一.what is open-close-principle
OCP 是指软件实体（类、模块、函数等）应该对扩展开放，对修改关闭。
二.OCP的设计准则
- 对扩展开放：当需求变化时，我们应该能够通过添加新的代码来满足需求，而不是修改现有的代码。比如添加新的功能、支持新的类型等。
- 对修改关闭：一旦软件实体（模块、函数等）被实现，就不应该对其进行修改。这是为了确保软件的稳定性和可靠性。
- 使用抽象：通过抽象（如接口、抽象类等）来定义软件实体的行为，使得具体实现可以独立于抽象而变化。
三.举例
3.1 设备驱动反例
每次新增一种设备类型（例如SPI、USB），都要修改这个 switch，违反开闭原则。
点击查看代码
typedef enum {DEV_UART,DEV_GPIO,DEV_I2C
} DevType;int dev_open(DevType type, const char *name) {switch (type) {case DEV_UART:// UART open logicbreak;case DEV_GPIO:// GPIO open logicbreak;case DEV_I2C:// I2C open logicbreak;default:return -1;}// 同理 read/write/close 都要有switch// 新增SPI或USB，就要改所有函数的switch
}
3.2 设备驱动正确实现
用结构体 + 函数指针表（vtable）模拟虚函数表，实现设备抽象。每个设备类型有一个“驱动”结构体，核心操作通过指针调用。
dev_abstraction.h
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// dev_abstraction.h
#ifndef DEV_ABSTRACTION_H
#define DEV_ABSTRACTION_H#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>// 设备句柄（opaque，用户不关心内部）
typedef struct dev_handle dev_handle_t;// 设备操作vtable（函数指针表）
typedef struct {int (*open)(dev_handle_t *h, const char *name, int flags);int (*read)(dev_handle_t *h, void *buf, size_t len);int (*write)(dev_handle_t *h, const void *buf, size_t len);void (*close)(dev_handle_t *h);int (*ioctl)(dev_handle_t *h, int cmd, void *arg); // 可选控制
} dev_ops_t;// 设备类型注册项
typedef struct dev_driver {const char *type_name; // "uart", "gpio", "i2c", "spi"...dev_ops_t ops;size_t priv_size; // 私有数据大小（每个驱动自定义）
} dev_driver_t;// 设备句柄结构（内部）
struct dev_handle {const dev_driver_t *driver;void *priv; // 驱动私有数据// ... 可加引用计数等
};// 核心API（永远不改）
void dev_register_driver(const dev_driver_t *driver);// 创建设备句柄（按类型名查找驱动）
dev_handle_t *dev_create(const char *type_name);// 通用操作（通过vtable调用）
int dev_open(dev_handle_t *h, const char *name, int flags);
int dev_read(dev_handle_t *h, void *buf, size_t len);
int dev_write(dev_handle_t *h, const void *buf, size_t len);
void dev_close(dev_handle_t *h);
int dev_ioctl(dev_handle_t *h, int cmd, void *arg);#endif
dev_abstraction.c
点击查看代码
// dev_abstraction.c
#include "dev_abstraction.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>#define MAX_DRIVERS 32static const dev_driver_t *drivers[MAX_DRIVERS] = {0};
static int driver_count = 0;void dev_register_driver(const dev_driver_t *driver)
{if (driver_count >= MAX_DRIVERS) {fprintf(stderr, "too many device drivers\n");return;}drivers[driver_count++] = driver;
}static const dev_driver_t *find_driver(const char *type_name)
{for (int i = 0; i < driver_count; i++) {const dev_driver_t *d = drivers[i];if (d && strcmp(d->type_name, type_name) == 0)return d;}return NULL;
}dev_handle_t *dev_create(const char *type_name)
{const dev_driver_t *d = find_driver(type_name);if (!d) {fprintf(stderr, "unknown device type: %s\n", type_name);return NULL;}dev_handle_t *h = malloc(sizeof(dev_handle_t));if (!h) return NULL;h->driver = d;h->priv = (d->priv_size > 0) ? calloc(1, d->priv_size) : NULL;return h;
}// 通用操作（委托给vtable，永远不改）
int dev_open(dev_handle_t *h, const char *name, int flags)
{if (!h || !h->driver || !h->driver->ops.open) return -1;return h->driver->ops.open(h, name, flags);
}int dev_read(dev_handle_t *h, void *buf, size_t len)
{if (!h || !h->driver || !h->driver->ops.read) return -1;return h->driver->ops.read(h, buf, len);
}int dev_write(dev_handle_t *h, const void *buf, size_t len)
{if (!h || !h->driver || !h->driver->ops.write) return -1;return h->driver->ops.write(h, buf, len);
}void dev_close(dev_handle_t *h)
{if (!h || !h->driver || !h->driver->ops.close) return;h->driver->ops.close(h);free(h->priv);free(h);
}int dev_ioctl(dev_handle_t *h, int cmd, void *arg)
{if (!h || !h->driver || !h->driver->ops.ioctl) return -1;return h->driver->ops.ioctl(h, cmd, arg);
}
新增UART驱动，不需要修改核心代码。
uart_driver.c
点击查看代码
// uart_driver.c
#include "dev_abstraction.h"
#include <stdio.h> // 模拟实际硬件操作typedef struct {FILE *fp; // 模拟串口文件
} uart_priv_t;static int uart_open(dev_handle_t *h, const char *name, int flags)
{uart_priv_t *p = h->priv;p->fp = fopen(name, "r+"); // 模拟 /dev/ttyS0return p->fp ? 0 : -1;
}static int uart_read(dev_handle_t *h, void *buf, size_t len)
{uart_priv_t *p = h->priv;return fread(buf, 1, len, p->fp);
}static int uart_write(dev_handle_t *h, const void *buf, size_t len)
{uart_priv_t *p = h->priv;return fwrite(buf, 1, len, p->fp);
}static void uart_close(dev_handle_t *h)
{uart_priv_t *p = h->priv;if (p->fp) fclose(p->fp);
}static int uart_ioctl(dev_handle_t *h, int cmd, void *arg)
{// 模拟设置波特率等return 0;
}static const dev_driver_t uart_driver = {.type_name = "uart",.ops = {.open = uart_open,.read = uart_read,.write = uart_write,.close = uart_close,.ioctl = uart_ioctl},.priv_size = sizeof(uart_priv_t)
};__attribute__((constructor))
static void register_uart(void)
{dev_register_driver(&uart_driver);
}
main.c
点击查看代码
int main(void)
{dev_handle_t *h = dev_create("uart");if (!h) return 1;if (dev_open(h, "/dev/ttyS0", 0) != 0) {dev_close(h);return 1;}char buf[128] = "Hello, device!";dev_write(h, buf, strlen(buf));memset(buf, 0, sizeof(buf));dev_read(h, buf, sizeof(buf));printf("Read: %s\n", buf);dev_ioctl(h, 0x1234, NULL); // 自定义命令dev_close(h);// 新增"spi"驱动后，直接dev_create("spi")即可，无需改mainreturn 0;
}