I think -1 and 0 are equally bad (as a caller might be unaware that the function also uses the output parameter as an error indicator and may end up calling malloc(utf8_strlen(x))).

I guess a better way would be?
uint32_t utf8_strlen(char *, uint8_t * errcode);

Returning 0 for invalid string is probably a bad idea. Shouldn't -1 be returned instead?

Re-published, thanks.

<stdint.h> should be included in Sample Tests.

I did, thanks.

Please update Sample Tests also.

Ok thanks.
I've republished it following your suggestions.

There exist some 64-bit platforms where int has 64 bits (ILP64 models). But my main concern is that there is a discrepancy between the description (where the function is defined as uint32_t one_at_a_time(int8_t *key, uint32_t len) and test code (where the prototype is defined as extern unsigned int jenkins_hash(char *, unsigned int)). I think it is better to be consistent.

int8_t makes sense - because of simmetry with the test definition.

But why is uint32_t preferred?
Is there any 64bit platform where unsigned int is not 4 bytes long?

Thank you.

Please also change the function signature in Sample Tests: extern uint32_t jenkins_hash(int8_t *, uint32_t); (and don't forget to include <stdint.h>).

Indeed, I agree with you, reason why I didn't raise this as an issue.

I was sure the C standard defines the default signedness (to signed), but I was wrong.
Thanks, changed to int8_t.

I've addressed both issues and re-published. Thanks.