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Editorial Team
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Asked: 2026-05-27T13:31:21+00:00

In the code given below: #include <stdio.h> #include <fcntl.h> #include <unistd.h> #include <stdlib.h> #include

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In the code given below:

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/disk.h>

int main(int argc, char **argv)
{
    int fd = open("/dev/disk0", O_RDONLY, 0);

    ...
}

fd has -1 for physical hard drives but it works fine for a USB/flash drive. Why is this happening?

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1 Answer

  1. Editorial Team

    This is almost certainly a permissions issue. Giving ordinary users any access to raw disks is a security hole (the user could circumvent all file permissions and read anything they want on the disk), so no OS gives raw disk access to normal users.

    In the example below, disk0 is my internal hard drive and disk3 is a USB flash drive. In OS X, even “administrators” aren’t part of the operator group that would allow raw disk access to fixed hard drives.


    pip$ ls -ltr /dev/disk*
    brw-r----- 1 root operator 14, 3 Dec 12 10:14 /dev/disk0s3
    brw-r----- 1 root operator 14, 2 Dec 12 10:14 /dev/disk0s2
    brw-r----- 1 root operator 14, 1 Dec 12 10:14 /dev/disk0s1
    brw-r----- 1 root operator 14, 0 Dec 12 10:14 /dev/disk0
    br--r----- 1 root operator 14, 4 Dec 14 15:15 /dev/disk1
    brw-r----- 1 root operator 14, 7 Dec 14 15:15 /dev/disk2s2
    br--r----- 1 root operator 14, 6 Dec 14 15:15 /dev/disk2s1
    brw-r----- 1 root operator 14, 5 Dec 14 15:15 /dev/disk2
    brw-r----- 1 pip operator 14, 9 Dec 14 15:18 /dev/disk3s1
    brw-r----- 1 pip operator 14, 8 Dec 14 15:18 /dev/disk3

    If your app really needs raw disk access you’ll either need to run it as root with sudo, add yourself to the operator group, or use the OS X APIs for privilege escalation.

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