Another way to phrase this question might be "How to end format specifiers in printf"
If I want to print microseconds in this format 100us using the following code...
long microseconds = 100L;
printf("%lus", microseconds);
only prints 100s because the u is combined with the %l and it interprets the format specifier as an unsigned long instead of a long
Just write
long microseconds = 100L;
printf("%ldus", microseconds);
Pay attention to that you may not use a length modifier without a conversion specifier.
How to separate format specifiers from characters with printf
If separation is important:
long microseconds = 100L;
printf("%ld" "us", microseconds);
Adjacent literal strings are concatenated. The above is equivalent to "%ldus".
// or maybe via macros
#DEFINE TIME_FMT "%ld"
#DEFINE TIME_SUFFIX "us"
...
printf(TIME_FMT TIME_SUFFIX, microseconds);