9.6.7.5 Defined Built-in Function

The DEFINED(symbol) built-in function return 1 if symbol is in the linker global symbol table and is defined; otherwise it returns 0. You can use this function to provide default values for symbols. For example, the following script fragment shows how to set a global symbol begin to the first location in the .text section, but if a symbol called begin already existed, its value is preserved:

SECTIONS { ...
  .text : {
   begin = DEFINED(begin) ? begin : . ;
   ...  
  }
  ...
}