Embedded Prep

1. Process and Thread Management in QNX (Overview)

QNX is a real-time, microkernel-based operating system, designed for deterministic and high-reliability environments like automotive, medical, and industrial systems.

In QNX:

• A process is a protected memory space that can have one or more threads of execution.
• The microkernel handles thread-level scheduling, not process-level, making threads the core unit of execution.

QNX uses:

• Preemptive multitasking
• Priority-based scheduling
• POSIX-compliant APIs for thread/process management

2. How Threads are Managed in QNX

In QNX, threads are managed directly by the microkernel, and they are:

• The basic unit of CPU execution
• Associated with a process’s address space
• Managed with thread control blocks (TCBs) in the kernel

Thread States in QNX:

• READY
• RUNNING
• BLOCKED (on I/O, synchronization, or time delay)
• DEAD

Thread Attributes:

• Priority
• Scheduling policy
• Stack size
• CPU affinity

Threads are managed via:

• pthread_create(), pthread_join(), pthread_cancel()
• sched_setscheduler(), sched_getscheduler()
• Kernel-level calls for priority and state management

3. Thread Scheduling Policies in QNX

QNX supports several POSIX-compliant thread scheduling policies:

Policy	Description
SCHED_FIFO	First-In-First-Out; real-time threads of same priority execute in arrival order. No timeslice.
SCHED_RR	Round-Robin; real-time threads with same priority take turns based on a timeslice.
SCHED_OTHER	Default non-real-time policy. Time-sharing, dynamic priority changes.
SCHED_SPORADIC	For periodic tasks with hard deadlines. Maintains CPU usage within a defined budget.

You set scheduling policies with pthread_attr_setschedpolicy() and set thread priorities with pthread_attr_setschedparam().

4. Priority Inheritance and Ceiling Protocols in QNX

QNX provides real-time synchronization protocols to avoid priority inversion — a situation where a low-priority thread holds a resource needed by a high-priority thread.

A. Priority Inheritance Protocol

• When a low-priority thread holds a mutex and a high-priority thread blocks on it, the kernel temporarily raises the low-priority thread’s priority to match the high-priority thread.
• Once the mutex is released, the thread’s priority returns to its original value.

Usage:

pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_INHERIT);
pthread_mutex_t mutex;
pthread_mutex_init(&mutex, &attr);

B. Priority Ceiling Protocol

• Each mutex is assigned a priority ceiling (the highest priority of any thread that will lock it).
• When a thread locks the mutex, its priority is immediately raised to the ceiling.
• Prevents deadlocks and priority inversion.

Usage:

pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_PROTECT);
pthread_mutexattr_setprioceiling(&attr, 20);
pthread_mutex_t mutex;
pthread_mutex_init(&mutex, &attr);

5. Creating and Managing Processes and Threads in QNX

A. Process Creation

• Processes are created using fork(), spawn() family, or exec() functions.

Example using spawn():

#include <spawn.h>
char *args[] = { "/bin/ls", NULL };
spawn(NULL, "/bin/ls", 0, NULL, args, NULL);

B. Thread Creation

• Threads are created using POSIX pthread API:

#include <pthread.h>

void* thread_func(void* arg) {
    // Your thread code
    return NULL;
}

int main() {
    pthread_t tid;
    pthread_create(&tid, NULL, thread_func, NULL);
    pthread_join(tid, NULL);
    return 0;
}

Thread attributes (like priority, stack size) can be set using pthread_attr_t.

C. Thread Priority Management

#include <sched.h>
struct sched_param param;
param.sched_priority = 10;
pthread_setschedparam(tid, SCHED_RR, &param);

6. Difference Between Process and Thread in QNX

Aspect	Process	Thread
Memory Space	Separate for each process	Shared within the same process
Overhead	Higher	Lower
Communication	Needs IPC (Message Passing)	Easier using shared variables
Scheduling	Not scheduled individually	Scheduled by microkernel
Crash Isolation	One process crash won’t affect others	One thread crash can affect others
Creation API	spawn(), fork(), exec()	pthread_create()

Summary

• Threads are the primary schedulable unit in QNX.
• QNX supports real-time scheduling (FIFO, RR, Sporadic).
• Use priority inheritance or ceiling protocols to prevent priority inversion.
• Processes are isolated memory spaces; threads share memory and execute concurrently within a process.
• You create processes using spawn() and threads using pthread_create().

Practical C code examples for QNX that demonstrate:

1. Creating a thread and setting its scheduling policy and priority
2. Using a mutex with priority inheritance protocol
3. Using a mutex with priority ceiling protocol

1. Thread Creation with Scheduling Policy and Priority

This example creates a thread with Round-Robin (SCHED_RR) scheduling and a priority of 10.

#include <pthread.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void* thread_function(void* arg) {
    printf("Thread is running with priority = %d\n", sched_get_priority_max(SCHED_RR));
    return NULL;
}

int main() {
    pthread_t thread;
    pthread_attr_t attr;
    struct sched_param param;

    // Initialize thread attributes
    pthread_attr_init(&attr);

    // Set scheduling policy to Round-Robin
    pthread_attr_setschedpolicy(&attr, SCHED_RR);

    // Set thread priority
    param.sched_priority = 10;
    pthread_attr_setschedparam(&attr, &param);
    pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);

    // Create thread
    if (pthread_create(&thread, &attr, thread_function, NULL) != 0) {
        perror("Failed to create thread");
        return 1;
    }

    pthread_join(thread, NULL);
    return 0;
}

2. Mutex with Priority Inheritance

This ensures that if a lower-priority thread holds a mutex needed by a higher-priority thread, its priority is temporarily elevated.

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

pthread_mutex_t mutex;

void* low_priority_task(void* arg) {
    pthread_mutex_lock(&mutex);
    printf("Low-priority thread acquired the mutex\n");
    sleep(3);  // Simulate long processing
    printf("Low-priority thread releasing the mutex\n");
    pthread_mutex_unlock(&mutex);
    return NULL;
}

void* high_priority_task(void* arg) {
    sleep(1);  // Ensure low-priority locks first
    printf("High-priority thread trying to acquire mutex\n");
    pthread_mutex_lock(&mutex);
    printf("High-priority thread acquired the mutex\n");
    pthread_mutex_unlock(&mutex);
    return NULL;
}

int main() {
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_INHERIT);
    pthread_mutex_init(&mutex, &attr);

    pthread_t low, high;
    pthread_create(&low, NULL, low_priority_task, NULL);
    pthread_create(&high, NULL, high_priority_task, NULL);

    pthread_join(low, NULL);
    pthread_join(high, NULL);
    return 0;
}

3. Mutex with Priority Ceiling

This raises the thread’s priority to the mutex ceiling as soon as it acquires the mutex.

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

pthread_mutex_t ceiling_mutex;

void* task(void* arg) {
    pthread_mutex_lock(&ceiling_mutex);
    printf("Thread entered critical section (priority elevated)\n");
    sleep(2);
    printf("Thread exiting critical section\n");
    pthread_mutex_unlock(&ceiling_mutex);
    return NULL;
}

int main() {
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_PROTECT);
    pthread_mutexattr_setprioceiling(&attr, 15); // Set ceiling

    pthread_mutex_init(&ceiling_mutex, &attr);

    pthread_t t;
    pthread_create(&t, NULL, task, NULL);
    pthread_join(t, NULL);

    return 0;
}

Tips:

• Always use PTHREAD_EXPLICIT_SCHED when assigning custom policies.
• Thread priorities in QNX range typically from 1 (lowest) to 255 (highest).
• Use sched_get_priority_min() and sched_get_priority_max() to verify valid priority ranges.

How to Debug & Verify Thread Priority and Mutex Behavior in QNX

1. Verify Thread Priorities at Runtime

You can verify thread scheduling attributes in two ways:

A. Programmatically (within your code)

Use pthread_getschedparam() to print the thread’s current scheduling policy and priority:

#include <pthread.h>
#include <sched.h>
#include <stdio.h>

void print_thread_info(pthread_t tid) {
    int policy;
    struct sched_param param;

    if (pthread_getschedparam(tid, &policy, &param) == 0) {
        printf("Thread Priority: %d\n", param.sched_priority);
        printf("Scheduling Policy: %s\n",
               policy == SCHED_FIFO ? "SCHED_FIFO" :
               policy == SCHED_RR   ? "SCHED_RR"   :
               policy == SCHED_OTHER ? "SCHED_OTHER" :
               "UNKNOWN");
    } else {
        perror("pthread_getschedparam");
    }
}

Call this inside or right after your thread function starts.

B. Using QNX ps and pidin utilities

Open a terminal or shell and use:

ps -t

Or to inspect details like priority:

pidin -p <PID> tid

This lists all threads under a process and their priority, state, and scheduling policy.

2. Verify Mutex Priority Protocol Behavior

A. Observe Inheritance or Ceiling Effects in Logs

Use printf() in your thread code to log:

• When mutex is acquired/released
• Which thread has the mutex
• Current thread priority before and after

For example, you might observe:

Low-priority thread acquired the mutex
High-priority thread trying to acquire mutex
Low-priority thread's priority temporarily raised!

This would confirm priority inheritance.

B. Use pidin to Check Thread Priority During Blocking

While the high-priority thread is blocked, run:

pidin -p <PID> tid

You should see the low-priority thread’s priority has been temporarily raised (for inheritance), or elevated to the ceiling (for ceiling protocol).

Example:

TID PRI STATE NAME
 1   15  READY  low_thread (priority raised to match ceiling/inheritance)
 2   10  BLOCKED high_thread (waiting for mutex)

3. Helpful Tools for Debugging in QNX

Tool	Use
pidin	View process/thread/mutex state and scheduling
ps	Basic process/thread info
top	Real-time CPU and priority monitoring
Momentics IDE	Graphical debugging and trace (if using QNX SDP)
System Profiler	Shows exact thread execution timelines, blocking, priorities, etc.

Summary

• Use pthread_getschedparam() and pidin to inspect thread priority and policy.
• Log thread actions to confirm correct mutex protocol behavior.
• Watch for temporary priority boosts (inheritance) or immediate elevation (ceiling).
• System Profiler (Momentics) is best for full trace and visualization.