ANSI-C: maximum number of characters printing a decimal int

Question

I\'d like to know if it is an easy way of determining the maximum number of characters to print a decimal int.

I know contains

Answer 1

If you assume CHAR_BIT is 8 (required on POSIX, so a safe assumption for any code targetting POSIX systems as well as any other mainstream system like Windows), a cheap safe formula is 3*sizeof(int)+2. If not, you can make it 3*sizeof(int)*CHAR_BIT/8+2, or there's a slightly simpler version.

In case you're interested in the reason this works, sizeof(int) is essentially a logarithm of INT_MAX (roughly log base 2^CHAR_BIT), and conversion between logarithms of different bases (e.g. to base 10) is just multiplication. In particular, 3 is an integer approximation/upper bound on log base 10 of 256.

The +2 is to account for a possible sign and null termination.

Answer 2

After accept answer (2+ yr)

The following fraction 10/33 exactly meets the needs for unpadded int8_t, int16_t, int32_t and int128_t. Only 1 char extra for int64_t. Exact or 1 over for all integer sizes up to int362_t. Beyond that may be more that 1 over.

#include <limits.h>
#define MAX_CHAR_LEN_DECIMAL_INTEGER(type) (10*sizeof(type)*CHAR_BIT/33 + 2)
#define MAX_CHAR_SIZE_DECIMAL_INTEGER(type) (10*sizeof(type)*CHAR_BIT/33 + 3)

int get_int( void ) {
                                            //   + 1 for the \n of fgets()
  char draft[MAX_CHAR_SIZE_DECIMAL_INTEGER(long) + 1];  //**

  fgets(draft, sizeof draft, stdin);
  return strtol(draft, NULL, 10);
}

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** fgets() typically works best with an additional char for the terminating '\n'.

Similar to @R.. but with a better fraction.

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Recommend using generous, 2x, buffers when reading user input. Sometimes a user adds spaces, leading zeros, etc.

  char draft[2*(MAX_CHAR_SIZE_DECIMAL_INTEGER(long) + 1)];
  fgets(draft, sizeof draft, stdin);