C标准库

一. 标准库 (Standard Library)

核心标准库头文件

#include <stdio.h>      // 输入输出
#include <stdlib.h>     // 内存管理、系统函数
#include <string.h>     // 字符串处理
#include <ctype.h>      // 字符处理
#include <math.h>       // 数学函数
#include <time.h>       // 时间日期
#include <assert.h>     // 断言调试

1. 输入输出库 (stdio.h)

#include <stdio.h>

// 文件操作
void file_operations() {
    FILE *file = fopen("test.txt", "w");
    if (file) {
        fprintf(file, "Hello World!\n");
        fclose(file);
    }
    
    // 读取文件
    file = fopen("test.txt", "r");
    if (file) {
        char buffer[100];
        while (fgets(buffer, sizeof(buffer), file)) {
            printf("Read: %s", buffer);
        }
        fclose(file);
    }
}

// 格式化输入输出
void formatted_io() {
    int age = 25;
    double salary = 50000.50;
    char name[] = "John";
    
    printf("Name: %s, Age: %d, Salary: %.2f\n", name, age, salary);
    
    // 字符串格式化
    char buffer[100];
    sprintf(buffer, "Formatted: %s-%d", name, age);
    printf("%s\n", buffer);
    
    // 从字符串读取
    char input[] = "100 200.5";
    int a;
    double b;
    sscanf(input, "%d %lf", &a, &b);
    printf("Parsed: %d, %.1f\n", a, b);
}

2. 字符串处理库 (string.h)

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

void string_operations() {
    char str1[50] = "Hello";
    char str2[50] = "World";
    char str3[50];
    
    // 字符串长度
    printf("Length: %zu\n", strlen(str1));
    
    // 字符串复制
    strcpy(str3, str1);
    printf("After copy: %s\n", str3);
    
    // 字符串连接
    strcat(str3, " ");
    strcat(str3, str2);
    printf("After concat: %s\n", str3);
    
    // 字符串比较
    int result = strcmp(str1, str2);
    printf("Compare result: %d\n", result);
    
    // 字符串查找
    char *pos = strstr(str3, "World");
    if (pos) {
        printf("Found 'World' at position: %ld\n", pos - str3);
    }
    
    // 内存操作
    char buffer[20];
    memset(buffer, 'A', 10);
    buffer[10] = '\0';
    printf("After memset: %s\n", buffer);
    
    // 内存复制
    char src[] = "Source";
    char dest[20];
    memcpy(dest, src, strlen(src) + 1);
    printf("After memcpy: %s\n", dest);
}

3. 内存管理库 (stdlib.h)

#include <stdlib.h>
#include <stdio.h>

void memory_operations() {
    // 动态内存分配
    int *numbers = (int*)malloc(10 * sizeof(int));
    if (numbers == NULL) {
        perror("malloc failed");
        return;
    }
    
    // 初始化内存
    for (int i = 0; i < 10; i++) {
        numbers[i] = i * i;
    }
    
    // 重新分配内存
    numbers = (int*)realloc(numbers, 20 * sizeof(int));
    if (numbers) {
        for (int i = 10; i < 20; i++) {
            numbers[i] = i * i;
        }
    }
    
    // 打印结果
    for (int i = 0; i < 20; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    
    // 释放内存
    free(numbers);
    
    // 其他实用函数
    int a = 5, b = 10;
    printf("abs(-10) = %d\n", abs(-10));
    printf("max(%d, %d) = %d\n", a, b, (a > b) ? a : b);
    
    // 随机数
    srand(time(NULL));  // 设置随机种子
    printf("Random number: %d\n", rand() % 100);
}

4. 数学库 (math.h)

#include <math.h>
#include <stdio.h>

void math_operations() {
    double x = 2.0, y = 3.0;
    
    printf("pow(%.1f, %.1f) = %.2f\n", x, y, pow(x, y));
    printf("sqrt(%.1f) = %.2f\n", x, sqrt(x));
    printf("sin(%.1f) = %.2f\n", x, sin(x));
    printf("cos(%.1f) = %.2f\n", x, cos(x));
    printf("log(%.1f) = %.2f\n", x, log(x));
    printf("exp(%.1f) = %.2f\n", x, exp(x));
    
    // 三角函数
    double angle = 45.0 * M_PI / 180.0;  // 转换为弧度
    printf("sin(45°) = %.2f\n", sin(angle));
    printf("cos(45°) = %.2f\n", cos(angle));
    
    // 取整函数
    double num = 3.7;
    printf("ceil(%.1f) = %.1f\n", num, ceil(num));
    printf("floor(%.1f) = %.1f\n", num, floor(num));
    printf("round(%.1f) = %.1f\n", num, round(num));
}

5. 时间和日期库 (time.h)

#include <time.h>
#include <stdio.h>

void time_operations() {
    // 获取当前时间
    time_t current_time = time(NULL);
    printf("Current timestamp: %ld\n", current_time);
    
    // 转换为可读格式
    char *time_str = ctime(&current_time);
    printf("Current time: %s", time_str);
    
    // 使用结构体获取详细时间
    struct tm *time_info = localtime(&current_time);
    printf("Year: %d\n", time_info->tm_year + 1900);
    printf("Month: %d\n", time_info->tm_mon + 1);
    printf("Day: %d\n", time_info->tm_mday);
    printf("Hour: %d\n", time_info->tm_hour);
    printf("Minute: %d\n", time_info->tm_min);
    printf("Second: %d\n", time_info->tm_sec);
    
    // 格式化时间输出
    char buffer[80];
    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", time_info);
    printf("Formatted: %s\n", buffer);
    
    // 计算程序运行时间
    clock_t start = clock();
    
    // 模拟一些工作
    for (long i = 0; i < 100000000; i++) {
        // 空循环
    }
    
    clock_t end = clock();
    double cpu_time_used = ((double)(end - start)) / CLOCKS_PER_SEC;
    printf("CPU time used: %.2f seconds\n", cpu_time_used);
}

6. 字符处理库 (ctype.h)

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

void character_operations() {
    char ch = 'A';
    char digit = '5';
    char space = ' ';
    char punct = '!';
    
    printf("isalpha('%c'): %d\n", ch, isalpha(ch));
    printf("isdigit('%c'): %d\n", digit, isdigit(digit));
    printf("isspace('%c'): %d\n", space, isspace(space));
    printf("isalnum('%c'): %d\n", ch, isalnum(ch));
    printf("ispunct('%c'): %d\n", punct, ispunct(punct));
    printf("isupper('%c'): %d\n", ch, isupper(ch));
    printf("islower('%c'): %d\n", ch, islower(ch));
    
    // 字符转换
    char lower = tolower(ch);
    char upper = toupper('b');
    printf("tolower('%c') = '%c'\n", ch, lower);
    printf("toupper('b') = '%c'\n", upper);
    
    // 实用示例：字符串处理
    char text[] = "Hello World 123!";
    printf("Original: %s\n", text);
    
    printf("Processed: ");
    for (int i = 0; text[i]; i++) {
        if (isalpha(text[i])) {
            putchar(toupper(text[i]));
        } else if (isdigit(text[i])) {
            putchar(text[i]);
        } else {
            putchar(' ');
        }
    }
    printf("\n");
}

7. 实用工具库 (stdlib.h 扩展)

#include <stdlib.h>
#include <stdio.h>

void utility_operations() {
    // 环境变量
    char *path = getenv("PATH");
    if (path) {
        printf("PATH: %s\n", path);
    }
    
    // 系统命令
    printf("System output:\n");
    system("ls -la");  // Linux/Mac
    // system("dir");   // Windows
    
    // 字符串转换
    char num_str[] = "12345";
    int number = atoi(num_str);
    printf("atoi('%s') = %d\n", num_str, number);
    
    char float_str[] = "3.14159";
    double pi = atof(float_str);
    printf("atof('%s') = %.5f\n", float_str, pi);
    
    // 更安全的转换函数
    char *endptr;
    long big_num = strtol("1234567890", &endptr, 10);
    printf("strtol result: %ld\n", big_num);
    
    // 退出程序
    // exit(EXIT_SUCCESS);  // 正常退出
    // exit(EXIT_FAILURE);  // 错误退出
}

8. 错误处理库 (errno.h)

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

void error_handling() {
    FILE *file = fopen("nonexistent_file.txt", "r");
    if (file == NULL) {
        printf("Error opening file: %s\n", strerror(errno));
        perror("fopen failed");
    }
    
    // 清除错误状态
    errno = 0;
    
    // 数学错误示例
    double result = sqrt(-1.0);
    if (errno != 0) {
        printf("Math error: %s\n", strerror(errno));
    }
}

9. 断言调试库 (assert.h)

#include <assert.h>
#include <stdio.h>

int divide(int a, int b) {
    assert(b != 0);  // 断言：除数不能为0
    return a / b;
}

void assertion_demo() {
    printf("Testing assertion...\n");
    
    // 正常情况
    int result = divide(10, 2);
    printf("10 / 2 = %d\n", result);
    
    // 错误情况（在调试模式下会终止程序）
    // result = divide(10, 0);  // 这会触发断言失败
    
    // 自定义错误检查
    int value = -5;
    assert(value >= 0 && "Value must be non-negative");
}