Question

MASSIVE EDIT:

I have a long int variable that I need to convert to a signed 24bit hexadecimal string without the “0x” at the start. The string must be 6 characters followed by a string terminator ‘\0’, so leading zeros need to be added.

Examples:
[-1 -> FFFFFF] —
[1 -> 000001] —
[71 -> 000047]

Answer
This seems to do the trick:

long int number = 37;
char string[7];

snprintf (string, 7, "%lX", number);

Answer 1

Because you only want six digits, you are probably going to have to do some masking to make sure that the number is as you require. Something like this:

sprintf(buffer, "%06lx", (unsigned long)val & 0xFFFFFFUL);

Be aware that you are mapping all long integers into a small range of representations. You may want to check the number is in a specific range before printing it (E.g. -2^23 < x < 2^23 – 1)