array

1. the array in C language actually is a sorted collection ,which can be indexed, every element only belong to each one of C Data types including compound type, Built-in type,even array type.

// 声明一个3行4列的二维数组
int matrix1[3][4]; 

// 初始化二维数组
int matrix1[2][3] = {
    {1, 2, 3},  // 第0行
    {4, 5, 6}   // 第1行
};

// 也可以省略第一维大小，由编译器推断
int matrix2[][3] = {{1,2,3}, {4,5,6}, {7,8,9}};
int val = matrix2[1][2]; // 访问第1行第2列的元素（值为6）
matrix2[0][1] = 10;      // 修改第0行第1列的元素

to save elements is in a row-by-row for multi-dimensional array.

 matrix[0][0], matrix[0][1], matrix[0][2]

matrix[1][0], matrix[1][1], matrix[1][2]
....

the array can be parameter of a function shown as follows.

void printMatrix(int arr[][3], int rows) {
    for(int i=0; i<rows; i++) {
        for(int j=0; j<3; j++) {
            printf("%d ", arr[i][j]);
        }
        printf("\n");
    }
}

the pointer to an array used to manipulate elements of the array more flexibly .

int matrix[2][3] = {
    {1, 2, 3},  // 第0行
    {4, 5, 6}   // 第1行
};
int (*ptr)[3] = matrix; // ptr是指向含有3个int元素的数组的指针
printf("%d", ptr[1][2]); // 等同于matrix[1][2]

by the way,the multi-dementional array can be dynamically created.

int rows = 3, cols = 4;
int **arr = (int **)malloc(rows * sizeof(int *));
for(int i=0; i<rows; i++) {
    arr[i] = (int *)malloc(cols * sizeof(int));
}

getchar() and putchar() in C

1. getchar() receive a single character from standard input.

int getchar(void);

2. putchar() output a single character to standard output

int putchar(int c);

#include <stdio.h>

int main() {
    int c;
    
    printf("Type some text (press Ctrl+D/Ctrl+Z to end):\n");
    
    // Read characters until EOF
    while ((c = getchar()) != EOF) {
        // Write each character back
        putchar(c);
    }
    
    return 0;
}

The ctype.h Header in C

this header file create a lot of functions to handle character.

    isalpha(c) - Checks if c is an alphabetic letter (a-z, A-Z)

    isupper(c) - Checks if c is an uppercase letter

    islower(c) - Checks if c is a lowercase letter
    
    isdigit(c) - Checks if c is a decimal digit (0-9)

    isxdigit(c) - Checks if c is a hexadecimal digit (0-9, a-f, A-F)
    
    isspace(c) - Checks if c is a whitespace character (space, \t, \n, etc.)
  

#include <stdio.h>
#include <ctype.h>

int main() {
    char c;
    
    printf("Enter a character: ");
    c = getchar();
    
    if (isalpha(c)) {
        printf("'%c' is a letter\n", c);
        printf("Uppercase: %c\n", toupper(c));
        printf("Lowercase: %c\n", tolower(c));
    }
    else if (isdigit(c)) {
        printf("'%c' is a digit\n", c);
    }
    else if (isspace(c)) {
        printf("You entered whitespace\n");
    }
    else {
        printf("'%c' is something else\n", c);
    }
    
    return 0;
}

logical operations

1. there are three logical operator in C language that are && which means and,|| which means or,! which means not.

• Logical AND (&&)

Returns true (1) only if both operands are true

Example: (a > b) && (c != d)

• Logical OR (||)

Returns true (1) if either operand is true

Example: (x == y) || (z < 0)

• Logical NOT (!)

Inverts the truth value (true becomes false, false becomes true)

Example: !(flag)

static

1. static variable,which are initialized only once, survived in the memory regardless of the scope of variable .

 #include <stdio.h>

void counter() {
    static int count = 0; // Static variable
    count++;
    printf("Count: %d\n", count);
}

int main() {
    counter(); // Output: Count: 1
    counter(); // Output: Count: 2
    counter(); // Output: Count: 3
    return 0;
}

2. static embellishes a global variable or function and restricts its visibility to the current file.

// File: file1.c
static int hiddenVar = 42; // Only visible in file1.c

void printHidden() {
    printf("%d\n", hiddenVar); // Works
}

// File: file2.c
extern int hiddenVar; // ERROR: Cannot access hiddenVar from file1.c

// File: utils.c
static void helper() { // Only visible in utils.c
    printf("Helper function\n");
}

void publicFunc() {
    helper(); // OK
}

// File: main.c
extern void helper(); // ERROR: Cannot call helper() from another file

malloc

1. malloc asks for memory allocation from OS,free releases the part of memory which was distributed by malloc.

#include <stdint.h>  // 使用标准整数类型

// 定义消息类型（可根据需求扩展）
typedef enum {
    MSG_TYPE_UNKNOWN = 0,
    MSG_TYPE_COMMAND,
    MSG_TYPE_DATA,
    MSG_TYPE_ACK,
    MSG_TYPE_ERROR
} MessageType;

// 定义消息头（固定部分）
typedef struct {
    uint32_t type;      // 消息类型（如命令、数据、ACK等）
    uint32_t length;    // 消息体的长度（字节数）
    uint64_t timestamp; // 可选：时间戳
    uint32_t checksum;  // 可选：校验和（用于数据完整性验证）
} MessageHeader;

// 定义完整消息结构（头 + 变长数据）
typedef struct {
    MessageHeader header;  // 消息头
    char payload[];       // 柔性数组（C99），存储实际数据（长度由 header.length 决定）
} Message;

#include <string.h>
#include <stdio.h>

// 动态创建消息（需手动管理内存）
Message* create_message(MessageType type, const char* data, uint32_t data_len) {
    // 分配内存（消息头 + 实际数据）
    Message* msg = malloc(sizeof(MessageHeader) + data_len);
    if (!msg) return NULL;

    // 填充消息头
    msg->header.type = type;
    msg->header.length = data_len;
    msg->header.timestamp = time(NULL); // 假设有 time() 函数
    msg->header.checksum = 0;           // 简化的示例，实际应计算校验和

    // 拷贝数据到 payload
    memcpy(msg->payload, data, data_len);
    return msg;
}

// 示例：发送一条命令消息
int main() {
    const char* cmd = "START_SERVER";
    Message* msg = create_message(MSG_TYPE_COMMAND, cmd, strlen(cmd) + 1);

    if (msg) {
        printf("Message created: type=%d, length=%d\n", 
               msg->header.type, msg->header.length);
        free(msg); // 释放内存
    }
    return 0;
}

how to handle file

1. to manipulate files involves opening, reading, writing, and closing files.

File Modes
Mode	Description
"r"	Read (file must exist)
"w"	Write (creates/overwrites file)
"a"	Append (creates/appends to file)
"r+"	Read/Write (file must exist)
"w+"	Read/Write (creates/overwrites)
"a+"	Read/Append (creates/appends)
"b"	Binary mode (e.g., "rb", "wb")

Function	Usage
fopen()	Open a file
fclose()	Close a file
fprintf()	Write formatted text
fscanf()	Read formatted text
fgetc()	Read a character
fputc()	Write a character
fgets()	Read a line
fputs()	Write a string
fread()	Read binary data
fwrite()	Write binary data
feof()	Check end-of-file
ferror()	Check file errors

#include <stdio.h>

typedef struct {
    int id;
    char name[50];
    float score;
} Student;

int main() {
    // Write binary data
    Student s1 = {1, "Alice", 95.5};
    FILE *file = fopen("students.dat", "wb");
    if (file == NULL) {
        perror("Error opening file");
        return 1;
    }
    fwrite(&s1, sizeof(Student), 1, file);
    fclose(file);

    // Read binary data
    Student s2;
    file = fopen("students.dat", "rb");
    fread(&s2, sizeof(Student), 1, file);
    printf("ID: %d, Name: %s, Score: %.2f\n", s2.id, s2.name, s2.score);
    fclose(file);

    return 0;
}

references

deepseek