ok...?

Runtime per se won't be a problem here (maybe it is about 0.4ms). The real trouble is, that this ends up in an infinite loop for values above the square closest to INT_MAX.

Hello siebenschlaefer,

is using
a locally allocated char array
which is returned via strdup(charray)
a form of memory leak?

thank you,
rowcased

Yes, this function allocates some memory and the caller is reposible for deallocation. That's common in C programs.
All alternatives either leak memory, cannot handle arbitrarily large results, and/or are not thread safe or reentrant.