的shm_open和ftruncate（）在共享内存编程 [英] shm_open and ftruncate() in Shared Memory Programming

问题描述

我想创建一个共享内存对象，并将其截断为特定的大小。

I want to create a Shared Memory Object and truncate it to a specific size.

SHMSIZE与512定义

SHMSIZE is defined with 512

模式设置与S_IRUSR | S_IWUSR | S_IWGRP | S_IRGRP | S_IWOTH | S_IROTH

MODE is set with S_IRUSR | S_IWUSR | S_IWGRP | S_IRGRP | S_IWOTH | S_IROTH

下面是我的code

char *shm_name = "SharedMemory";    
int fd; 

/* Open an Shared Memory Object for Read-/Write-Access */    
if((fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE) < 0)) {     
     perror("\nshm_open() in Caretaker failed");    
     exit(EXIT_FAILURE);
}

/* Truncate Shared Memory Object to specific size */
if((ftruncate(fd, SHMSIZE) < 0)) {
    perror("\nftruncate() in Caretaker failed");
    exit(EXIT_FAILURE);
}

在调试我看了那的shm_open（）的返回值为0每一次，但我可以看到的/ dev / shm的这个对象。但在执行ftruncate（）返回错误无效的参数每一次。

While Debugging i watched that the return value of shm_open() is 0 every time, but i can see this object in /dev/shm. And while executing ftruncate() it returns every time with error "invalid argument".

为什么fd是0每次和为什么ftruncate不起作用？
我应该怎么办？

Why fd is 0 every time and why ftruncate doesn't work? What should i do?

推荐答案

业务在该语句的顺序是靠不住的：

The order of operations in this statement is wonky:

if((fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE) < 0)) {

您正在分配的shm_open（...）LT的结果; 0 到 FD ，这绝对不是你想要的。

You're assigning the result of shm_open(...) < 0 to fd, which is definitely not what you want.

移动比较​​括号外：

if((fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE)) < 0) {
                                                   ^^^

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