How large buffer do I need?

Question

For a double value stored in x, how large does buffer need to be in the function call below?

sprintf(buffer, "%.*g", DBL_DIG, x);

Answer 1

The worst case will be :

• one minus - if number if negative
• one decimal point .
• DBL_DIG decimal digits
• the exponent part that should not be greater that e+999 (*)
• the terminating null

So the size of buffer should be DBL_DIG + 8.

(*) According to wikipedia page on [IEEE floating point] the exponent part for a double is at most 2¹⁰²³ < 10³⁰⁸. So the decimal representation of the exponent need at most 3 digits.

Of course above stuff only has sense for IEEE754 compliant floating point implementations (thanks to Basile Starynkevitch for noticing)