STM32 printf problem summary semihosting microlib understanding

background ： Use STM32F4 When developing board serial port , Want to use c The language comes with printf Function to output the string to the serial port debugging assistant .

1. First, we need to redefine fputc()、fgetc() function

/// Redirect c Library function printf To serial port DEBUG_USART, After resetting, you can use printf function 
int fputc(int ch, FILE *f)
{
	/*  Send a byte of data to the serial port DEBUG_USART */
	HAL_UART_Transmit(&huart1, (uint8_t *)&ch, 1, 1000);	
	
	return (ch);
}

/// Redirect c Library function scanf To serial port DEBUG_USART, Override backward to use scanf、getchar Such as function 
int fgetc(FILE *f)
{
	int ch;
	HAL_UART_Receive(&huart1, (uint8_t *)&ch, 1, 1000);	
	return (ch);
}

2. Check options for Target–>Target–>Use MicroLIB, Use MicroLIB library .
   -microLIB It's a standard. C An alternative library to the Library , Used in embedded hardware with very little memory .
   -microLIB Standard libraries are not supported
   - Mainly complete the following work ：
   - Create an executable C The environment of the program ： Create a stack 、 Create a heap （ if necessary ）、 The library used by the initialization program
   - call main（） Start execution

3. Half host （semi-hosting）： It's a mechanism , Make it run in ARM The code on the target can communicate , And use Input/Output Tools

   - At the beginning of development , Developers may not know what to do ARM What input and output devices are there on the device , The semi host mechanism allows developers not to know ARM Device peripherals , Input and output debugging can be realized by using the peripherals of the host computer .

   - If you want to use ARM Input on the device 、 Output devices , First of all, turn off semihosting. Then redirect the input and output to ARM On the device .
   
   -microLIB Compile time through Low-Level Functions Connect with hardware , Adopt abridged version high-level functions, useless system I/O functions.

4. If not used microLIB library , Use the standard library （ By default semihosting）, call printf（） Functions like that , Will cause a crash . resolvent ：
   - Add compilation settings to the program , Don't use semihosting

//C In language 
#pragma import(__use_no_semihosting_swi)
// In assembly language 
IMPORT __use_no_semihosting_swi

Reference resources ：

1. KeilMDK In the configuration item Use MicroLIB
2. I/O retargeting