In this lesson, you will learn how to interact with hardware peripherals from user space by treating them as standard files within the Linux filesystem.
The Unix Philosophy: Everything is a File
In an Embedded Linux environment, the kernel abstracts hardware complexity through the Virtual Filesystem (VFS). For character devices—peripherals that stream data byte-by-byte, like UARTs, sensors, or GPIO controllers—the kernel exposes an interface located under the /dev directory.
When you write to or read from these "device files," the kernel intercepts those system calls and translates them into specific driver operations. You don't need to know the underlying memory-mapped I/O addresses or handle complex register bit-masking in your application; the driver handles that heavy lifting.
Communicating via read() and write()
Because these devices appear as files, you interact with them using standard POSIX system calls. Opening /dev/my_sensor returns a file descriptor, which you then use just as you would with a text file on a disk.
Consider a scenario where you are reading temperature data from a sensor driver:
#include <fcntl.h>
#include <unistd.h>
int fd = open("/dev/temp_sensor", O_RDONLY);
if (fd >= 0) {
char buffer[8];
// This call triggers the driver's read function
read(fd, buffer, sizeof(buffer));
close(fd);
}
When read() is called, the kernel suspends your user-space process and switches context to the kernel-space driver. The driver then communicates with the physical hardware (perhaps via I2C or SPI), retrieves the data, copies it into your buffer, and returns control to your application.
Why Use Character Device Files?
This abstraction is powerful because it decouples your application logic from the hardware implementation. If you replace your current sensor with a different model, you simply update the kernel module or Device Tree configuration. As long as the new driver exposes the same character device interface, your user-space code remains completely unchanged. This layering is the cornerstone of portability in professional embedded systems.
Key Takeaway: User-space applications interface with hardware by performing standard file I/O operations on device nodes in /dev, allowing the kernel to abstract the complexities of hardware-specific register communication.