C Embedded Real Time (Coding)

 60 Minutes
 4 Questions


This assessment is designed to evaluate a candidate's ability to write efficient, correct, and hardware-conscious C code within the constraints of embedded and real-time systems. It tests key concepts like bitwise manipulation, memory safety, task scheduling, buffer management, and deadline-aware logic-all of which are fundamental in low-level firmware or bare-metal development environments.
What This Assesses:

- Embedded mindset: working without dynamic memory or OS support
- Strong C fundamentals (types, structs, pointers, array logic)
- Real-time thinking: time budget, task deadlines, cooperative execution
- Defensive coding: buffer limits, return validation
- System-level problem solving: scheduling, resource constraints


Example Question:

Coding
Coding Challenge: Real-Time Scheduler with Deadline Awareness

Problem Description:

You're working on a cooperative task scheduler for a real-time embedded system. Each task can be ready or not ready, and each one reports:
  • How much time it needs to run (duration)
  • Its hard deadline (an absolute timestamp in milliseconds)

Your job is to implement a function that selects the best task to run next, based on these rules:
  • The task must be marked as ready
  • The task must be able to be completed before or at its deadline
  • Among all eligible tasks, choose the one with the earliest deadline.
  • If no task is ready or none can be completed on time, return NULL.

Function Signature

const Task* schedule_next(uint32_t current_time_ms);

current_time_ms: the current system time in milliseconds

Task Descriptor

Each task is represented as a structure:

typedef struct {
  uint8_t id;
  const char* name;
  uint8_t ready;      // 1 = task is ready to run, 0 = not ready
  uint32_t deadline_ms;   // absolute time (ms) when task must be done
  uint32_t duration_ms;   // how long the task takes to run
} Task;

All 4 tasks are stored in a global array:

extern Task task_table[4];

The function return value:
  • Return a pointer to the selected Task
  • Return NULL if no task can be scheduled

Constraints:
  • All times are given in milliseconds
  • The scheduler is non-preemptive (once a task starts, it runs to completion)
  • No dynamic memory is allowed
  • There are exactly 4 tasks in task_table[]

Example

current_time_ms = 1000;

task_table = {
 {0, "SensorRead", 1, 1100, 50},  // OK - earliest deadline
 {1, "Display",  1, 1020, 30},  // X - Cannot finish before deadline; 1000 + 30 > 1020
 {2, "Logger",   1, 1200, 250},  // X Cannot finish before deadline
 {3, "Idle",    0, 1300, 100}  // X Not ready
};

The correct task to run is "SensorRead" (ID = 0), as it's ready, can finish by the deadline, and has the earliest deadline of all valid options.