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Re: sprintf() heap usage

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.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library

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