What is the minimum buffer size for sprintf with %g?

Question

The problem is to statically allocate a buffer large enough to fit a printed double, formatted with %g at maximum precision. This seems like a simple enough task, bu I'm having trouble. The best I have come up with (assuming the number to be printed is x) is

char buf[1 + DBL_DIG + DBL_DIG + 1 + 1 + 1 + DBL_DIG + 1];
int len = sprintf(buf, "%.*g", DBL_DIG, x);

The DBL_DIG macro is from float.h, and apparently it is supposed to indicate the maximum precision for the type double. We need:

• 1 byte for a negative sign
• enough bytes to capture the significant digits
• at most one 'separator' char (comma, etc.) per digit
• 1 byte for a decimal point
• 1 byte for 'e'
• 1 byte for the sign on the exponent
• some bytes for the exponent
• 1 byte for the trailing null written by sprintf.

I'm using the number of significant digits as an upper bound on the number of digits in the exponent. Have I made any errors? Is there a better solution? Should I just allocate 64, 128, or 256 bytes and hope for the best?

Answer 1

You cannot pre-calculate the size at compile time. The %g formatter takes the locale into account (for the 1000's separator etc.) See http://linux.die.net/man/3/sprintf for a description on how to calculate the size safely.