Guide: Reading Linux Directories

Everything in Linux is a file. This statement says that the Linux OS treats every entry in a file system (regular file, directory, block device, char device, link, UNIX socket) as a file. This unified approach simplifies file handling, allowing a single interface to interact with various types of entries. Let’s see how this works in practice:

1. Navigate to guides/reading-linux-dirs/support/ and checkout dir_ops.c. This code creates a directory dir, if it does not exists, and attempts to open it the same way we would open a regular file. Compile and run the code.

   student@os:~/.../reading-linux-dirs/support$ ./dir_ops
   12:45:34 FATAL dir_ops.c:17: fopen: Is a directory
   

   The error message is crystal clear: we cannot use fopen() on directories. So the FILE structure is unsuited for directories. Therefore, this handler is not generic enough for a regular Linux filesystem, and we have to use a lower-level function.

   Quiz - What syscall does fopen() use?

2. Now that we know that fopen() relies openat(), let’s try using open(), which wraps openat() but offers a simpler interface.

   Inspect, compile and run the code dir_ops_syscalls.c.

   student@os:~/...reading-linux-dirs/support$ ./dir_ops_syscalls
   Directory file descriptor is: 3
   

   This output proves that the open() syscall is capable of also handling directories, so it’s closer to what we want.

   Note: that it is rather uncommon to use open() for directories. Most of the time, opendir() is used instead.

In conclusion, the key difference between fopen() and open() is in the type of handler they return. The FILE structure from fopen() is suited only for regular files, while the file descriptor returned by open() is more flexible. The differences between the two handlers are explored in the file descriptors section.