uint64_t string conversion and extraction of first 4 characters

Question

I've been handed this function that checks if a given 4 digit PIN number is correct. This PIN is a string of length 5 (eg "1234\0"). The real PIN is obtained after some calculations, by converting a uint64_t to string and extracting the first 4 characters:

uint8_t pin_verification(uint64_t number, uint8_t *pin)
{
  uint8_t string[14];

  // ASCII string conversion
  sprintf((char *)(string), "%llu", number);

  // Pin verification
  if (memcmp(pin, string, 4))
  {
    return 0;
  }
  else
  {
    return 1;
  }
}

I've been told that it works, but I'm trying to run this on a STM32 chip and the sprintf function doesn't work properly. I've tried solutions like using the PRIu64 modifier from the inttypes.h library, but it still doesn't work.

I don't mind to change this function if there is a way to avoid the use of sprintf .

Thanks!

Answer 1

run this on a STM32 chip and the sprintf function doesn't work properly

You are using standard C library implementation (most probably newlib in it's "nano") version that does not support long long printf format specifiers.

Either:

• do not use long long format specifier and find other way
• provide your own library for printing long long numbers
• use a C standard library that supports long long printing
  • ie. use full newlib version, ie. remove -specs=nano.specs or -lnano from your compiler command line

Note that:

• told that it can be 14 digits long maximum then 14 byte buffer is too short to store a string with 14 digits.
• prefer snprintf to protect against buffer overflows