Segfault When Setting Up Multiple Callbacks While GPIO Events Are Firing
Introduction
The Raspberry Pi is a popular single-board computer that has been widely adopted for various applications, including robotics, home automation, and IoT projects. The GPIO (General Purpose Input/Output) pins on the Raspberry Pi provide a way to interact with the physical world, making it an ideal platform for projects that require real-time input and output. However, when working with GPIO events, developers may encounter issues such as segfaults, which can be challenging to debug.
Background
In this article, we will explore a specific issue that was encountered while setting up multiple callbacks for GPIO events on a Raspberry Pi Zero 2W. The issue was caused by a segfault in the pigpiod library, which is a C library that provides a way to interact with the GPIO pins on the Raspberry Pi. We will provide a step-by-step guide on how to reproduce the issue and discuss possible solutions.
Reproducing the Issue
To reproduce the issue, follow these steps:
Step 1: Start I2C Communication
Start some intensive I2C communication using the hardware chip and without using pigpiod. This will generate lots of events on pins 2 & 3.
Step 2: Compile the C Program
Compile the C program below:
#include <pigpio.h>
void callback1(int gpio, int level, uint32_t tick) {
printf("Callback 1: GPIO %d, Level %d, Tick %d\n", gpio, level, tick);
}
void callback2(int gpio, int level, uint32_t tick) {
printf("Callback 2: GPIO %d, Level %d, Tick %d\n", gpio, level, tick);
}
int main() {
if (gpioInitialise() < 0) {
printf("Failed to initialize GPIO\n");
return 1;
}
gpioSetMode(2, PI_INPUT);
gpioSetMode(3, PI_INPUT);
gpioWatchdogStart(2, 1000);
gpioWatchdogStart(3, 1000);
gpioSetAlertFunc(2, callback1);
gpioSetAlertFunc(3, callback2);
while (true) {
if (!gpioRead(2)) {
printf("GPIO 2 is low\n");
} else {
printf("GPIO 2 is high\n");
}
if (!gpioRead(3)) {
printf("GPIO 3 is low\n");
} else {
printf("GPIO 3 is high\n");
}
sleep(1);
}
gpioTerminate();
return 0;
}
Step 3: Run the Loop
Run the loop:
while true; do if ! ./test-gpio-callbacks; then break; fi; done
On our system, this reliably segfaults within about 20 iterations.
Analysis
The segfault is caused by the fact that the gpioWatchdogStart function is called multiple times in the loop, which can lead to a race condition. When the gpioWatchdogStart function is called, it sets up a watchdog timer that will trigger a callback function when the specified time period has elapsed. However, if the gpioWatchdogStart function is called multiple times in quick succession, the watchdog timers may overlap, causing the callback functions to be called multiple times simultaneously. This can lead to a segfault.
Solution
To fix the issue, we can use a single watchdog timer and call the callback functions only once. We can also use a mutex to synchronize access to the GPIO pins and prevent the watchdog timers from overlapping.
Here is the modified C program:
#include <pigpio.h>
#include <pthread.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void callback1(int gpio, int level, uint32_t tick) {
printf("Callback 1: GPIO %d, Level %d, Tick %d\n", gpio, level, tick);
}
void callback2(int gpio, int level, uint32_t tick) {
printf("Callback 2: GPIO %d, Level %d, Tick %d\n", gpio, level, tick);
}
int main() {
if (gpioInitialise() < 0) {
printf("Failed to initialize GPIO\n");
return 1;
}
gpioSetMode(2, PI_INPUT);
gpioSetMode(3, PI_INPUT);
pthread_mutex_lock(&mutex);
gpioWatchdogStart(2, 1000);
gpioWatchdogStart(3, 1000);
pthread_mutex_unlock(&mutex);
gpioSetAlertFunc(2, callback1);
gpioSetAlertFunc(3, callback2);
while (true) {
if (!gpioRead(2)) {
printf("GPIO 2 is low\n");
} else {
printf("GPIO 2 is high\n");
}
if (!gpioRead(3)) {
printf("GPIO 3 is low\n");
} else {
printf("GPIO 3 is high\n");
}
sleep(1);
}
gpioTerminate();
return 0;
}
Conclusion
Introduction
In our previous article, we explored a specific issue that was encountered while setting up multiple callbacks for GPIO events on a Raspberry Pi Zero 2W. The issue was caused by a segfault in the pigpiod library, which was triggered by a race condition between the gpioWatchdogStart function and the callback functions. In this article, we will provide a Q&A section to help developers understand the issue and its solutions.
Q: What is a segfault?
A segfault, also known as a segmentation fault, is a type of error that occurs when a program attempts to access a memory location that it is not allowed to access. This can happen when a program tries to read or write to a memory location that is outside the bounds of the program's memory space.
Q: What is a race condition?
A race condition is a situation where two or more threads or processes are competing for access to a shared resource, and the outcome depends on the order in which they access the resource. In the case of the segfault, the gpioWatchdogStart function and the callback functions are competing for access to the GPIO pins, and the outcome depends on the order in which they access the pins.
Q: How can I prevent a segfault when setting up multiple callbacks for GPIO events?
To prevent a segfault when setting up multiple callbacks for GPIO events, you can use a single watchdog timer and call the callback functions only once. You can also use a mutex to synchronize access to the GPIO pins and prevent the watchdog timers from overlapping.
Q: What is a mutex?
A mutex, short for mutual exclusion, is a synchronization primitive that allows only one thread or process to access a shared resource at a time. By using a mutex, you can prevent multiple threads or processes from accessing the GPIO pins simultaneously, which can help prevent a segfault.
Q: How can I use a mutex to prevent a segfault?
To use a mutex to prevent a segfault, you can create a mutex object and lock it before calling the gpioWatchdogStart function. You can then unlock the mutex after calling the gpioWatchdogStart function. This will ensure that only one thread or process can access the GPIO pins at a time, which can help prevent a segfault.
Q: What is the difference between a mutex and a semaphore?
A mutex and a semaphore are both synchronization primitives that can be used to prevent multiple threads or processes from accessing a shared resource simultaneously. However, a mutex is a more lightweight synchronization primitive that is designed for use in a single process, while a semaphore is a more heavyweight synchronization primitive that is designed for use in multiple processes.
Q: How can I debug a segfault?
To debug a segfault, you can use a debugger such as gdb to examine the program's memory and stack trace. You can also use a tool such as valgrind to detect memory leaks and other memory-related issues.
Q: What are some best practices for preventing segfaults when working with GPIO events?
Some best practices for preventing segfaults when working with GPIO events include:
- Using a single watchdog timer and calling the callback functions only once
- Using a mutex to synchronize access to the GPIO pins
- Avoiding the use of multiple threads or processes to access the GPIO pins simultaneously
- Using a debugger such as
gdbto examine the program's memory and stack trace - Using a tool such as
valgrindto detect memory leaks and other memory-related issues
Conclusion
In this article, we provided a Q&A section to help developers understand the issue of segfaults when setting up multiple callbacks for GPIO events on a Raspberry Pi Zero 2W. We discussed the causes of segfaults, including race conditions and memory-related issues, and provided solutions such as using a single watchdog timer and a mutex to synchronize access to the GPIO pins. By following these best practices, developers can help prevent segfaults and ensure that their GPIO events are handled correctly.