Anonymous Structures in C

Anonymous Structures in C Language with Examples

In this article, I will discuss Anonymous Structures in C Language with Examples. Please read our previous article discussing Alignment Support in C Language with Examples. At the end of this article, you will understand the following pointers:

  1. What are Anonymous Structures in C Language?
  2. Anonymous Structure in a Struct 
  3. Anonymous Structure in a Union
  4. Configuration Structures Example Using Anonymous Structures in C
  5. Hardware Register Mapping Example Using Anonymous Structures in C
  6. Protocol Data Units Example Using Anonymous Structures in C
  7. Union Type Punning Example Using Anonymous Structures in C
  8. When to Anonymous Structures in C?
  9. Advantages and Disadvantages of Anonymous Structures in C
What are Anonymous Structures in C Language?

Anonymous structures in C allow us to define structures without naming them, which is particularly useful in nested structures or unions, where naming the inner structure isn’t necessary. They were introduced in the C11 standard, enhancing the language’s flexibility and reducing the need for unnecessary naming.

  • Definition: An anonymous structure is a struct definition without a tag or a typedef, usually nested within another struct or union.
  • Access: Members of an anonymous struct can access the parent struct directly, simplifying notation.
  • Use Cases: They are especially useful in organizing complex data structures, such as nested configurations or hardware register mappings, allowing for more straightforward and readable code.
Example: Anonymous Structure in a Struct 
#include <stdio.h>

typedef struct {
    struct {  // Anonymous struct
        int x;
        int y;
    double radius;
} Circle;

int main() {
    Circle c;
    c.x = 10;  // Direct access to 'x' and 'y'
    c.y = 20;
    c.radius = 5.0;

    printf("Circle center: (%d, %d), radius: %.1f\n", c.x, c.y, c.radius);
    return 0;

Output: Circle center: (10, 20), radius: 5.0

In this example, the Circle struct contains an anonymous struct with x and y coordinates. These members can be accessed directly through a Circle variable.

Example: Anonymous Structure in a Union
#include <stdio.h>

typedef union {
    struct {  // Anonymous struct
        int intValue;
        float floatValue;
    char strValue[20];
} Data;

int main() {
    Data d;
    d.intValue = 42;
    d.floatValue = 3.14;

    printf("Integer: %d, Float: %.2f\n", d.intValue, d.floatValue);
    return 0;

Output: Integer: 42, Float: 3.14

Here, the Data union contains an anonymous struct with an int and a float alongside a char array. The anonymous struct allows for accessing the intValue and floatValue without an additional struct name.

Anonymous Structures Real-Time Examples in C

Anonymous structures in C can be particularly useful in real-time systems and embedded programming, where efficiency and direct access to data are crucial. Here are a few practical examples that demonstrate how anonymous structures can be used effectively in real-world scenarios:

Configuration Structures Example Using Anonymous Structures in C

Using anonymous structures in C for defining configuration structures is a neat and organized way to represent various configuration parameters that might be grouped logically. This approach is especially useful in embedded systems, where configuration settings are often numerous and diverse. Here’s an example demonstrating how you might define a configuration structure for a hypothetical device using anonymous structures:

#include <stdbool.h>

typedef struct {
    struct {
        unsigned int baudRate;    // Baud rate for communication
        unsigned int dataBits;    // Number of data bits
        bool parity;              // Parity bit setting
    } serialConfig;

    struct {
        bool isEnabled;           // Is logging enabled?
        unsigned int logLevel;    // Level of logging detail
    } loggingConfig;

    struct {
        unsigned int sampleRate;  // Rate of sampling data
        bool isContinuous;        // Continuous or single-shot sampling
    } sensorConfig;
} DeviceConfig;

int main() {
    DeviceConfig config;

    // Initialize serial configuration
    config.serialConfig.baudRate = 9600;
    config.serialConfig.dataBits = 8;
    config.serialConfig.parity = false;

    // Initialize logging configuration
    config.loggingConfig.isEnabled = true;
    config.loggingConfig.logLevel = 2; // Assuming 2 is some predefined log level

    // Initialize sensor configuration
    config.sensorConfig.sampleRate = 1000; // Sample rate in Hz
    config.sensorConfig.isContinuous = true;

    // Now the config structure is ready to be used
    // configureDevice(&config);

    return 0;

In this example:

  • DeviceConfig is a structure representing the entire configuration for a device.
  • Inside DeviceConfig, there are three anonymous structures: serialConfig, loggingConfig, and sensorConfig. Each of these represents a different aspect of the device configuration.
  • The main function shows an example of how you might initialize this configuration structure. The configuration parameters are set according to the needs of the application.
  • After initializing, the configuration can be passed to a function like configureDevice, which would apply these settings to the device.

This method of structuring configuration data is beneficial as it groups related settings together, making the code more readable and easier to maintain. Also, using anonymous structures helps logically separate configuration aspects without the overhead of defining separate structures for each group.

Hardware Register Mapping Example Using Anonymous Structures in C

Creating a hardware register map using anonymous structures in C is a common technique in embedded system programming. This approach allows you to represent hardware registers as structured data, making your code more readable and maintainable. Here’s an example to illustrate this concept:

typedef struct {
    volatile uint32_t CONTROL; // Control Register
    volatile uint32_t STATUS;  // Status Register
    volatile uint32_t DATA;    // Data Register
} PeripheralRegs;

#define PERIPHERAL ((PeripheralRegs *)0x40000000U) // Base address of the peripheral

int main() {
    // Set a value to the control register

    // Read the status register
    uint32_t status = PERIPHERAL->STATUS;

    // Write to the data register
    PERIPHERAL->DATA = 0x12345678;

    return 0;

In this example:

  • PeripheralRegs is a structure representing a set of registers for a peripheral.
  • Each structure member corresponds to a register (like CONTROL, STATUS, and DATA) and is declared as volatile uint32_t. The volatile keyword tells the compiler that the variable’s value may change at any time without any action being taken by the code the compiler finds nearby. This is essential for hardware registers.
  • The #define PERIPHERAL line defines a macro that casts the base address of the peripheral (here represented as 0x40000000U) to a pointer to PeripheralRegs. This allows for easy access to the registers using the arrow operator (->).

When using this technique, it’s important to ensure that the structure accurately represents the hardware register layout and that the base address is correct for the target hardware. Also, remember that the hardware manufacturer usually provides hardware-specific details like register addresses and bitfields, so it’s essential to consult the relevant documentation or header files.

Protocol Data Units Example Using Anonymous Structures in C

Protocol Data Units (PDUs) are used in network communications to encapsulate the data transferred over the network. In C programming, particularly in the context of network protocols, PDUs can be represented using structures to define their format. When dealing with PDUs, it’s common to encounter various fields like headers, payloads, and trailers, which can be nicely mapped using anonymous structures for clarity and ease of access. Here’s an example to demonstrate how you might define a PDU for a simple network protocol using anonymous structures in C:

#include <stdint.h>

typedef struct {
    struct {
        uint8_t version;   // Version of the protocol
        uint8_t type;      // Type of the message
        uint16_t length;   // Length of the payload
    } header;

    uint8_t payload[256]; // Payload data

    struct {
        uint16_t checksum; // Checksum for error detection
    } trailer;
} ProtocolPDU;

int main() {
    ProtocolPDU pdu;

    // Initialize PDU
    pdu.header.version = 1;
    pdu.header.type = 0x02;
    pdu.header.length = sizeof(pdu.payload);

    // Set payload data
    // ... populate pdu.payload ...

    // Set trailer
    pdu.trailer.checksum = calculateChecksum(pdu.payload, sizeof(pdu.payload));

    // Now pdu can be sent over the network
    // sendPDU(&pdu);

    return 0;

In this example:

  • ProtocolPDU is a structure representing the PDU of a hypothetical network protocol.
  • The header and trailer are defined as anonymous structures within ProtocolPDU. This grouping makes logical sense and aids in readability.
  • The payload is an array of bytes, the size of which depends on the protocol’s specification.
  • In the main function, the PDU is initialized, populated with data, and prepared for transmission. This would include setting the header values, filling the payload, and calculating the checksum for the trailer.

Note that this is a simplified example. In real-world applications, you might need to deal with issues like byte order (endianness), padding, and alignment, particularly when sending data over a network. Also, the actual implementation of functions like calculateChecksum and sendPDU would depend on the specifics of the protocol and the platform you’re working on.

Union Type Punning Example Using Anonymous Structures in C

Type punning using unions in C is a technique to access the same memory location in multiple ways. This is often used for low-level programming tasks such as hardware register manipulation or protocol handling, where you need to access the same data as different types. Here’s an example that uses a union with anonymous structures for type punning:

#include <stdio.h>
#include <stdint.h>

typedef union {
    struct {
        uint8_t byte1;
        uint8_t byte2;
        uint8_t byte3;
        uint8_t byte4;

    uint32_t word;
} Register;

int main() {
    Register reg;

    // Accessing the register as a 32-bit word
    reg.word = 0xAABBCCDD;

    // Accessing individual bytes
    printf("Byte 1: %02X\n", reg.byte1);
    printf("Byte 2: %02X\n", reg.byte2);
    printf("Byte 3: %02X\n", reg.byte3);
    printf("Byte 4: %02X\n", reg.byte4);

    return 0;

In this example:

  • A Register union is defined with two members: a 32-bit word (uint32_t) and an anonymous structure containing four 8-bit bytes (uint8_t).
  • When reg.word is set to 0xAABBCCDD, this 32-bit value is stored in the memory area of the union.
  • Due to the nature of unions, the same memory location can be accessed as either a 32-bit word or as four separate bytes. Thus, when the individual bytes are printed, you’ll see the corresponding parts of the 32-bit value.
  • The ordering of bytes (byte1, byte2, byte3, byte4) depends on the endianness of the machine. The above code assumes little-endian architecture, where the least significant byte is stored first.
  • Type punning with unions is generally considered safe in C, but it’s important to be aware of the endianness and the size of types on your specific platform to avoid unexpected results.
  • This technique should be used judiciously and with a clear understanding of the underlying hardware architecture.
When to Anonymous Structures in C?

Using anonymous structures in C can be advantageous in several scenarios, but it’s essential to consider the specific context and the trade-offs involved. Here are some situations where anonymous structures might be particularly useful:

  • Simplifying Access to Nested Members: When you have nested structures and you frequently need to access deep members, anonymous structures can flatten the access pattern. This is particularly useful in scenarios where the nested hierarchy is an implementation detail rather than something that contributes to understanding the code.
  • Hardware Register Mapping: In low-level programming, such as device driver development, you often need to map hardware registers to structures. Anonymous structures can logically group related registers without introducing additional naming layers.
  • Union with Discriminated Types: When using a union for type-punning (i.e., accessing the same data in multiple ways), anonymous structures can be used for more convenient access to different data views.
  • Configuration Structures: For complex configurations where different settings are closely related, anonymous structures can make the configuration more readable and easier to initialize.
  • Reducing Namespace Pollution: In cases where you have a lot of small structures that are only relevant within the context of a larger structure, anonymous structures can keep the global or local namespace cleaner.
Advantages and Disadvantages of Anonymous Structures in C

Anonymous structures in C, introduced in the C11 standard, offer advantages and disadvantages. Their use can make code more concise and reduce namespace clutter, but there are also considerations regarding readability and compatibility.

Advantages Of Anonymous Structures in C
  • Simplicity and Conciseness: Anonymous structures simplify the declaration of nested structures by removing the need for a separate named struct. This can make the code more concise and easier to write.
  • Direct Member Access: Members of anonymous structures can be accessed directly without needing to use the name of the inner structure, which can simplify code access patterns and improve readability in certain contexts.
  • Reduced Namespace Pollution: By not requiring names for inner structures, anonymous structures help reduce namespace clutter. This is especially beneficial in large codebases where managing names can be challenging.
  • Convenience in Union Usage: In unions, anonymous structures provide a way to group related data and access these members directly, which can be more intuitive than named structures in some cases.
Disadvantages Of Anonymous Structures in C
  • Compatibility Issues: Anonymous structures are a part of the C11 standard. If you work with compilers or systems that do not fully support C11, you may encounter compatibility issues.
  • Potential for Confusion: Direct access to members of anonymous structures can lead to confusion, especially in large structures or when multiple anonymous structures are used. It can be unclear which structure a member belongs to, affecting code readability.
  • Debugging Challenges: Some debuggers might have difficulty displaying members of anonymous structures or may display them less intuitively, complicating the debugging process.
  • Limitations in Design: Using anonymous structures can sometimes lead to design limitations. For instance, you cannot use forward declarations of an anonymous structure, which can restrict their use in header files and API design.

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

Leave a Reply

Your email address will not be published. Required fields are marked *