Static Assertions in C

Static Assertions in C Language with Examples

In this article, I will discuss Static Assertions in C Language with Examples. Please read our previous article discussing Generic Selection in C Language with Examples. At the end of this article, you will understand the following pointers:

  1. What are Static Assertions in C Language?
  2. Data Type Size Validation Example Using Static Assertions in C
  3. Ensuring Correct Struct Size Example Using Static Assertions in C
  4. Validating Alignment Requirements Example Using Static Assertions in C
  5. Verifying Relationships Between Constants Example Using Static Assertions in C
  6. Ensuring Enum Values Example Using Static Assertions in C
  7. When to Use Static Assertions in C?
  8. Advantages and Disadvantages of Using Static Assertions in C
What are Static Assertions in C Language?

Static assertions in C, introduced in the C11 standard, allow us to perform compile-time checks of certain conditions. These assertions are useful for validating assumptions about program elements such as the size of data types, struct layouts, or other compile-time constants. This can significantly improve the robustness of your code.

Syntax: _Static_assert(expression, message);

  • expression: The compiler evaluates this constant expression at compile time. If the expression evaluates to 0 (false), the compilation will fail, and the compiler will display the specified message.
  • message: A string literal that serves as the error message displayed when the assertion fails.
Data Type Size Validation Example Using Static Assertions in C

Data type size validation is a common use case for static assertions in C. By ensuring that certain data types have the expected sizes, you can avoid potential bugs and ensure consistency across different platforms or compilers. Here’s an example to illustrate how you can use static assertions for this purpose:

#include <assert.h>
#include <stdint.h>

int main() {
    // Validate the size of basic data types
    static_assert(sizeof(int) == 4, "Expected int to be 4 bytes");
    static_assert(sizeof(short) == 2, "Expected short to be 2 bytes");
    static_assert(sizeof(long) >= 4, "Expected long to be at least 4 bytes");

    // Validate the size of fixed-width integers from stdint.h
    static_assert(sizeof(uint8_t) == 1, "Expected uint8_t to be 1 byte");
    static_assert(sizeof(uint16_t) == 2, "Expected uint16_t to be 2 bytes");
    static_assert(sizeof(uint32_t) == 4, "Expected uint32_t to be 4 bytes");
    static_assert(sizeof(uint64_t) == 8, "Expected uint64_t to be 8 bytes");

    // Additional checks for specific platform requirements
    static_assert(sizeof(float) == 4, "Expected float to be 4 bytes");
    static_assert(sizeof(double) == 8, "Expected double to be 8 bytes");

    return 0;
  • Basic Data Types: The size of int, short, and long are checked. These sizes can vary between platforms and compilers, so asserting their sizes is important, especially if your program depends on these sizes for functionality or memory layout.
  • Fixed-Width Integers: For fixed-width integer types like uint8_t, uint16_t, uint32_t, and uint64_t, defined in stdint.h, static assertions ensure that they have the correct sizes. These types are commonly used in programs that require exact-width integer arithmetic.
  • Floating-Point Types: Sizes of float and double are checked. Like integer types, the size of floating-point types can also vary, especially on different hardware architectures.
Ensuring Correct Struct Size Example Using Static Assertions in C

Ensuring the correct size of a struct is a common and important use of static assertions in C, especially when dealing with low-level programming such as hardware interfacing, binary file formats, or network protocols. Here’s an example that illustrates how to use static assertions to ensure the correct size of a struct:

#include <assert.h>
#include <stdint.h>

// Define a struct
struct ExampleStruct {
    uint32_t field1;
    uint16_t field2;
    uint8_t field3;
    // potentially some padding here

int main() {
    // Static assertion to ensure the struct is of the expected size
    static_assert(sizeof(struct ExampleStruct) == 8, "Size of ExampleStruct is not 8 bytes");

    return 0;

Struct Definition: struct ExampleStruct is defined with a uint32_t (4 bytes), a uint16_t (2 bytes), and a uint8_t (1 byte). Depending on the platform and compiler, padding might be added to align the data structure in memory.

Static Assertion:

  • The static_assert checks that the size of struct ExampleStruct is exactly 8 bytes. This size is based on the sum of the sizes of its fields, but it’s important to remember that the compiler might add padding for alignment purposes, which can affect the total size of the structure.
  • If the actual size of the struct differs from what is expected (8 bytes in this case), the compilation will fail with the message “Size of ExampleStruct is not 8 bytes”. This clearly indicates that the struct’s memory layout does not match the expectations, which could be crucial in certain applications.
Validating Alignment Requirements Example Using Static Assertions in C

Validating alignment requirements of structures or specific members within structures is crucial in C programming, especially in systems programming, embedded systems, and interfacing with hardware where specific memory alignment is often necessary. Static assertions can be effectively used to ensure that these alignment requirements are met. Here’s an example demonstrating how to use static assertions for validating alignment:

#include <assert.h>
#include <stddef.h>
#include <stdint.h>

struct MyStruct {
    uint32_t a;
    uint16_t b;
    uint8_t c;

int main() {
    // Ensure that 'struct MyStruct' is aligned to a 4-byte boundary
    static_assert(alignof(struct MyStruct) == 4, "struct MyStruct must be aligned to 4 bytes");

    // Validate the alignment of specific members within the struct
    static_assert(offsetof(struct MyStruct, a) % 4 == 0, "Member 'a' is not 4-byte aligned");
    static_assert(offsetof(struct MyStruct, b) % 2 == 0, "Member 'b' is not 2-byte aligned");

    return 0;

Struct Definition: struct MyStruct is defined with members uint32_t a, uint16_t b, and uint8_t c. These members’ sizes and alignment requirements might lead to automatic padding inserted by the compiler.

Static Assertions for Alignment:

  • The first static_assert checks the alignment of the entire struct. The alignof operator ensures that struct MyStruct is aligned to a 4-byte boundary, which is typical for uint32_t on most platforms.
  • The following static_assert statements check the alignment of individual members within the struct. They use offsetof to ensure that members a and b are properly aligned according to their respective data types (4-byte alignment for uint32_t and 2-byte alignment for uint16_t).
Verifying Relationships Between Constants Example Using Static Assertions in C

Verifying relationships between constants using static assertions is a powerful technique in C programming. It ensures that certain invariants or conditions related to constant values are maintained, which is particularly useful in scenarios where the relationship between different constants is crucial for the program’s correctness. Here’s an example that demonstrates how to use static assertions for this purpose:

#include <assert.h>

#define BUFFER_SIZE 1024
#define MAX_ITEMS 100

// Ensure that ITEM_SIZE is a whole number and not zero
static_assert(ITEM_SIZE > 0, "ITEM_SIZE must be greater than 0");
static_assert(BUFFER_SIZE % MAX_ITEMS == 0, "BUFFER_SIZE must be a multiple of MAX_ITEMS");

#define TIMEOUT_MS 5000
#define POLL_INTERVAL_MS 100

// Ensure that POLL_INTERVAL_MS divides TIMEOUT_MS evenly
static_assert(TIMEOUT_MS % POLL_INTERVAL_MS == 0, "TIMEOUT_MS must be a multiple of POLL_INTERVAL_MS");

int main() {
    // Your code here
    return 0;

Buffer and Item Size Checks:

  • BUFFER_SIZE is defined as the total size of a buffer, and MAX_ITEMS is the maximum number of items that can fit into this buffer. ITEM_SIZE is calculated based on these two constants.
  • The first static_assert ensures that ITEM_SIZE is greater than 0, which is important to avoid division by zero or having items with no size.
  • The second static_assert checks that BUFFER_SIZE is an exact multiple of MAX_ITEMS, ensuring that the buffer can be evenly divided into items without any leftover space.

Timeout and Poll Interval Checks:

  • TIMEOUT_MS and POLL_INTERVAL_MS are constants representing a timeout value and a polling interval, respectively.
  • The static_assert ensures that TIMEOUT_MS is a multiple of POLL_INTERVAL_MS. This is important for scenarios where you might want to poll regularly within a timeout period without any remainder time.
Ensuring Enum Values Example Using Static Assertions in C

Ensuring specific values or properties of enumeration (enum) constants in C is another practical use of static assertions. This can be particularly important when enums are used for indexing arrays, interfacing with hardware, or communicating with external systems where specific values are expected. Here’s an example that demonstrates how to use static assertions to ensure certain properties of enum values:

#include <assert.h>

// Define an enum for error codes
typedef enum {
    ERROR_NONE = 0,
} ErrorCode;

// Static assertions to ensure specific properties of the enum
static_assert(ERROR_NONE == 0, "ERROR_NONE must be 0");
static_assert(ERROR_UNKNOWN == 3, "ERROR_UNKNOWN must be 3");

// Example of using the enum
int main() {
    ErrorCode code = ERROR_FILE_NOT_FOUND;

    // Your code handling different error codes
    return 0;

Enum Definition: The enum ErrorCode is defined with specific error codes. The first value, ERROR_NONE, is explicitly set to 0, and the other values are automatically assigned incrementally.

Static Assertions:

  • The first static_assert checks that ERROR_NONE is 0. This is often a convention in error handling, where 0 represents no error.
  • The second static_assert verifies that ERROR_UNKNOWN is 3. This kind of check is useful if the code’s logic relies on ERROR_UNKNOWN being the last error code or having a specific value.
When to Use Static Assertions in C?

Static assertions in C are an invaluable tool for ensuring certain conditions at compile time, which can prevent bugs and maintain consistency across platforms and compilers. Here are some scenarios where static assertions are particularly useful:

Data Type Size Validation
  • To ensure portability: If your code assumes a specific size for standard data types (like int, char, etc.), a static assertion can verify this assumption. This is crucial for code that might run on different architectures where the size of these types can vary.
  • Struct Size and Alignment: To guarantee that a struct has the expected size and alignment, which might be critical for binary compatibility or interfacing with hardware or other languages.
Checking Relationships Between Constants
  • Validating Constant Expressions: Ensuring that certain defined constants in your program maintain expected relationships, like MAX_SIZE being greater than MIN_SIZE.
  • Array Size Checks: When you have arrays whose size depends on constants or other compile-time expressions, static assertions can ensure they are within expected bounds.
Compile-Time Logic Validation
  • Feature Checks: Static assertions can validate these features if your program depends on certain compiler or environment features (like a specific C standard version).
  • Conditional Compilation: In conjunction with preprocessor directives, static assertions can validate assumptions made for different compilation conditions or platform-specific code.
Interface and API Constraints
  • Ensuring API Consistency: If you’re developing a library, static assertions can be used to ensure that the client code meets certain compile-time conditions.
  • Version Compatibility: To check if the right versions of libraries or headers are being used, especially when backward compatibility is a concern.
Debugging and Development Aids
  • Sanity Checks: During development, static assertions can act as sanity checks, verifying that certain refactorings or changes haven’t broken basic assumptions in the code.
  • Documenting Assumptions: They serve as a form of documentation, clearly stating non-obvious assumptions or requirements in the code.

Advantages and Disadvantages of Using Static Assertions in C

Using static assertions in C programming, introduced with the C11 standard, offers several advantages and disadvantages. Here’s a detailed look at both:

Advantages of Using Static Assertions in C
Compile-Time Error Checking:
  • Early Detection of Errors: Static assertions allow for detecting errors at compile time rather than at runtime. This early detection can save time and resources since issues are caught before the program runs.
  • Ensures Assumptions: They help ensure that certain assumptions about the program (like size of data types, structure alignment, etc.) are correct. This is particularly useful in cross-platform development, where these assumptions might vary.
Code Documentation and Readability:
  • Self-Documenting Code: Static assertions can serve as a form of documentation, clearly stating the programmer’s assumptions or requirements right in the code.
  • Improved Readability: They make the code more readable and understandable by explicitly stating certain conditions that must hold true for the code to work correctly.
Helpful in Cross-Platform Development:
  • Platform-Specific Checks: Static assertions can be used to ensure that the code adheres to certain platform-specific constraints or requirements.
Disadvantages of Using Static Assertions in C
Limited to Compile-Time Checks:
  • Cannot Assert Runtime Conditions: Static assertions are limited to compile-time checks. They cannot be used to assert conditions that are only known at runtime.
  • Restricted Scope: They can only be used to check conditions that can be evaluated during compilation, such as sizes of types, structure offsets, or values of compile-time constants.
Potential Overuse or Misuse:
  • Clutter and Overhead: Overuse of static assertions can make the code harder to read, especially if used excessively for conditions that are unlikely to fail.
  • False Confidence: Relying too much on static assertions might give a false sense of security, leading developers to pay less attention to runtime checks and validations.
Error Messages Can Be Cryptic:
  • Complexity in Understanding Errors: The error messages generated by static assertions can be cryptic or hard to understand, especially for less experienced programmers.
Version Compatibility:
  • Not Available in Older Standards: Static assertions are not available in C standards prior to C11, limiting their use in legacy codebases or with compilers that do not support the latest standards.

In the next article, I will discuss Alignment Support in C Language. In this article, I explain Static Assertions in C Language with Examples. I hope you enjoy this Static Assertions in C Language with Examples article. I would like to have your feedback. Please post your feedback, questions, or comments about this article.

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