9.6.7.11 Sizeof Built-in Function

The SIZEOF(section) built-in function return the size in bytes of the named section, if that section has been allocated. If the section has not been allocated when this is evaluated, the linker will report an error. In the following example, symbol_1 and symbol_2 are assigned identical values:

SECTIONS{ ...
  .output {
    .start = . ;
    ...   
    .end = . ;
    }
  symbol_1 = .end - .start ;
  symbol_2 = SIZEOF(.output);
...
}