larsks / readdir-vs-stat

Overview

This repository contains two programs that will tell you the "type" (regular file, socket, block/char device, etc) of a given path:

• get_filetype_readdir -- use the readdir(3) function to determine the file type.
• get_filetype_stat -- use the stat(2) function to determine the file type.

They generally result in the same output:

$ ./get_filetype_readdir /tmp /run/docker.sock /etc/motd /dev/null
path /tmp type directory
path /run/docker.sock type socket
path /etc/motd type file
path /dev/null type character device

$ ./get_filetype_stat /tmp /run/docker.sock /etc/motd
path /tmp type directory
path /run/docker.sock type socket
path /etc/motd type file
path /dev/null type character device

Compiling the code

Run make to build the two programs:

$ make
cc    -c -o get_filetype_readdir.o get_filetype_readdir.c
cc -o get_filetype_readdir get_filetype_readdir.o
cc    -c -o get_filetype_stat.o get_filetype_stat.c
cc -o get_filetype_stat get_filetype_stat.o

The problem

In the overview, the two programs consistently produced the same results. If we run these commands in a rootless Podman container, they produce different output. Using get_filetype_stat, we see:

$ podman run -it --rm -v $PWD:/code -w /code fedora:latest
[root@d7096e4278a7 code]# ./get_filetype_stat /dev/null
path /dev/null type character device

But using get_filetype_readdir, we see:

$ podman run -it --rm -v $PWD:/code -w /code fedora:latest
[root@052dd40d2fd9 code]# ./get_filetype_readdir /dev/null
path /dev/null type file

What's going on?