How can I compare floating point numbers in a Makefile?

dwhitney67 · 02-17-2023, 05:18 PM

Below is a basic Makefile where I am attempting to ascertain if I have a 'modern' g++ compiler that supports C++11. In the past, this worked fine, but now that I have version g++ 11.3.0, it does not.

Code:

ifndef CXX
CXX = g++
endif

GCC_VERSION := $(shell $(CXX) -dumpversion)
ifeq ($(shell expr $(GCC_VERSION) '>=' 4.5), 1)
CXXFLAGS  += -std=c++11
endif

foo:
    @echo "GCC_VERSION is: $(GCC_VERSION)"
    @echo "CXXFLAGS are  : $(CXXFLAGS)"

I've tried using 'test' in lieu of 'expr', but it only allows numeric (integer) values.

Is there a better way to ascertain if the compiler supports C++11?

pan64 · 02-18-2023, 01:56 AM

There are two ways.
1. split version string to major/minor version numbers and compare them one by one
2. use something which can do that, but actually these are not floating numbers (may contain two dots for example), so better to use the previous solution.

dwhitney67 · 02-18-2023, 09:14 AM

I ended up relying on a combination of 'bc' (to perform a mathematical computation) and 'expr':

Code:

GCC_VERSION := $(shell $(CXX) -dumpversion)
CXX11_SUPPORTED := $(shell echo "$(GCC_VERSION) - 4.5 >= 0" | /usr/bin/bc -l)

ifeq ($(shell expr $(CXX11_SUPPORTED) '==' 1), 1)
    CXXFLAGS  += -std=c++11
endif

I wish there were something better than this kludge.