sprintf() heap usage

Joel Sherrill joel.sherrill@oarcorp.com
Tue Jul 16 15:23:00 GMT 2013

On 7/16/2013 10:18 AM, Eric Blake wrote:
> On 07/16/2013 03:39 AM, Vasili Galka wrote:
>> Hi,
>> I've been surprised to discover that using sprintf() leads to
>> requirement of sbrk(). Can anyone please explain me why?
>> For gods sake, the function already has output buffer provided. The
>> lifetime of the function is well defined and it has stack. Why would
>> it require heap!?
> Computation of %g and friends can require allocation in order to safely
> convert a power-of-two floating-point number into a power-of-10 string
> representation (particularly for numbers on the extreme small end, like
> 1e-300).  Even if the final representation fits in the buffer passed
> into snprintf, the intermediate conversion steps require more bytes than
> can be safely allocated within a single page of the stack, and as newlib
> cannot assume a large stack size, it is easier to implement the
> conversion process using the heap.
And none of this requires virtual memory, paging, or an MMU. It is simply
making the heap larger.

Joel Sherrill, Ph.D.             Director of Research & Development
joel.sherrill@OARcorp.com        On-Line Applications Research
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