C Structs Explained: From Basics to Embedded Bit-Fields

C Structs Explained: From Basics to Embedded Bit-Fields

In C, a struct (structure) is a user-defined composite data type that groups variables of different types under a single name.
Structs are foundational in embedded systems, operating systems, and driver development, where they are commonly used to model hardware registers, protocol packets, and complex state objects.

🧱 Defining Struct Variables

#

There are several valid ways to define and instantiate a structure in C.

Define and Instantiate Together

#

struct string
{
    char name[8];
    int age;
    char sex[2];
    char depart[20];
    float wage1, wage2, wage3, wage4, wage5;
} person;   // Defines the struct and creates variable 'person'

This approach is concise and commonly used when the struct type is only needed once.

Define First, Instantiate Later

#

struct string {
    char name[8];
    int age;
    char sex[2];
    char depart[20];
};

struct string person;   // Create variable using the struct template

This is preferred when the structure type will be reused across multiple variables or translation units.

Anonymous Structs

#

Anonymous structs omit the structure name and are useful for one-off use cases:

struct {
    int id;
    char name[16];
} Liming, Liuqi;

These are typically used for local data grouping inside functions.

🔍 Accessing and Assigning Members

#

Struct members are accessed using the dot operator (.).

#include <stdio.h>

int main(void) {
    struct {
        char *name;
        int age;
        char group;
    } stu1;

    stu1.name = "Tom";
    stu1.age = 18;
    stu1.group = 'A';

    printf("%s is %d years old in group %c\n",
           stu1.name, stu1.age, stu1.group);

    return 0;
}

The dot operator works only with actual struct variables, not pointers.

📚 Struct Arrays

#

Struct arrays are ideal for managing collections of related objects, such as students, devices, or packets.

struct {
    char name[8];
    char sex[2];
    int age;
    char addr[40];
} student[40];   // Array of 40 student structures

student[0].age = 18;

Each element in the array is a fully independent struct.

👉 Struct Pointers

#

For large structures or dynamically allocated memory, pointers to structs are preferred.

#include <stdlib.h>
#include <string.h>

struct string {
    char name[8];
    int age;
};

struct string *student;

student = (struct string *)malloc(sizeof(struct string));

strcpy(student->name, "Jack");
student->age = 18;

When accessing members through a pointer, use the arrow operator (->).

student->age is shorthand for (*student).age

🧩 Bit-Fields (位结构)

#

Bit-fields allow precise control over individual bits within a struct. This is extremely common in embedded programming when mapping C structures directly to hardware registers.

struct {
    unsigned incon   : 8;  /* bits 0–7  */
    unsigned txcolor : 4;  /* bits 8–11 */
    unsigned bgcolor : 3;  /* bits 12–14 */
    unsigned blink   : 1;  /* bit 15 */
} ch;

Why Bit-Fields Matter

#

• Efficient memory usage
• Clear mapping to register layouts
• Improved readability vs manual bit masking

⚠️ Note: Bit-field layout and alignment can be compiler- and endian-dependent. Always verify with the target compiler and hardware.

🧾 Using typedef with Structs

#

Using typedef simplifies syntax and improves readability—especially in large codebases.

typedef struct person {
    int age;
    char *name;
} student;

student stu1;   // No need to write 'struct person'

This pattern is widely used in:

• Kernel code
• Device drivers
• Embedded firmware APIs

🎯 Key Takeaways

#

• struct groups heterogeneous data into a single logical unit
• Use arrays and pointers for scalability and performance
• Bit-fields are essential for hardware-level programming
• typedef struct dramatically improves code clarity

A solid understanding of C structs is non-negotiable for anyone working close to the hardware or performance-critical systems.