STM32 CDC_Transmit_FS：为什么在使用 sprintf() 或 strcat() 时会出现 '\\x'？

Question

I am trying to send values from ADC through USB using "CDC_Transmit_FS()"

On the receiving side, I am receiving data using readline() and decoding the 'string' to 'int'

The code works fine but occasionally I receive for example, b'\\x00234\\n' instead of b'1234\\n', which raises decoding error.

Do you know why does '\\x' appear?

One more question is: Is there any smarter method to send ADC values through USB instead of converting int values to string?

I want to make the transmission faster. thanks in advance!

uint32_t adcbuff[sample];
char endofpacket[5] = {'9', '9', '9', '9', '\n'};
char txbuff[sample*5];

while(1)
{

    HAL_ADC_Start_DMA(&hadc2,(uint32_t*)adcbuff, sample);

    for(i = 0; i < sample; i++)
    {
     sprintf (tempbuff,  "%u\n", ((adcbuff[i] * 5000) / 0xFFFF)-2000); 
     strcat( txbuff,tempbuff);
    }
        
    strcat( txbuff,endofpacket);
    CDC_Transmit_FS( (uint8_t*)txbuff, strlen(txbuff));  
    strcpy(txtbuff,"");

}

Answer 1

not enough rep to post as a comment

Usually \\x is an indication of a hexadecimal value. Could it be that a non alphanumeric value is being received? For troubleshooting, I would temporarily change

sprintf (tempbuff, "%u\\n", ((adcbuff[i] * 5000) / 0xFFFF)-2000); to

sprintf (tempbuff, "%s\\n", ((adcbuff[i] * 5000) / 0xFFFF)-2000); to see what kind of characters are being sent over. (Maybe sprintf to a tmp file instead.)

Answer 2

b' \\x00 234\\n' - This means that first byte is 0! Not ASCII 0 = 0x30, but just 0. Probably this is effect of strcat - after concatenating this function adds '\\0' at the end of string.

Instead of using sprintf, just redirect stdout to USB-CDC and use printf:

int _write(int file, char *ptr, int len)
{
    UNUSED(file);
    CDC_Transmit_FS((uint8_t*)ptr, len);
    while (hcdc->TxState != 0);
    return len;
}

If you want to send all at once use setvbuf for stdout with _IOFBF and call fflush(stdout);