check.py

import serial
import struct

s = serial.Serial('/dev/ttyACM1', 460800, timeout=1.0)
print('Opened port %s', s)

s.write(b'1')
ok = 0

while True:
    buf = s.read(1)
    (msgLen, ) = struct.unpack('B', buf)
    buf = s.read(msgLen)
    sm = 0
    for c in buf[:-1]:
        sm = (sm + c) % 256
    if sm != buf[-1]:
        print("wrong sum", sm, buf[-1])
    else:
        ok += 1
    if ok % 10 == 1:
        print("ok", ok)
    s.write(buf)

main.cpp

#include "mbed.h"

BufferedSerial com(USBTX, USBRX, 460800);

void check(BufferedSerial *com) {
  uint8_t u;
  com->read(&u, 1);
  uint8_t buf[256];
  while(1) {
    uint8_t len = rand() % 254 + 1;
    uint8_t sum = 0;
    for (int i = 0; i < len; i++) {
      buf[i] = rand() % 256;
      sum = (sum + buf[i]) % 256;
    }

    buf[len] = sum;
    len += 1;
    com->write(&len, 1);
    com->write(buf, len);

    int rd = 0;
    while (rd < len) {
      int x = com->read(&buf, len);
      rd += x;
    }
    sum = 0;
    for (int i = 0; i < len-1; i++) {
      sum = (sum + buf[i]) % 256;
    }
    if (sum != buf[len-1]) {
      while(1) {}
    }
  }
}

int main() {
  check(&com);
}

mbed_app.bad.json

{
    "requires": ["bare-metal"],
    "target_overrides": {
       "*": {
            "target.c_lib": "small"
        },
        "K64F": {
            "platform.stdio-baud-rate": 9600
        }
    }
}

mbed_app.good.json

{
    "target_overrides": {
       "*": {
            "target.c_lib": "small"
        },
        "K64F": {
            "platform.stdio-baud-rate": 9600
        }
    }
}