Motivating Example
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
void foo(){
const size_t N = 100'000'000;
std::vector<int> v(N);
// fill v with 1,2,3,...
std::iota(v.begin(), v.end(), 1);
// sum in a tight loop
long long sum = 0;
for (size_t i = 0; i < N; ++i) {
sum += v[i];
}
std::cout << "sum = " << sum << "\n";
}
int add(int a, int b) {
return a + b;
}
int main() {
foo();
return 0;
}
- Compile to an object file
1
g++ -c example.cpp -o example.o
- Inspect raw (mangled) symbols
nm example.o, you might see:
1
2
0000000000000000 T _Z3foov
000000000000015c T _Z3addii
_Z3foov→ mangled for foo()_Z3addii→ mangled for add(int,int)
If we compile this file to an object file example.o:
1
g++ -C example.cpp
- We can see symbols with
nm:nm example.o:
1
2
000000000000015c T _Z3barv
0000000000000000 T _Z3foov
-
Demangle a symbol
echo '_Z3addii' | c++filt, output:add(int, int) -
Demangle in-place with nm
nm -C example.o. Example output:
1
2
3
4
5
6
7
8
0000000000000000 T foo() ← foo() at offset 0x0 in .text
000000000000015c T add(int, int) ← add() at offset 0x15c in .text
U __stack_chk_fail ← undefined symbol
0000000000000000 W __gnu_cxx::new_allocator<int>::allocate(...)
← weak symbol (address 0 until link)
0000000000000000 r __pstl::execution::v1::seq
← read-only data at offset 0 in .rodata
nmis a tool in GNUBinutilsthat lists symbol table of an.o,.so, and static library.a, or executable- Symbol types are:
T= global (external) function in .textr= read-only dataW= weak symbol
- Symbol Interpretation
foois the first symbol in this section, so it lives at memory0000000000000000__gnu_cxx::new_allocator<int>...is a weak symbol, meaning it has a default value of 0 until the final link decides their address.- Once you’ve linked into an executable or
.som these offsets become real address.
