Symbol Table Section

// one symbol table entry.
typedef struct {
	Elf32_Word		st_name; // an index into the symbol name stored in symbol string table section.
	Elf32_Addr		st_value; // the value of the associated symbol. may be an address, absolute value, etc.
	Elf32_Word		st_size; // symbol's size, e.g. data object's size; 0 if unknown, or no size.
	uint8_t			st_info;  // symbol's type and binding attributes.
	uint8_t			st_other;
	Elf32_Half		st_shndx; // Every symbol is defined in relation to some section; this memober holds the relavant section header table index.
} Elf32_Sym;

// lld/ELF/Symbols.h

// Represents a common symbol.
//
// On Unix, it is traditionally allowed to write variable definitions
// without initialization expressions (such as "int foo;") to header
// files. Such definition is called "tentative definition".
//
// Using tentative definition is usually considered a bad practice
// because you should write only declarations (such as "extern int
// foo;") to header files. Nevertheless, the linker and the compiler
// have to do something to support bad code by allowing duplicate
// definitions for this particular case.
//
// Common symbols represent variable definitions without initializations.
// The compiler creates common symbols when it sees varaible definitions
// without initialization (you can suppress this behavior and let the
// compiler create a regular defined symbol by -fno-common).
//
// The linker allows common symbols to be replaced by regular defined
// symbols. If there are remaining common symbols after name resolution is
// complete, they are converted to regular defined symbols in a .bss
// section. (Therefore, the later passes don't see any CommonSymbols.)
class CommonSymbol : public Symbol {
public:
  CommonSymbol(InputFile *file, StringRefZ name, uint8_t binding,
               uint8_t stOther, uint8_t type, uint64_t alignment, uint64_t size)
      : Symbol(CommonKind, file, name, binding, stOther, type),
        alignment(alignment), size(size) {}

  static bool classof(const Symbol *s) { return s->isCommon(); }

  uint32_t alignment;
  uint64_t size;
};