prepare (1/2): 
finished

make (2/2): 
g++ -g -Wall -O2 -o c2.o -c c2.cpp 
c2.cpp: In function 'void fence_decrypt(std::__cxx11::string&, int, int)':
c2.cpp:43:9: warning: unused variable 'row' [-Wunused-variable]
     int row = key;
         ^~~
c2.cpp: In function 'int main(int, char**)':
c2.cpp:66:25: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
     char* short_options="ge:d:k";
                         ^~~~~~~~
g++ -g -Wall -O2 -o c2 c2.o
finished

all tasks finished, build successfully