来自文件 fd 的文件系统 fd

Question

My program opens a file and wants to set O_DIRECT for it. But the program assumes block size of 512. So, I need to check for that size. man 2 open suggests to do it like this ioctl(filesystem_fd, BLKSSZGET, &block_size) .

And my question is: how to get this filesystem_fd from fd of an ordinary file?

Answer 1

You don't need to do that. If you want to know the block size of the filesystem in which your file resides, just look in st_blksize. That is:

#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char **argv)
{
        struct stat sb;
        char *name = argc > 1 ? argv[1] : argv[0];

        if( stat(name, &sb) == -1 ) {
                perror(name);
                return EXIT_FAILURE;
        }

        printf("Block size: %d\n", (int)sb.st_blksize);
        return EXIT_SUCCESS;
}