mpaland/printf

Incorrect test case

akbertram opened this issue · 2 comments

akbertram commented

The "float" test suite contains the following assertions:

test::sprintf(buffer, "%.9f", 42.8952);
REQUIRE(!strcmp(buffer, "42.895200000"));

test::sprintf(buffer, "%.10f", 42.895223);
REQUIRE(!strcmp(buffer, "42.895223000"));

The first assertion looks correct, but the second assertion appears to be wrong: the precision is set to 10 and there are only 9 digits after the decimal point.

I've checked this with GLIBC 2.23-0ubuntu10 and indeed, 10 digits appear after the decimal point.

mpaland commented

Hello Alex,
thanks a lot for your investigation and bringing this to attention.

I know that the float conversion ain't really pretty and even lacks exponential support. It's still a kind of workaround and perhaps I get some help from @ckormanyos in the wintertime to improve the floating-point conversion.

So, yeah, the testcase you mentioned is actually a valid one for this printf implementation, because the precision of 10 is silently limited to 9 digits, so having "%.10f" returns 9 digits after the point. That is expected here - but I agree, this is not the correct way and may be considered as a bug.
And I should mention this explicit in the readme.

Anyway, I fix this and return the given number of precision digits while returning zeros after the 9th digit.
The testcase above will become

  test::sprintf(buffer, "%.12f", 42.89522312345678);
  REQUIRE(!strcmp(buffer, "42.895223123000"));

then.
This is not perfect as well, but better than returning a wrong precision.

akbertram commented

Thanks for your response, and for the test suite! The tests have proven very useful in verifying our Java implementation of sprintf()!

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