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

To just obtain the length you can write:

int length = snprintf(NULL, 0, "{data:%d}", 12312);

Note that the return type is int. It may return -1 in case of some sort of error. Make sure your input data doesn't include long strings that might cause the total length to exceed INT_MAX !