What is the purpose and best usage scenarios of the const keyword in C language?

const Keyword Purpose:

Constant Variables

c
const int MAX_SIZE = 100;
// MAX_SIZE value cannot be modified
// MAX_SIZE = 200;  // Compilation error

Pointers and const

c
// Pointer to constant (content cannot be modified)
const int *ptr1 = &value;
// *ptr1 = 10;  // Compilation error
ptr1 = &other;  // Legal

// Constant pointer (address cannot be modified)
int *const ptr2 = &value;
*ptr2 = 10;     // Legal
// ptr2 = &other;  // Compilation error

// Constant pointer to constant
const int *const ptr3 = &value;
// *ptr3 = 10;  // Compilation error
// ptr3 = &other;  // Compilation error

Function Parameters

c
// Prevent function from modifying parameters
void print_string(const char *str) {
    printf("%s\n", str);
    // str[0] = 'X';  // Compilation error
}

// Return constant pointer
const char* get_version() {
    return "1.0.0";
}

Structure Members

c
struct Point {
    const int x;
    const int y;
};

struct Point p = {10, 20};
// p.x = 30;  // Compilation error

Best Usage Scenarios:

Function Parameter Protection

c
// Don't modify input data
size_t string_length(const char *str) {
    size_t len = 0;
    while (str[len] != '\0') len++;
    return len;
}

// Don't modify array contents
int array_sum(const int *arr, size_t size) {
    int sum = 0;
    for (size_t i = 0; i < size; i++) {
        sum += arr[i];
    }
    return sum;
}

Global Constants

c
// Define in header file
extern const int CONFIG_MAX_CONNECTIONS;
extern const char* CONFIG_LOG_FILE;

// Implement in source file
const int CONFIG_MAX_CONNECTIONS = 100;
const char* CONFIG_LOG_FILE = "app.log";

Enum Replacement

c
// Use const instead of macro definitions
const int ERROR_NONE = 0;
const int ERROR_INVALID_PARAM = -1;
const int ERROR_OUT_OF_MEMORY = -2;

Class Constants (C++ style)

c
struct Config {
    const int timeout;
    const int max_retries;
};

struct Config config = {
    .timeout = 30,
    .max_retries = 3
};

const vs #define Comparison:

Type Safety

c
// const - Has type checking
const int BUFFER_SIZE = 1024;

// #define - No type checking
#define BUFFER_SIZE 1024

Debugging Support

c
// const - Debugger can view
const int value = 100;

// #define - Debugger cannot view
#define value 100

Scope Control

c
// const - Follows scope rules
void function() {
    const int local = 10;
}

// #define - Globally effective
#define local 10

Important Considerations:

const Initialization

c
// Must initialize
const int value;  // Error
const int value = 10;  // Correct

const and Pointers

c
// Distinguish const position
const int *p1;      // *p1 is const
int const *p2;      // *p2 is const (same as above)
int *const p3;      // p3 is const
const int *const p4; // Both are const

const and Arrays

c
// Array elements cannot be modified
const int arr[] = {1, 2, 3};
// arr[0] = 10;  // Compilation error

const and Function Return Values

c
// Return const pointer to prevent modification
const char* get_string() {
    return "Hello";
}

// Caller cannot modify returned string
const char* str = get_string();
// str[0] = 'X';  // Compilation error