Why open() system call does not open swapfile in Ubuntu?

Question

I have a question about open() system call. I am writing a C code, in which I need to open all files from a provided directory in read only mode. Before the open call I check if the user has read permission to this file. Although when the provided directory is /, a file there swapfile cannot open and system call fails, while user has read permission.

Can someone tell me why this happens or how to correct it?

As you can see below user has read access, so the file should open.

-rw-------   1 root root 2147483648 Οκτ  22 18:58 swapfile

The code which is responsible for the error:

int fd = open(path, O_RDONLY);
if (fd == -1)
{
    perror("open");
    exit(1);
}        

The error:

open: Permission denied

Answer 1

You are trying to open the file as a non-root user.

The permissions on the file allows the owner of the file to open it for reading (and writing), and denies any sort of access to the group and to "others". This is what your ls -l output shows.

The owner of the file is the root user, and you are executing your code as user which is not root. This is why you get a "permission denied" error.