Updated: April 19, 2023

Guard a critical section in an interrupt handler


#include <sys/neutrino.h>

void InterruptLock( intrspin_t* spinlock );


The spinlock (a variable shared between the interrupt handler and a thread) to use.
Note: If spinlock isn't a static variable, you must initialize it by calling:
memset( spinlock, 0, sizeof( *spinlock ) );

before using it with InterruptLock().



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The InterruptLock() function guards a critical section by locking the specified spinlock. You can call this function from a thread or from an interrupt handler. Before calling this function:

If you don't do these things, the thread might SIGSEGV when it calls InterruptLock().

This function tries to acquire the spinlock (a variable shared between the interrupt handler and a thread) while interrupts are disabled. The code spins in a tight loop until the lock is acquired. It's important to release the lock as soon as possible. Typically, this is a few lines of code without any loops:

InterruptLock( &spinner );

/* ... critical section */

InterruptUnlock( &spinner );

InterruptLock() solves a common need in many realtime systems to protect access to shared data structures between an interrupt handler and the thread that owns the handler. The traditional POSIX primitives used between threads aren't available for use by an interrupt handler.

The InterruptLock() and InterruptUnlock() functions work on single-processor or multiprocessor machines.

  • You should disable interrupts for as little time as possible (i.e., the minimum time you need to access or deal with the hardware). Failure to do so may result in increased interrupt latency and the inability to meet realtime deadlines.
  • Some kernel calls and library routines reenable interrupts. Masked interrupts aren't affected.
  • This function doesn't support nesting. If a specific thread calls InterruptLock() multiple times with different spinlocks and then calls InterruptUnlock() the same number of times and in reverse order with the locks, the first InterruptUnlock() call reenables interrupts and the critical section is no longer protected.


QNX Neutrino

Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes