That means that you use function strfind, but its definition is missing -- it's not in your code or the libraries you link. You need to link the library which contains the strfind function. I've never used the strfind function and I don't know which library is it in. Is it this function
So either you will have to link a library that contains the function, write the function yourself, or use a different function, such as strstr() from string.h, if it fits you needs.
what is option -lgen and where can i find it .
-lgen tells the linker to link a libgen.so library. I think the functionality of libgen is provided by glibc in linux.