How many chars do I need to print a size_t with sprintf? [duplicate]

Question

I want to do something like this:

char sLength[SIZE_T_LEN];

sprintf(sLength, "%zu", strlen(sSomeString));

That is, I want to print in a buffer a value of type size_t. The question is: what should be the value of SIZE_T_LEN? The point is to know the minimum size required in order to be sure that an overflow will never occur.

Answer 1

sizeof(size_t) * CHAR_BIT (from limits.h) gives the number of bits. For decimal output, use a (rough) lower estimate of 3 (4 for hex) bits per digit to be on the safe side. Don't forget to add 1 for the nul terminator of a string.

So:

#include <limits.h>

#define SIZE_T_LEN ( (sizeof(size_t) * CHAR_BIT + 2) / 3 + 1 )

This yields a value of type size_t itself. On typical 8/16/32/64 bit platforms + 2 is not required (the minimum possible size of size_t is 16 bits). Here the error is already large enough to yield a correctly truncated result.

Note that this gives an upper bound and is fully portable due to the use of CHAR_BIT. To get an exact value, you have to use log10(SIZE_MAX) (see JohnBollinger's answer for this). But that yields a float and might be calculated at run-time, while the version above is compile-time evaluated (and likely costs more stack than the rough estimate already). Unless you have a very RAM constrained system, that is ok. And on such a system, you should refrain from using stdio anyway.

To be absolutely on the safe side, you might want to use snprintf (but that is not necessary).