c - aio_read from OS X 上的文件错误

Question

以下代码:

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <aio.h>
#include <errno.h>

int main (int argc, char const *argv[])
{
  char name[] = "abc";
  int fdes;
  if ((fdes = open(name, O_RDWR | O_CREAT, 0600 )) < 0)
    printf("%d, create file", errno);

  int buffer[] = {0, 1, 2, 3, 4, 5};
  if (write(fdes, &buffer, sizeof(buffer)) == 0){
    printf("writerr\n");
  }

  struct aiocb aio;
  int n = 2;
  while (n--){
    aio.aio_reqprio = 0;
    aio.aio_fildes = fdes;
    aio.aio_offset = sizeof(int);
    aio.aio_sigevent.sigev_notify = SIGEV_NONE; 

    int buffer2;
    aio.aio_buf = &buffer2;
    aio.aio_nbytes = sizeof(buffer2);

    if (aio_read(&aio) != 0){
      printf("%d, readerr\n", errno);
    }else{
      const struct aiocb *aio_l[] = {&aio};
      if (aio_suspend(aio_l, 1, 0) != 0){
         printf("%d, suspenderr\n", errno);
      }else{
        printf("%d\n", *(int *)aio.aio_buf);
      }
    }
  }

  return 0;
}

在 Linux 上工作正常(Ubuntu 9.10，使用 -lrt 编译)，打印

1
1

但在 OS X(10.6.6 和 10.6.5，我已经在两台机器上测试过)上失败了:

1
35, readerr

这可能是由于 OS X 上的某些库错误，还是我做错了什么？

Answer 1

您需要调用aio_return(2)每个异步 I/O 操作恰好一次。根据该手册页上的注释，不这样做会泄漏资源，而且显然也会导致您的问题。在调用 aio_suspend 等待 I/O 完成后，确保调用 aio_return 以获取读取的字节数，例如:

const struct aiocb *aio_l[] = {&aio};
if (aio_suspend(aio_l, 1, 0) != 0)
{
  printf("aio_suspend: %s\n", strerror(errno));
}
else
{
  printf("successfully read %d bytes\n", (int)aio_return(&aio));
  printf("%d\n", *(int *)aio.aio_buf);
}

另请记住 aio_read(2) 中的这些重要说明手册页(强调我的):

The Asynchronous I/O Control Block structure pointed to by aiocbp and the buffer that the aiocbp->aio_buf member of that structure references must remain valid until the operation has completed. For this reason, use of auto (stack) variables for these objects is discouraged.

The asynchronous I/O control buffer aiocbp should be zeroed before the aio_read() call to avoid passing bogus context information to the kernel.