ftruncate不工作的POSIX共享内存在Mac OS X [英] ftruncate not working on POSIX shared memory in Mac OS X

问题描述

我已经写在Mac OS X中的code使用POSIX共享内存，如下所示：

I have written a code on Mac OS X to use POSIX shared memory as shown below:

#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>

int main() {
    int fileHandle = shm_open("TW_ShMem1",O_CREAT|O_RDWR, 0666);

    if(fileHandle==-1) {
       //error.

    } else {
        //Here, it is failing on Mac OS X
        if(-1==ftruncate(fileHandle, 8192)) {
            shm_unlink("TW_ShMem1");
            fileHandle = -1;
        } else {
            return 0;
        }
    }

    return 1;
}

ftruncate Linux上的工作没有任何问题。在Mac OS X，它返回-1和错误号是 EINVAL （如被看见在调试器）。

ftruncate on Linux is working without any problem. On Mac OS X, it is returning -1 and errno is EINVAL (as seen in the debugger).

为什么会失败？这里正在错失？

Why is it failing? What is being missed here?

推荐答案

这看起来像OSX行为 - ftruncate仅在段的初始的创建的工作一次。任何后续调用失败以这种方式。该最早提到我能找到这是一个后苹果的邮件列表。

This looks like OSX behaviour - ftruncate only works once on the initial creation of the segment. Any subsequent calls fail in this manner. The earliest reference I can find to this is a post to the apple mailing list.

如果我把一个 shm_unlink 的的shm_open 的ftruncate都能正常工作了。

If I put an shm_unlink before the shm_open the ftruncate works consistently.

假设你只是想调整共享内存段的一次，你可以包装在ftruncate一个 FSTAT 来确定当前的大小和的情况下调整它的 st_size == 0

assuming that you only want to resize the shared memory segment the once, you could wrap the ftruncate in an fstat to determine the current size and resize it in the case that st_size == 0

例如

struct stat mapstat;
if (-1 != fstat(fileHandle, &mapstat) && mapstat.st_size == 0) {
    ftruncate(fileHandle, 8192);
}

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