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

Instead of using sprintf, you could use asprintf. This allocates a buffer of the correct size to fit your string.

Answer 2

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.

Answer 3

Use snprintf() to find out how many characters you need:

#include <float.h> /* DBL_DIG */
#include <stdio.h>
#include <stdlib.h>

int main(void) {
  double x = rand() / (double)RAND_MAX;
  char find_len[1];
  int need_len;
  char *buf;

  need_len = snprintf(find_len, 1, "%.*g", DBL_DIG, x);
  buf = malloc(need_len + 1);
  if (buf) {
    int used = sprintf(buf, "%.*g", DBL_DIG, x);
    printf("need: %d; buf:[%s]; used:%d\n", need_len, buf, used);
    free(buf);
  }
  return 0;
}

You need a C99 compiler for snprintf().
snprintf() was defined by the C99 standard. A C89 implementation is not required to have snprintf() defined, and if it has as an extension, it is not required to "work" as described by the C99 Standard.

Answer 4

Two things: %g does not show all of the representable digits, %g shows a nice-for-humans rounded result. You can specify the precision using %f or %e if you would like a different result.

Never use sprintf() rather than using snprintf(). In your case: int len = snprintf(buf, dimensionof(buf), "%.*f", DBL_DIG, x);

Answer 5

Should I just round up to 64, 128, or 256 and hope for the best?

Yes, just do that -.-