No rule to make target 'openmp'?

问题

The following is defined in the makefile:

CXX = g++
CXXFLAGS = -std=c++11

I would like to compile my code with OpenMP directives without changing the original makefile (which runs perfectly). The manual suggests a way to do this, is by changing it on the command line. However, when I run,

 make CXXFLAGS=-std=c++11 -fopenmp

the error mentioned before pops up. Can someone help me understand what I'm doing wrong?

回答1:

The problem here is the space between -std=c++11 and -fopenmp. It splits these two arguments up and the -fopenmp is interpreted as the option -f then openmp is interpreted as a makefile that make attempts to build because it can't find it in the current directory.

You need to execute make with quotes (") like

$ make CXXFLAGS="-std=c++11 -fopenmp"

this will pass CXXFLAGS= -std=c++11 -fopenmp to make. We can see this behaviour with the following simple makefile.

CXX = g++
CXXFLAGS = -std=c++11

all:
    @echo $(CXXFLAGS)

Running make will produce the output

$ make
-std=c++11

Running make CXXFLAGS="-std=c++11 -fopenmp" will produce

$ make
-std=c++11 -fopenmp

the correct output and what we were hoping to achieve.

As an aside the command, make CXXFLAGS= -std=c++11 -fopenmp, in your question should produce more errors than just

make: openmp: No such file or directory
make: *** No rule to make target 'openmp'.  Stop.

because of the space between CXXFLAGS= and -std=c++11. I get

$ make CXXFLAGS= -std=c++11 -fopenmp
make: invalid option -- =
make: invalid option -- c
make: invalid option -- +
make: invalid option -- +
make: invalid option -- 1
make: invalid option -- 1

If for instance you want to compile with -std=c++14 instead of -std=c++11 you would need to execute make with