C++ internals. Francis Visoiu Mistrih - 1 / 41

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1 C++ internals Francis Visoiu Mistrih - francis@lse.epita.fr 1 / 41

2 Language features Standard Standard library ISO C++ Committee 2 / 41

3 ABI Specifies an interface between object files Implementation details: runtime functions, memory layout, etc. Depends on: Platform, architecture N4028 aims at defining a portable ABI We'll focus on the Itanium C++ ABI. 3 / 41

4 Example The standard allows virtual member function calls The ABI specifies that virtual calls should use a vtable 4 / 41

5 Name mangling void foo(int i) } void bar(int i) } C T bar T foo C T _Z3bari T _Z3fooi 5 / 41

6 Name mangling - Overloading void foo(int i) } void foo(float i) } C error: conflicting types for 'foo' C T _Z3foof T _Z3fooi 6 / 41

7 Name mangling - Overloading void foo(int); _Z3fooi void foo(float); _Z3foof 7 / 41

8 $ c++filt _ZNSt6vectorIiSaIiEEC2Ev $ llvm-cxxfilt _ZNSt6vectorIiSaIiEEC2Ev Name mangling - more std::vector<int, std::allocator<int>>::vector() Functions, templates, types, operators, namespaces vtables, rtti, thunks, ctor, dtor decltype, lambdas, member pointers, etc. 8 / 41

9 Class members struct str str(int field) : field_field} } void change_field(int new_field) field_ = new_field; } int field_; 9 / 41

10 Class members void str::change_field(int new_field) field_ = new_field; } first argument -> %rdi second argument -> %rsi <str::change_field(int)>: 0: mov %esi,(%rdi) 2: c3 retq 10 / 41

11 Class members void str::change_field(str *this, int new_field) this->field_ = new_field; } first argument -> %rdi (this) second argument -> %rsi (new_field) <str::change_field(int)>: 0: mov %esi,(%rdi) 2: c3 retq 11 / 41

12 Member pointers struct str int field_; int field2_; int str::*field_ptr = &str::field_; int str::*field2_ptr = &str::field2_; str s; s.*field_ptr = 30; s.*field2_ptr = 42; A pointer to data member is an offset from the base address of the class object containing it, represented as a ptrdiff_t. field_ptr == 0x0 field2_ptr == 0x4 12 / 41

13 Simple inheritance struct base int field_; struct derived : base int field2_; 13 / 41

14 Multiple inheritance struct base1 int field1_; struct base2 int field2_; struct derived : base1, base2 int field3_; 14 / 41

15 Simple inheritance - Cast derived d; base *b = &d; assert((void*)&d == (void*)&b); 15 / 41

16 Multiple inheritance - Cast derived d; base1 *b = &d; assert((void*)&d == (void*)&b); 16 / 41

17 Multiple inheritance - Cast derived d; base2 *b = &d; assert((void*)&d!= (void*)&b); push mov sub lea [...] add pop retq %rbp %rsp,%rbp $0x10,%rsp -0xc(%rbp),%rdi $0x10,%rsp %rbp 17 / 41

18 Virtual struct base int field_; virtual void foo() } 18 / 41

19 vtable 19 / 41

20 Virtual struct base int field_; virtual void foo() } struct derived : base int field2_; void foo() override } 20 / 41

21 vtable - override base *b = new base; 21 / 41

22 vtable - override base *b = new derived; 22 / 41

23 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; 23 / 41

24 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; 24 / 41

25 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; 25 / 41

26 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; 26 / 41

27 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } derived d; base2 &b = b; b.bar(); struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; 27 / 41

28 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } derived d; base2 &b = b; b.bar(); struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; void bar() override } 28 / 41

29 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } derived d; base2 &b = b; b.bar(); b.vptr[0](&b); decltype(b) == base2& struct derived : base1, base2 int field3_; void bar() override } 29 / 41

30 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; void bar() override } derived d; base2 &b = b; b.bar(); b.vptr[0](&b); decltype(b) == base2& this!= &d Inside derived::bar 30 / 41

31 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } derived d; base2 &b = b; b.bar(); b.vptr[0](&b); decltype(b) == base2& Inside derived::bar struct derived : base1, base2 int field3_; void bar() override } this!= &d Thunks. 31 / 41

32 Virtual - multiple inheritance struct base1 int field_; virtual void foo() } struct base2 int field2_; virtual void bar() } struct derived : base1, base2 int field3_; void bar() override } 32 / 41

33 Thunks thunk to derived::bar() addq $-16, %rdi jmp derived::bar() thunk to derived::bar() this -= 16 derived::bar(this) } 33 / 41

34 Object files ELF sections struct str void foo() } void bar() } Sections [4].group GROUP [5].text._ZN3str3foo PROGBITS a 0 AXG [6].group GROUP [7].text._ZN3str3bar PROGBITS a 0 AXG / 41

35 Object files Symbols struct str void foo() } void bar() } nm -C W str::bar() W str::foo() When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error. 35 / 41

36 Object files Symbols struct str void foo(); void bar(); void str::foo() void str::bar() nm -C T str::bar() T str::foo() ODR: one definition rule 36 / 41

37 Object files Symbols template <typename T> struct str T foo(); template <typename T> T str<t>::foo() return } str<int> s; s.foo(); W str<int>::foo() Templates are weak. 37 / 41

38 Where is the vtable? In the code segment (read + executable) Avoid runtime modifications The virtual table for a class is emitted in the same object containing the definition of its key function 38 / 41

39 The virtual table for a class is emitted in the same object containing the definition of its key function Key function The first non-pure virtual function that is not inline struct str virtual void foo() } virtual void bar(); void str::bar() } If there is no key function, it is emitted everywhere used struct str virtual void foo() } 39 / 41

40 More Virtual-base inheritance, exceptions, SEH, RTTI, unwind tables, ABI runtime support, new/delete, dynamic_cast, etc. 40 / 41

41 References Any Slideshow created using remark. 41 / 41

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