I am trying to develop a sensor driver on the STM32F401CCU6. I have already worked out the sensor data acquisition on the STM. The problem I am facing is getting that over USB to my host system. Before I start sending the actual sensor data over, I am trying to send a simple "Hello World!\n" over to my host computer. I want to first figure out the correct way to receive and decode the data on my host system.

Here is how I have declared the string to send:

const char* start_byte = "Hello World!\n";

Here is how I am sending it over USB to the host system in the main() function:

while (1)

    /* USER CODE BEGIN 3 */
      CDC_Transmit_FS((uint16_t*)start_byte, sizeof(start_byte));

When I read the serial data on my host system (using python, CoolTerm, Putty), it only shows the string "Hell".

Here is the sample output on the serial monitors:


The string is never-ending. It terminates after the second 'L'. Any help would be appreciated. Thanks in advance!

  • 2
    \$\begingroup\$ You're taking the sizeof a pointer to your string, not of the string itself. Declare your string as an array like const char start_byte[] = "Hello World!\n"; and it'll work. \$\endgroup\$
    – brhans
    Sep 30, 2022 at 13:50
  • \$\begingroup\$ @brhans thanks! I changed sizeof to strlen and it worked. I also declared the start_byte as uint8_t* instead of const char* \$\endgroup\$
    – Harit
    Sep 30, 2022 at 14:09
  • \$\begingroup\$ Yeah that'll work just as well. Maybe better to use strlen since then you don't unnecessarily send the string's \0 terminator byte. \$\endgroup\$
    – brhans
    Sep 30, 2022 at 14:17
  • \$\begingroup\$ @brhans Thanks for that bit of information! I was unaware of the \0 terminator for strings. Now the raw data makes more sense to me. I was seeing this terminator throughout my raw data and was so confused. Appreciate your insights :) \$\endgroup\$
    – Harit
    Oct 4, 2022 at 11:34

1 Answer 1


Thanks to @brhans for pointing out my mistake. When I use sizeof, I was getting the size of the pointer to my string in return. Instead, I needed to use strlen to get the length of the string. With strlen, the correct number of bytes are transmitted and I am able to receive my string on the host machine.

  • 1
    \$\begingroup\$ Note: you can mark your own answer as accepted to indicate the problem has been solved. \$\endgroup\$
    – Glorfindel
    Oct 4, 2022 at 12:52

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