Make parson -Wfloat-equal compatible

[msteinbeck]

I try to include parson into a project which compiles the source files with -Wfloat-equal. Unfortunately, parson compares double values: https://github.com/kgabis/parson/blob/master/parson.c#L1367. As far as I can see this is the only line.

[kgabis]

Should be fixed now (if your standard library defines isnan and isinf).

[msteinbeck]

You patch, unfortunately, does not work. GCC and Clang on Linux, at least on my system, do not provide isnan and isinf. How about the following snippet:

#ifdef __GNUC__
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wfloat-equal"
#endif
    if ((number * 0.0) != 0.0) { /* nan and inf test */
#ifdef __GNUC__
#pragma GCC diagnostic pop
#endif

It temporarily disables the warning and works with GCC and Clang.

[kgabis]

What compiler (version) on what sytem are you using and what flags do you use to compile parson?

[msteinbeck]

I'm running Slackware 14.2 with gcc-5.5.0, clang-3.8.0, and glibc-2.23. I attached a small example, containing my library (tinyspline), parson (the latest commit), and an example which prints the resulting JSON String to stdout. I use the following command to compile the program:

gcc -std=c89 -Werror -Wall -Wextra -Wfloat-equal -pedantic -o json_export json_export.c parson.c tinyspline.c -lm

I get the following error:

parson.c: In function ‘json_value_init_number’:
parson.c:59:43: error: comparing floating point with == or != is unsafe [-Werror=float-equal]
 #define IS_NUMBER_INVALID(x) (((x) * 0.0) != 0.0)
                                           ^
parson.c:1373:9: note: in expansion of macro ‘IS_NUMBER_INVALID’
     if (IS_NUMBER_INVALID(number)) {
         ^
cc1: all warnings being treated as errors

Contrary to my previous tests, Clang compiles without any issues:

clang -std=c89 -Werror -Wall -Wextra -Wfloat-equal -pedantic -o json_export json_export.c parson.c tinyspline.c -lm

Attachment: parson.zip

EDIT: Update zip file.

[kgabis]

I’m unable to download your attachment, could you mail it to me?

[msteinbeck]

You should now be able to download the attachment. Maybe uMatrix blocked Github's upload service. Anyhow, I mailed you the attachment.

[kgabis]

As far as I understand some versions of libc don't provide isnan unless you compile your code using c99 standard or newer (#define isnan is guarded by #ifdef __USE_ISOC99). On my mac there is no such guard, but when I compiled it on gnu/linux the problem appeared. clang on the other hand doesn't generate a warning when comparing to a float constant that's fully representable.
Example:

int comp(double x, double y)
{
        int test1 = x == 0.0;
        int test2 = x == 0.1; /* warning */
        int test3 = x == 1.0;
        int test4 = x == 2.0;
        int test5 = x == 3.0;
        int test6 = x == 4.0;
        int test7 = x == 4.4; /* warning */
        int test8 = x == y; /* warning */
        return test1 || test2 || test3 || test4 || test5 || test6 || test7 || test8;
}

It's a bit worrying because no warning is generated for this function:

int comp()
{
        double acc = 0.0;
        for (int x = 0; x < 10; x++) {
                acc += 0.1;
        }
        return acc == 1.0; /* no warning in clang */
}

Anyway, I'd rather not add #pragma GCC diagnostic ignored "-Wfloat-equal" because it's feels like a hack and it won't scale if other people start reporting such problems. Is there a way for you to disable this warning when compiling parson.c (such as -w flag in gcc)?

[msteinbeck]

Thank you for your work.

Is there a way for you to disable this warning when compiling parson.c (such as -w flag in gcc)?

I don't want to compile external libraries (currently it's only parson) separately and, thus, use the same compiler flags for all source files. This decision is the basis of the fact that I want to keep the option open that others can simply include tinyspline's source files into their projects without depending on tinyspline's build system (CMake)---similar to parson. That's why I compile tinyspline with -Werror -Wall -Wextra -Wfloat-equal -pedantic for GCC and Clang, and /Wall /WX for MSVC. Warnings that can't be fixed are disabled temporarily. It ensures that the source files are as portable as possible. Otherwise, projects using tinyspline may be in the same situation as I am with parson; including the source files requires some extra effort.

Anyhow, I can still maintain a modified version of parson in tinyspline's repository. The required modifications aren't too invasive.

Finally, I would like to thank you for this great library. You did a great job!