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makefileopenmpcompiler-flags

No rule to make target 'openmp'?

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?

Solution

  • 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

    $ make CXXFLAGS=-std=c++14

    note: no space, or equivalently with

    $ make CXXFLAGS="-std=c++14"

    again without a space.