10.6.7.6 DEFINED(symbol)

Return 1 if symbol is in the linker global symbol table and is defined; otherwise return 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 : . ;
    ...
  }
  ...
}