Strict Aliasing and Alignment: Avoiding Hidden Embedded C Bugs

Strict aliasing and memory alignment are two important C programming rules that help compilers and processors access data correctly.

Strict aliasing determines which pointer types can legally access an existing object’s stored value.

Memory alignment determines where an object can be placed in memory so that its address satisfies the alignment requirements of its data type.

Violating these rules can cause undefined behaviour, incorrect calculations, unexpected compiler optimizations, or processor exceptions.

For example, accessing an integer object through an incompatible float * pointer can violate strict aliasing. Similarly, accessing a 32-bit value through an address that does not satisfy its alignment requirement can cause problems on certain embedded processors.

In simple words: Strict aliasing is about accessing data using the correct type, while alignment is about accessing data from a suitable memory address.

Introduction

Have you ever written Embedded C code that works perfectly during debugging but produces unexpected results after compilation with optimization enabled?

Have you experienced a firmware crash after changing a pointer, modifying a structure, or accessing a memory buffer?

These problems can sometimes be connected to two important concepts:

• Strict aliasing.
• Memory alignment.

Many beginners learn pointers, arrays, structures, and typecasting without understanding the rules that control how C programs access memory.

This becomes especially important in embedded systems because firmware communicates directly with hardware and often operates with limited memory and processor resources.

For example, an embedded engineer might work with:

• Sensor data buffers.
• Communication packets.
• Hardware registers.
• DMA memory buffers.
• ARM microcontroller memory.
• Network protocol structures.
• Firmware data structures.

Incorrect pointer operations in these areas can introduce difficult-to-find bugs.

Understanding strict aliasing and alignment helps developers write more predictable, portable, and reliable Embedded C programs.

Let’s understand both concepts with practical examples.

1. What Is Strict Aliasing in C?

Strict aliasing is a rule in the C programming language that controls how an object can be accessed through different types of expressions.

In simple terms, you cannot freely use any pointer type to read or modify an object.

The C standard defines which types are permitted to access an object’s stored value.

For example, consider:

#include <stdio.h>

int main(void)

{

int number = 100;

int *ptr = &number;

printf(“%d\n”, *ptr);

return 0;

}

This is valid because the pointer type is compatible with the object type.

Here:

• number is an integer object.
• ptr is an integer pointer.
• *ptr accesses the integer object.

Now consider a different example.

int number = 100;

float *ptr = (float *)&number;

float result = *ptr;

This code converts an integer object’s address into a float * and accesses the object through that incompatible type.

The access violates the strict aliasing rules and results in undefined behaviour.

The important point is that a pointer cast does not automatically change the actual type or representation of the object.

The object is still an integer.

2. Why Does Strict Aliasing Exist?

You might wonder why C has this rule.

The main reason is compiler optimization.

Compilers use information about data types to understand how a program accesses memory.

If two pointers have incompatible types, the compiler can sometimes assume that they do not refer to the same object.

This allows the compiler to optimize instructions.

For example, it may:

• Remove unnecessary memory reads.
• Reuse previously loaded values.
• Reorder certain operations.
• Combine instructions.
• Improve execution efficiency.

These optimizations are useful in embedded systems because developers often need efficient firmware.

However, when code violates strict aliasing rules, the compiler may make assumptions that do not match the programmer’s intention.

As a result, the firmware may behave unexpectedly.

3. Understanding a Strict Aliasing Violation with an Example

Consider the following code:

#include <stdio.h>

int main(void)

{

int value = 10;

float *float_ptr = (float *)&value;

*float_ptr = 5.5f;

printf(“%d\n”, value);

return 0;

}

A beginner might expect the program to modify the integer variable.

But this is not a valid way to convert a floating-point value into an integer.

The program accesses an integer object through an incompatible floating-point pointer.

That violates strict aliasing rules.

The result is undefined behaviour.

Important: Undefined behaviour does not mean the program must crash. It means the C standard does not require any particular result.

The program might appear to work, display an unexpected value, or behave differently after optimization.

This is why engineers should never depend on such pointer conversions.

4. What Is Memory Alignment in Embedded C?

Memory alignment means placing data at an address that satisfies the alignment requirement of its type.

Different data types can have different alignment requirements.

For example, on a common 32-bit embedded platform:

Data Type

Example Size

Illustrative Alignment

char

1 byte

1 byte

uint16_t

2 bytes

2 bytes

uint32_t

4 bytes

4 bytes

uint64_t

8 bytes

Platform-dependent

These are illustrative common requirements, not universal rules.

The actual alignment requirements depend on the compiler, target architecture, and data type.

Simple example

Imagine a 32-bit integer requires a four-byte aligned address.

These addresses satisfy that requirement:

0x20000000

0x20000004

0x20000008

0x2000000C

These addresses do not:

0x20000001

0x20000002

0x20000003

A processor or compiler may handle unaligned accesses differently depending on the target architecture and instruction.

5. What Happens When Memory Is Not Properly Aligned?

Unaligned memory access can create different problems.

Problem 1: Processor exception

Some processors or instructions cannot perform particular unaligned accesses.

The processor may generate an alignment fault or exception.

Problem 2: Unexpected performance

Some processors support unaligned accesses but may require additional memory operations.

This can affect execution performance.

Problem 3: Incorrect assumptions in firmware

Developers sometimes assume that every processor handles unaligned memory in the same way.

That assumption is dangerous.

Problem 4: Hardware-specific restrictions

Certain peripheral registers, DMA engines, and memory regions may have stricter access requirements than ordinary RAM.

Therefore, engineers must check the target hardware documentation.

6. Strict Aliasing vs Memory Alignment

These concepts are related to memory access, but they solve different problems.

Feature

Strict Aliasing

Memory Alignment

Main purpose

Controls valid object access types

Controls valid object addresses

Main concern

Pointer type

Memory address

Common mistake

Incompatible pointer access

Misaligned pointer dereference

Possible result

Undefined behaviour

Undefined behaviour or hardware fault

Common solution

Use compatible types or memcpy

Ensure correct alignment

Important in

Compiler optimization

Processor memory access

A program can satisfy alignment requirements but still violate strict aliasing.

Similarly, a program can use compatible pointer types but still have an invalid alignment problem.

Both must be considered during Embedded C development.

7. What Is Type Punning in C?

Type punning means interpreting the same underlying bytes as different data types.

It is commonly used in low-level programming, communication protocols, and embedded firmware.

For example, a developer may want to inspect the binary representation of a floating-point number.

However, not every type-punning technique is valid in standard C.

Consider this unsafe approach:

float temperature = 25.5f;

uint32_t *ptr = (uint32_t *)&temperature;

uint32_t bits = *ptr;

This accesses a floating-point object through an incompatible integer pointer type.

It can violate strict aliasing.

Safer method: Use memcpy()

#include <stdint.h>

#include <string.h>

float temperature = 25.5f;

uint32_t bits;

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memcpy(&bits, &temperature, sizeof(bits));

This copies the object representation into an integer object.

It avoids accessing the float through an incompatible integer lvalue.

However, the exact interpretation of the resulting bits depends on the floating-point representation and platform.

Also, ensure the source and destination sizes are appropriate for the intended operation.

This is a widely useful technique when working with binary data.

8. How to Avoid Strict Aliasing Problems

Follow these practices when writing Embedded C.

Method 1: Use compatible pointer types

Use a pointer type that is compatible with the object being accessed.

int number = 100;

int *ptr = &number;

*ptr = 200;

This is a straightforward and valid approach.

Method 2: Use unsigned char for byte access

C allows an object’s representation to be inspected or copied through character types.

For example:

#include <stdint.h>

uint32_t value = 0x12345678U;

unsigned char *bytes = (unsigned char *)&value;

unsigned char first_byte = bytes[0];

This is a valid way to inspect an object’s byte representation.

However, the actual byte value depends on the target’s byte ordering.

Method 3: Use memcpy() for type conversion

When you need to copy an object’s representation between different types, use memcpy().

#include <stdint.h>

#include <string.h>

float input = 10.5f;

uint32_t output;

memcpy(&output, &input, sizeof(output));

This avoids an incompatible pointer dereference.

Method 4: Understand union-based type punning

C supports accessing the stored value of a union through its members, with specific rules.

Example:

union Data

{

uint32_t integer;

float decimal;

};

union Data data;

data.integer = 0x3F800000U;

float result = data.decimal;

In C, reading a different union member reinterprets the stored representation. The resulting value is implementation-dependent and may be a trap representation for some types.

Do not assume this is a portable numerical conversion.

For predictable representation copying, memcpy() is often the clearer choice.

9. How to Avoid Memory Alignment Problems

Method 1: Use properly declared variables

Let the compiler allocate ordinary variables according to their type alignment requirements.

uint32_t sensor_data = 100;

Method 2: Check pointer alignment

C provides _Alignof in C11 and later.

You can use it to inspect alignment requirements.

#include <stdint.h>

#include <stdalign.h>

size_t alignment = _Alignof(uint32_t);

For an actual pointer, you can inspect its address using uintptr_t where supported:

#include <stdint.h>

uintptr_t address = (uintptr_t)ptr;

if ((address % _Alignof(uint32_t)) == 0U)

{

/* Address satisfies uint32_t alignment */

}

This checks the alignment of the address.

It does not prove that a valid uint32_t object exists there or that accessing it is otherwise permitted.

Method 3: Avoid unsafe pointer casting

Do not assume that converting a byte pointer into a larger integer pointer makes the address properly aligned.

For example:

unsigned char buffer[8];

uint32_t *ptr = (uint32_t *)&buffer[1];

This is unsafe because the resulting address may not satisfy the alignment requirement of uint32_t.

The cast itself can produce undefined behaviour if the converted pointer is not correctly aligned for the target type.

Method 4: Use memcpy() to read unaligned data

For a byte buffer containing external data, memcpy() is a safer way to copy bytes into an appropriately aligned local object.

#include <stdint.h>

#include <string.h>

unsigned char buffer[8] = {

0x78, 0x56, 0x34, 0x12,

0x00, 0x00, 0x00, 0x00

};

uint32_t value;

memcpy(&value, &buffer[1], sizeof(value));

This example copies four bytes starting at offset 1.

The destination variable is properly aligned according to its declaration.

However, the resulting numerical value depends on the target’s byte ordering.

The memcpy() approach avoids directly dereferencing a potentially unaligned uint32_t *.

10. Why Are Strict Aliasing and Alignment Important in ARM Microcontrollers?

ARM-based microcontrollers are widely used in:

• Automotive electronics.
• Industrial automation.
• IoT devices.
• Medical equipment.
• Consumer electronics.
• Robotics.

Developers working with ARM Cortex-M processors must understand memory access requirements.

Different ARM architectures and instruction sets have different alignment behaviours.

For example, certain ARMv7 and later configurations support unaligned access for some load and store instructions.

However, this does not mean every unaligned access is supported.

Some instructions require natural alignment, and some memory regions or system configurations impose additional restrictions.

For example:

• Exclusive load/store instructions have alignment requirements.
• Doubleword operations may have stricter requirements.
• DMA transfers can have hardware-specific alignment restrictions.
• Peripheral registers may require specific access widths.

Always check the technical reference manual for the exact processor.

11. Common Strict Aliasing and Alignment Mistakes

Mistake

Why It Is Dangerous

Better Practice

Casting int pointer to float pointer

May violate strict aliasing

Use memcpy

Reading packed data through an ordinary pointer

May cause alignment issues

Copy into aligned storage

Assuming all ARM CPUs support unaligned access

Behaviour varies

Check architecture documentation

Ignoring compiler optimization

Undefined behaviour may become visible

Test optimized builds

Accessing structures through incompatible pointers

May violate aliasing

Use compatible types

Ignoring DMA alignment

May violate hardware requirements

Follow DMA documentation

Assuming pointer casting changes object type

It does not

Understand object representation

12. Practical Learning Exercise

You can understand these concepts through a simple Embedded C exercise.

Exercise 1: Understand object addresses

Create an integer variable.

Print its address.

Check whether the address satisfies its alignment requirement.

Exercise 2: Understand byte representation

Create a 32-bit integer.

Access its bytes through an unsigned character pointer.

Observe the byte ordering.

Exercise 3: Practise memcpy()

Copy a floating-point object’s representation into an integer object.

Compare the resulting bits using a debugger.

Exercise 4: Investigate compiler behaviour

Compile a test program with different optimization levels.

Compare the generated assembly and output.

Do not rely on a program with undefined behaviour to produce a predictable result.

Exercise 5: Test on an ARM development board

Use a suitable debugger to inspect memory addresses and investigate alignment behaviour supported by the target.

Conclusion

Strict aliasing and memory alignment are important concepts for writing reliable Embedded C firmware.

Strict aliasing controls how different pointer types can access objects, while memory alignment ensures that data is accessed from addresses satisfying the required alignment rules.

Ignoring these concepts can introduce hidden bugs, compiler optimization problems, unexpected processor exceptions, and firmware reliability issues.

Embedded engineers should understand pointers, object representations, memory addresses, compiler behaviour, and ARM alignment requirements before working on low-level firmware.

The most important lesson is simple: Never assume that a pointer cast automatically makes an invalid memory access safe.

To strengthen your Embedded C, ARM programming, memory management, and firmware debugging knowledge, explore the technical learning resources at Embedded Hash.

Frequently Asked Questions

1. What is strict aliasing in C?

Strict aliasing is a C language rule that determines which types of expressions can legally access an object’s stored value.

2. What happens when strict aliasing is violated?

Violating strict aliasing results in undefined behaviour. The compiler may make optimizations based on assumptions that the program has broken.

3. What is memory alignment in Embedded C?

Memory alignment means placing an object at an address that satisfies the alignment requirements of its data type.

4. Why is memory alignment important in ARM processors?

Some ARM instructions and memory regions require particular alignment. Incorrect access can result in additional memory operations, incorrect behaviour, or an alignment exception.

5. What is the difference between strict aliasing and alignment?

Strict aliasing concerns the type used to access an object. Alignment concerns whether the object’s address satisfies the required alignment.

6. How can I avoid strict aliasing problems?

Use compatible pointer types, character pointers for byte access, and memcpy() when copying object representations between different types.

7. Can memcpy() solve unaligned memory access problems?

Yes. Copying bytes from an unaligned source buffer into a properly declared local object avoids directly dereferencing an incorrectly aligned typed pointer. The copy length and buffer boundaries must still be valid.

8. Does pointer casting change the original data type?

No. A pointer cast changes the pointer’s type, not the actual declared type or stored representation of the object.

9. Are unaligned accesses always invalid on ARM?

No. Support depends on the processor architecture, instruction, configuration, and memory region. Some operations support unaligned access while others require alignment.

10. Why should embedded engineers learn strict aliasing?

It helps developers avoid undefined behaviour, understand compiler optimizations, and write reliable firmware for microcontrollers and embedded processors.