On few systems double is same as long double. How can I detect if long double is of extended precision than double at compile time and use it to conditional compile.
I see there are predefined macros present in libgcc SIZEOF_DOUBLE and SIZEOF_LONG_DOUBLE But there are not portable across different toolchains.
Is there C way to do this?
You could compare DBL_MANT_DIG and LDBL_MANT_DIG from float.h.
You can test e.g.
#if DBL_MANT_DIG < LDBL_MANT_DIG
or similar values defined in float.h
The "correct" solution to this problem (as used by many projects) is to create a configure script.
The configure script runs various tests that include compiling and running small programs to determine compiler and system properties. The script then writes out it's findings as a header file, or a makefile, or both. Of course, yours can do anything you like.
There are tools some tools to do this sort of thing semi-automatically, but they're probably overkill for you. If you'd like to take a look the names are autoconf and automake. Beware, they're not simple to learn, but they generate configure scripts and makefiles that should work on just about any platform as long as it has a unix-style shell, and GNU make.
Why do you want long double? For precision. So go straight to the core of the issue and test for precision, specified by EPSILON:
#include <float.h>
printf("Info: long double epsilon = %10.4Lg\n", LDBL_EPSILON);
if (LDBL_EPSILON > 1.2e-19) {
printf("Insufficient precision of long double type\n");
return 1;
}
This is a test at run time though, not at configure or compile time.
Put it either in a unit test, or in a little test program to be run from CMake (as proposed in the answer by ams).
...at compile time and if (LDBL_EPSILON > 1.2e-19) is a runtime expression and you can't compare floats at compile time (in preprocessor)./
sizeof(double) > 8. Although not portable either, it'll probably still work in most cases.sizeof(double) < sizeof(long double)or am I missing something?sizeof.