Converting long long / uint64_t to string or char

Projects Programming
1

Struggling to convert this and the only other question I can find was this fairly unhelpful one

I get the unix time (in s) but because I take many samples per second I convert this to ms then add millis() to it. This works fine as far as getting a unique timestamp goes, but it needs to be converted into a string for MQTT to handle.

I have tried some of the solutions in the linked thread such as this

void publishJson() {
  char buffer[15];
  measurement_time.toCharArray(buffer, 13);

  String jsonString = "{";
  jsonString += "\"measurement_time\":" + String(buffer) + ",";
  jsonString += "}";
  Serial.println(jsonString);
}

and get the complie error -
error: request for member 'toCharArray' in 'measurement_time', which is of non-class type 'uint64_t {aka long long unsigned int}' measurement_time.toCharArray(buffer, 13);

my original error is:

Compilation error: call of overloaded 'String(uint64_t)' is ambiguous
2

Try using sprintf to see if unsigned long long is supported.

void setup(){
  char  buffer[21]; //maximum value for uint64_t is 20 digits
  sprintf(buffer, "%llu", number);
3

You could just do it manually

uint64_t val = measurement_time;
char buffer[15];
uint8_t ndx = 15;
while (ndx) {
  ndx--;
  uint8_t v = val % 10;
  val = val  / 10; 
  buffer[ndx]  = v + '0';
}
4

Be careful to include the terminating null at the end of the buffer, variables declared inside a function will not be automatically initialized to all zeros.

5

Ah sorry, good point ! , so that would be

uint64_t val = measurement_time;
char buffer[15];
uint8_t ndx = 14;
buffer[ndx] = '\0';
while (ndx) {
  ndx--;
  uint8_t v = val % 10;
  val = val  / 10; 
  buffer[ndx]  = v + '0';
}

and afterwards

if (val) ERROR
6

Tried sprintf on a Mega and an UNO R4 WiFi, does not support unsigned long long on either.

7

Need to be careful of the buffer size, and allow for the number of digits being less than the full buffer. Safer to use a buffer that will fix the maximum size of a uint64_t.

  uint64_t val = measurement_time;
  char buffer[21];
  char* ndx = &buffer[sizeof(buffer) - 1];
  *ndx = '\0';
  do {
    *--ndx = val % 10 + '0';
    val = val  / 10;
  } while (val != 0);
  Serial.println(ndx);

This is a reduced form of the code used by Print for uint32_t

size_t Print::printNumber(unsigned long n, uint8_t base)
{
  char buf[8 * sizeof(long) + 1]; // Assumes 8-bit chars plus zero byte.
  char *str = &buf[sizeof(buf) - 1]; 

  *str = '\0';

  // prevent crash if called with base == 1
  if (base < 2) base = 10; 

  do {
    char c = n % base;
    n /= base;

    *--str = c < 10 ? c + '0' : c + 'A' - 10; 
  } while(n);

  return write(str);
}
8

Works on an ESP32:

void setup() {
  Serial.begin(115200);
  delay(2000);

  uint64_t x = 17179869184;
  Serial.printf("x = %llu\n", x);
}

void loop() {
}

x = 17179869184
11

@trap-fish, if the code in Post #8 doesn't work on your platform, then you can use one of the roll-your-own solutions presented above to get a character string decimal representation.

There's also a more efficient option if you're OK with time stamps presented as a character string HEX representation:

void setup() {
  Serial.begin(115200);
  delay(2000);

  uint64_t x = 4330422113570437784;
  uint32_t lower = x & 0xFFFFFFFF;
  uint32_t upper = (x >> 32) & 0xFFFFFFFF;
  Serial.printf("x = 0x%08X%08X\n", upper, lower);
}

void loop() {
}

x = 0x3C18BFF7FEDCBA98

However, I think I've seen some posts that say AVR-based platforms have a problem shifting 64-bit integers. If you run across that, then you could go with:

void setup() {
  Serial.begin(115200);
  delay(2000);
  uint32_t lower;
  uint32_t upper;

  uint64_t x = 4330422113570437784;
  uint8_t *ptr = reinterpret_cast<uint8_t *>(&x);
  memcpy(&upper, ptr + sizeof(uint32_t), sizeof(uint32_t));
  memcpy(&lower, ptr, sizeof(uint32_t));
  Serial.printf("x = 0x%08X%08X\n", upper, lower);
}

void loop() {
}

x = 0x3C18BFF7FEDCBA98

As I mentioned above, either of the HEX solutions are more efficient than trying to get a decimal representation.

12

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