Sysfspwm Library

A C++ library for the Linux kernel's PWM device class.

What It's For

You might have used the PWM interface on a single-board computer running Linux, such as a Raspberry Pi or other embedded system. If so, you might have seen the Linux Kernel's sysfs PWM interface

echo -n 0 > /sys/class/pwm/pwmchip0/export
echo -n 1 > /sys/class/pwm/pwmchip0/pwm0/enable
echo -n 1000000 > /sys/class/pwm/pwmchip0/pwm0/period 
echo -n 500000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle

This is a quick little C++ library for that built-in kernel interface, so you don't have to mess with writing strings to magical files. Instead, you can do things like:

vector<PWMChip> pwmchip_list = PWMChip::getSystemPWMChips();
PWMChip mypwmchip = pwmchip_list[0];
PWM mypwm = mypwmchip.export_pwm(0);

or

mypwm.set_frequency_and_ratio(1000, 0.5);
mypwm.set_enabled(true);

How it works

Under the hood, this is using libudevpp, a C++ wrapper for libudev, which is itself a C library for manipulating entires in /sys.

This uses CMake as the build system, so you'll need it installed, as well as libudev. libudevpp will get downloaded automatically. These dependancies might require you to do something like:

sudo apt install libudev-dev cmake build-essential

You can try this out by cloning, building, and running the example program:

git clone https://github.com/zeroping/sysfspwm.git
cd sysfspwm
mkdir build
cd build
cmake ../
make

Then, to list available PWM chips on a single board computer:

./example/pwm_control

And to set up a 2 KHz square wave on the 0th PWM on the 0th PWM chip:

./example/pwm_control /sys/class/pwm/pwmchip0 0 2000 0.5

Work From Here

There are a few rough edges. One known annoyance is that the /sys/class/pwm/pwmchip0/export sysfs attribute can't be written under libudevpp right now, so that's currently accomplished with a file stream.