :  %s\n",size);
        exit(0);
    }
}

输出不变。那么为什么要使用shm_get？

谢谢

解决方法

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