I usually received free() error when my output was bad somehow. Inspect carefully what your code is doing and what it is trying to return (I think the culprit may simply be the absence of null pointer at the end of the string, but I can't tell for sure).

I keep getting this error:

*** Error in `./test': free(): invalid next size (normal): 0x00000000025bd990 ***

It also says signal code: 6. One test is passed but one is failed and there is one error. Any ideas what this means?