Cross-compilation mental model in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of cross-compiled embedded C code with different endianness

Consider this embedded C code compiled on a host machine (little-endian) but running on a target machine (big-endian). What will be the output when the program prints the integer value stored in memory?

Embedded C

#include <stdio.h>

int main() {
    unsigned int x = 0x12345678;
    unsigned char *p = (unsigned char *)&x;
    printf("%02x %02x %02x %02x\n", p[0], p[1], p[2], p[3]);
    return 0;
}

A 00 00 00 00

B 78 56 34 12

C Error: incompatible types

D 12 34 56 78

💡 Hint

The host endianness does not affect runtime memory layout; consider the target's big-endian storage.

The program executes on the big-endian target. The constant 0x12345678 is stored with most significant byte first: 12 34 56 78. Printing bytes in memory order (p[0] to p[3]) outputs 12 34 56 78. Host endianness is irrelevant at runtime.

🧠 Conceptual

intermediate

Understanding cross-compilation toolchain components

Which component in a cross-compilation toolchain is responsible for converting source code into machine code for the target architecture?

A Cross-compiler

B Target assembler

C Host linker

D Host debugger

💡 Hint

Think about which tool translates code to target machine instructions.

The cross-compiler compiles source code into machine code for the target architecture. The host linker and debugger run on the host, and the target assembler converts assembly code to machine code but is usually part of the cross-compiler toolchain.

🔧 Debug

advanced

Debugging a cross-compiled embedded program with wrong output

You cross-compiled a program for an embedded device. The program reads sensor data but always prints zero. Which of these is the most likely cause?

Embedded C

#include <stdio.h>

volatile int *sensor = (int *)0x40000000;

int main() {
    int value = *sensor;
    printf("Sensor value: %d\n", value);
    return 0;
}

A The variable 'value' is not declared volatile.

B The printf format specifier %d is wrong for int type.

C The sensor address is incorrect for the target device memory map.

D The program lacks a main function.

💡 Hint

Check if the memory address matches the target device's hardware layout.

If the sensor address is wrong, reading from it returns zero or garbage. The format specifier %d is correct for int. Declaring 'value' volatile is not necessary here. The program has a main function.

📝 Syntax

advanced

Identifying syntax error in cross-compiled embedded C code

Which option contains a syntax error that will prevent cross-compilation?

Embedded C

void setup() {
    int x = 10
    if (x > 5) {
        x = x + 1;
    }
}

A Incorrect braces placement in if statement

B Missing semicolon after 'int x = 10'

C Using 'void' as return type in setup function

D Using 'x' without declaration

💡 Hint

Look carefully at the end of each statement.

The line 'int x = 10' is missing a semicolon, causing a syntax error. Using 'void' as return type and braces placement are correct. 'x' is declared properly.

🚀 Application

expert

Determining the number of instructions generated by cross-compilation

Given this simple embedded C function, how many assembly instructions will the cross-compiler likely generate for the return statement on a typical ARM Cortex-M target?

Embedded C

int add_one(int a) {
    return a + 1;
}

A 2 instructions

B 1 instruction

C 3 instructions

D 4 instructions

💡 Hint

Consider loading the argument, adding 1, and returning the result.

Typically, the compiler generates one instruction to add 1 to the input register and one instruction to return (branch). So, about 2 instructions.