get physical block size of a device

Question

In the below code I'm trying to get the physical block size of a device via file/directory path residing in the file system mount point of that device. I am supposed to get the same info as the command sudo blockdev --getpbsz /dev/sdc, but the ioctl call is getting failed with error message Inappropriate ioctl for device.

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fs.h>
#include <sys/ioctl.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

int main() {
    const int fd = open("/tmp", O_DIRECTORY | O_RDONLY);
    if (fd == -1) {
        printf("error %s\n", strerror(errno));
        return EXIT_FAILURE;
    }

    unsigned long num_blocks = 0;
    if (ioctl(fd, BLKPBSZGET, &num_blocks) == -1) {
        printf("error %s\n", strerror(errno));
        return EXIT_FAILURE;
    }
    printf("physical block size: %lu", num_blocks);

    close(fd);

    return EXIT_SUCCESS;
}

Output

error Inappropriate ioctl for device 

Answer 1

The ioctl call BLKPBSZGET is meant to be used on block device files for exemple on /dev/sdc, not on a file or directory residing in a filesystem /tmp/

Determine the block device associated with the filesystem where your target directory resides with df and get the filesystem

$ df /tmp/
Filesystem      1K-blocks     Used Available Use% Mounted on
/dev/sdc       1055762868 26105084 975954312   3% /

So your code should be like that:

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fs.h>
#include <sys/ioctl.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

int main() {
    const int fd = open("/dev/sdc", O_RDONLY);
    if (fd == -1) {
        printf("Error: %s\n", strerror(errno));
        return EXIT_FAILURE;
    }

    unsigned int block_size = 0;
    if (ioctl(fd, BLKPBSZGET, &block_size) == -1) {
        printf("ioctl error: %s\n", strerror(errno));
        close(fd);
        return EXIT_FAILURE;
    }

    printf("Physical block size: %u bytes\n", block_size);

    close(fd);
    return EXIT_SUCCESS;
}

And give me the write output:

$ gcc main.c
$ ./a.out
Error: Permission denied
$ sudo !!
sudo ./a.out
Physical block size: 4096 bytes

Oups, I missed to give you my sources (https://forums.raspberrypi.com/viewtopic.php?t=263949)