Can't run mex-built application in Ubuntu 18.04

I followed this example to compile a mex application in Ubuntu 18.04. I successfully built engdemo object as shown in the output of mex:

mex -v -client engine engdemo.cpp
Verbose mode is on.
... Looking for compiler 'g++' ...
    ... Executing command 'which g++' ...Yes ('/usr/bin/g++').
    ... Executing command 'g++ -print-file-name=libstdc++.so' ...Yes ('/usr/lib/gcc/x86_64-linux-gnu/7/libstdc++.so').
    ... Executing command 'g++ -dumpversion' ...Yes ('7').
    ... Executing command 'which g++' ...Yes ('/usr/bin/g++').
    ... Looking for folder '/usr' ...Yes.
    ... Executing command 'g++ -dumpmachine' ...Yes ('x86_64-linux-gnu').
    Found installed compiler 'g++'.
Set INCLUDE = /usr/lib/gcc/x86_64-linux-gnu/7/include;/usr/include/c++/7;/usr/include/c++/7/x86_64-linux-gnu;/usr/include/c++/7/backward;
Options file details
-------------------------------------------------------------------
Compiler location: /usr/bin/g++
Options file: /home/tley7b/.matlab/R2021b/mex_C++_glnxa64.xml
CMDLINE1 : /usr/bin/g++ -c -DMATLAB_DEFAULT_RELEASE=R2017b  -DUSE_MEX_CMD   -D_GNU_SOURCE -DMATLAB_MEX_FILE  -I"/usr/local/MATLAB/R2021b/extern/include" -I"/usr/local/MATLAB/R2021b/simulink/include" -fexceptions -fPIC -fno-omit-frame-pointer -pthread -std=c++11 -O2 -fwrapv -DNDEBUG "/home/tley7b/test_mex/engdemo.cpp" -o /tmp/mex_20753447411181_1313/engdemo.o
CMDLINE2 : /usr/bin/g++ -pthread -Wl,--no-undefined   -O  /tmp/mex_20753447411181_1313/engdemo.o   "/usr/local/MATLAB/R2021b/sys/os/glnxa64/orig/libstdc++.so.6"  $MW_GLIBC_SHIM -Wl,--as-needed -Wl,-rpath-link,/usr/local/MATLAB/R2021b/extern/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/extern/bin/glnxa64" -Wl,-rpath-link,/usr/local/MATLAB/R2021b/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/bin/glnxa64" -leng -lMatlabEngine -lMatlabDataArray -lmx -lmat -lm -o engdemo
CXX : /usr/bin/g++
DEFINES : -DMATLAB_DEFAULT_RELEASE=R2017b  -DUSE_MEX_CMD   -D_GNU_SOURCE -DMATLAB_MEX_FILE 
MATLABMEX : -DMATLAB_MEX_FILE 
CFLAGS : -fexceptions -fPIC -fno-omit-frame-pointer -pthread
CXXFLAGS : -fexceptions -fPIC -fno-omit-frame-pointer -pthread -std=c++11
INCLUDE : -I"/usr/local/MATLAB/R2021b/extern/include" -I"/usr/local/MATLAB/R2021b/simulink/include"
CXXOPTIMFLAGS : -O2 -fwrapv -DNDEBUG
CXXDEBUGFLAGS : -g
LDXX : /usr/bin/g++
LDFLAGS : -pthread -Wl,--no-undefined 
LDTYPE : 
LINKEXPORT : 
LINKEXPORTVER : 
LINKLIBS : "/usr/local/MATLAB/R2021b/sys/os/glnxa64/orig/libstdc++.so.6"  $MW_GLIBC_SHIM -Wl,--as-needed -Wl,-rpath-link,/usr/local/MATLAB/R2021b/extern/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/extern/bin/glnxa64" -Wl,-rpath-link,/usr/local/MATLAB/R2021b/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/bin/glnxa64" -leng -lMatlabEngine -lMatlabDataArray -lmx -lmat -lm
LDOPTIMFLAGS : -O
LDDEBUGFLAGS : -g
MWCPPLIB : "/usr/local/MATLAB/R2021b/sys/os/glnxa64/orig/libstdc++.so.6"
OBJEXT : .o
LDEXT : 
SETENV : CC="gcc"
CXX="/usr/bin/g++"
CFLAGS="-fexceptions -fPIC -fno-omit-frame-pointer -pthread -DMATLAB_DEFAULT_RELEASE=R2017b  -DUSE_MEX_CMD   -D_GNU_SOURCE -DMATLAB_MEX_FILE "
CXXFLAGS="-fexceptions -fPIC -fno-omit-frame-pointer -pthread -std=c++11 -DMATLAB_DEFAULT_RELEASE=R2017b  -DUSE_MEX_CMD   -D_GNU_SOURCE -DMATLAB_MEX_FILE "
COPTIMFLAGS="-O2 -fwrapv -DNDEBUG"
CXXOPTIMFLAGS="-O2 -fwrapv -DNDEBUG"
CDEBUGFLAGS="-g"
CXXDEBUGFLAGS="-g"
MW_GLIBC_SHIM="$MW_GLIBC_SHIM"
LD="gcc"
LDXX="/usr/bin/g++"
LDFLAGS="-pthread -Wl,--no-undefined   "/usr/local/MATLAB/R2021b/sys/os/glnxa64/orig/libstdc++.so.6"  $MW_GLIBC_SHIM -Wl,--as-needed -Wl,-rpath-link,/usr/local/MATLAB/R2021b/extern/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/extern/bin/glnxa64" -Wl,-rpath-link,/usr/local/MATLAB/R2021b/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/bin/glnxa64" -leng -lMatlabEngine -lMatlabDataArray -lmx -lmat -lm "
LDDEBUGFLAGS="-g"
GCC : /usr/bin/g++
CPPLIB_DIR : /usr/lib/gcc/x86_64-linux-gnu/7/libstdc++.so
VER : 7
GCCDIR : /usr
GCC_TARGET : x86_64-linux-gnu
MATLABROOT : /usr/local/MATLAB/R2021b
ARCH : glnxa64
SRC : "/home/tley7b/test_mex/engdemo.cpp"
OBJ : /tmp/mex_20753447411181_1313/engdemo.o
OBJS : /tmp/mex_20753447411181_1313/engdemo.o 
SRCROOT : /home/tley7b/test_mex/engdemo
DEF : /tmp/mex_20753447411181_1313/engdemo.def
EXP : "engdemo.exp"
LIB : "engdemo.lib"
EXE : engdemo
ILK : "engdemo.ilk"
MANIFEST : "engdemo.manifest"
TEMPNAME : engdemo
EXEDIR : 
EXENAME : engdemo
OPTIM : -O2 -fwrapv -DNDEBUG
LINKOPTIM : -O
-------------------------------------------------------------------
Building with 'g++'.
/usr/bin/g++ -c -DMATLAB_DEFAULT_RELEASE=R2017b  -DUSE_MEX_CMD   -D_GNU_SOURCE -DMATLAB_MEX_FILE  -I"/usr/local/MATLAB/R2021b/extern/include" -I"/usr/local/MATLAB/R2021b/simulink/include" -fexceptions -fPIC -fno-omit-frame-pointer -pthread -std=c++11 -O2 -fwrapv -DNDEBUG "/home/tley7b/test_mex/engdemo.cpp" -o /tmp/mex_20753447411181_1313/engdemo.o
/usr/bin/g++ -pthread -Wl,--no-undefined   -O  /tmp/mex_20753447411181_1313/engdemo.o   "/usr/local/MATLAB/R2021b/sys/os/glnxa64/orig/libstdc++.so.6"  $MW_GLIBC_SHIM -Wl,--as-needed -Wl,-rpath-link,/usr/local/MATLAB/R2021b/extern/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/extern/bin/glnxa64" -Wl,-rpath-link,/usr/local/MATLAB/R2021b/bin/glnxa64 -L"/usr/local/MATLAB/R2021b/bin/glnxa64" -leng -lMatlabEngine -lMatlabDataArray -lmx -lmat -lm -o engdemo
MEX completed successfully.

I also setup LD_LIBRARY_PATH, PATH and search path correctly. However, I can't get the application to run by calling

./engdemo

The error is

./engdemo
↑
Invalid use of operator.

Any suggestion?

Answer 1

[Varun]

Hi Tuan,

Looks like you are trying to run the executable "engdemo" from MATLAB commad window. You are facing the following error:

./engdemo
↑
Invalid use of operator.

"./engdemo" is not the correct way to run any exetuable as long as you are using MATLAB commad window. This will work only outside the MATLAB i.e. Linux bash shell, windows command promt, etc.

To run such exetuables using MATLAB command window, you can use below methods:

1) Using "!":

>> !engdemo

2) Using "system":

>> system('engdemo');

Also, refer to the following documentation on running External Commands, Scripts, and Programs from the MATLAB:

https://www.mathworks.com/help/matlab/matlab_external/run-external-commands-scripts-and-programs.html

Hope it helps.