C++ Symbol Resolution

Symbol Resolution

Symbol resolution is the linker's process of matching undefined symbols (like function calls) with their definitions across object files.

Symbol Types

Strong Symbols

Initialized globals and functions:

int x = 10;        // Strong
void func() { }    // Strong

Weak Symbols

Uninitialized globals:

int y;             // Weak
__attribute__((weak)) int z = 5;  // Explicitly weak

Undefined Symbols

External references:

extern int external_var;  // Undefined
void external_func();     // Undefined

Resolution Rules

1. Multiple strong symbols → Error
2. One strong, multiple weak → Choose strong
3. Multiple weak symbols → Choose any
4. No definition found → Undefined reference error

Name Mangling

C++ encodes function signatures for overloading:

void func(int)     // _Z4funci
void func(double)  // _Z4funcd
void func(int, int) // _Z4funcii

// Prevent mangling with C linkage
extern "C" void c_func();

Common Errors & Solutions

Undefined Reference

undefined reference to `function()'

Solutions:

• Ensure function is implemented
• Link the correct library
• Check name mangling (C vs C++)

Multiple Definition

multiple definition of `variable'

Solutions:

• Use extern in headers
• Use inline for C++17
• Use anonymous namespaces

Viewing Symbols

# List symbols
nm object.o

# Demangle C++ symbols
nm object.o | c++filt

# Undefined symbols only
nm -u object.o

Next Steps

• Understand Dynamic Linking
• Learn about GOT/PLT
• Explore Memory Layout