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Step 1: add

target_link_libraries(${PROJECT_NAME} 
  i2c
)

To your CMakeList.txt.

Step 2: Declare the headers as "C",

extern "C"
{
    #include<linux/i2c-dev.h>
    #include <i2c/smbus.h>
}

Because libi2c.so is a dynamic library compiled with gcc.

If you does not declare it explicitly, the compiler of ROS , which is g++, will generate different function names in the symbol table thus the ld command cannot find any match in the i2c library.

Referece: https://en.wikipedia.org/wiki/Name_mangling