How to calculate the length of output that sprintf will generate?

Question

Goal: serialize data to JSON.

Issue: i cant know beforehand how many chars long the integer is.

i thought a good way to do this

Answer 1

Printf supports %n format parameter, which means "write position of %n in output string to int value pointed by x-parameter), so:

int x;snprintf(NULL, 0, "Message: %s%n", "Error!",&x);

Should works!