RT-Thread 入门学习笔记：vsnprintf来替代rt_vsnprintf来打印浮点

前言

• 有童鞋说在gcc下，vsnprintf来替代rt_vsnprintf来打印浮点会引发死机
• 经过实际验证，没有发现死机
• 但并不建议直接用vsnprintf来替代rt_vsnprintf，打印浮点多半用于调试，可以独立成一个专门的打印函数
• vsnprintf只是用于把浮点格式化到字符串，只要字符串长度不溢出，理论上不会造成死机
• 如果发现死机，需要确认打印的环境，如中断中，尤其在串口输出的情况下

测试例程

• RT-Thread Studio arm-none-eabi-gcc 环境：未发现死机现象
• Keil MDK5 环境：未发现死机现象
• cygwin gcc 环境：未发现死机现象
• 可以正常的用于浮点数的调试打印

/*
 * Copyright (c) 2006-2018, RT-Thread Development Team
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Change Logs:
 * Date           Author       Notes
 * 2018-11-06     SummerGift   change to new framework
 */

#include 
#include 
#include 
#include "drv_gpio.h"
/* defined the LED0 pin: PA5 */
#define LED0_PIN    GET_PIN(A, 5)

#include 

#define DBG_BUFF_MAX_LEN          256

/* debug print : support float double */
int dbg_printf(const char *fmt, ...)
{
    va_list args;
    static char rt_log_buf[DBG_BUFF_MAX_LEN] = { 0 };

    va_start(args, fmt);
    int length = vsnprintf(rt_log_buf, sizeof(rt_log_buf) - 1, fmt, args);

    rt_kputs(rt_log_buf);

    return length;
}

int main(void)
{
    double a = 334455.00123345;
    double b = 667788.14234234;
    dbg_printf("double a = %lf\r\n", a);
    int count = 1;
    /* set LED0 pin mode to output */
    rt_pin_mode(LED0_PIN, PIN_MODE_OUTPUT);

    while (count++)
    {
        rt_pin_write(LED0_PIN, PIN_HIGH);
        dbg_printf("double a + b = %lf\r\n", a+b+count);
        rt_thread_mdelay(1000);
        rt_pin_write(LED0_PIN, PIN_LOW);
        rt_thread_mdelay(2000);
    }

    return RT_EOK;
}

• cygwin的测试

/* vsnprintf example */
#include 
#include 

#define DBG_BUFF_MAX_LEN            256

/* debug print : support float double */
int dbg_printf(const char *fmt, ...)
{
    va_list args;
    static char rt_log_buf[DBG_BUFF_MAX_LEN] = { 0 };

    va_start(args, fmt);
    int length = vsnprintf(rt_log_buf, sizeof(rt_log_buf) - 1, fmt, args);

    printf("%s\r\n", rt_log_buf);

    return length;
}
 
int main ()
{
    double a = 123453.0001444;
    dbg_printf("%lf", a);
    return 0;
}

输出效果

msh >
 \ | /
- RT -     Thread Operating System
 / | \     4.1.0 build Feb  3 2022 21:11:28
 2006 - 2022 Copyright by RT-Thread team
double a = 334455.001233
double a + b = 1002245.143576
msh >double a + b = 1002246.143576
double a + b = 1002247.143576
double a + b = 1002248.143576
double a + b = 1002249.143576
double a + b = 1002250.143576
double a + b = 1002251.143576
double a + b = 1002252.143576
double a + b = 1002253.143576
double a + b = 1002254.143576
double a + b = 1002255.143576
double a + b = 1002256.143576
double a + b = 1002257.143576
double a + b = 1002258.143576
double a + b = 1002259.143576
double a + b = 1002260.143576
double a + b = 1002261.143576

• cygwin的打印：

$ ./a.exe
123453.000144

小结

• rt_kprintf，采用不依赖标准C库的方式，主要目的是为了减少ROM（Flash）的占用
• 如果实际中需要浮点打印（如打印到串口、打印到文件），可以基于vsnprintf编写一个简单的全功能格式打印函数
• RT-Thread ulog可以开启浮点支持，开启后使用：vsnprintf