The linker is complaining that you didn't link in the execprocess.obj object data.

If you are using the GCC from the command-line, make sure to compile as:
g++ -Wall -o extProg extProg.cpp execprocess.cpp

If you are using VC++ or C++Turbo or some other IDE, make sure you add execprocess.cpp and execprocess.hpp to the project before compiling.

It looks cryptic, but ld is the name of the linker on *nix systems (and also with the GCC on all supported systems). A non-zero exit status means something went wrong...

The complaints about "undefined reference to X" means that the compiler knows what X is, but it cannot find X. Since X is defined in another cpp file, that file must be compiled and linked to the cpp file containing main().

For example:
g++ -Wall -c execprocess.cpp (compile to "execprocess.o")
g++ -Wall -c extProg.cpp (compile to "extProg.o")
g++ extProg.o execprocess.o (have GCC tell the linker connect the two object files into a single executable file)
Since extProg.o uses stuff in execprocess.o, the linker puts them together so that it will work.

Hope this helps.