What is ABI?

Learn the ABI (Application Binary Interface) concept.

What is ABI?

The ABI (Application Binary Interface) defines how compiled object files talk to each other. It is not source syntax but binary compatibility: calling convention, struct layout, name mangling and vtable.

If two compilers follow the same ABI, one can ship a compiled library and the other a main program and they link. When ABIs differ, even identical source code yields mismatched symbol names and link errors. This is why GCC and Clang share the Itanium ABI; MSVC uses a different one.

ABI components

The four core ABI components are: calling convention (how parameters are passed via registers or stack), struct layout (how members are arranged in memory), name mangling (encoding the function signature into a symbol name) and vtable (the virtual function table).

If any of these four changes, the ABI breaks. For example, adding a new virtual function to a class changes the vtable; old compiled code would call the wrong function. This is why library authors must keep ABI stable.

Exercise Write the compute function

Write int compute(int x) with body return x * 3;. In main write std::cout << compute(4);.

Show example solution

#include <iostream>

int compute(int x) {
    return x * 3;
}

int main() {
    std::cout << compute(4);
    return 0;
}

Test yourself

What does ABI define?
```
g++ -c lib.cpp
clang++ main.cpp lib.o
```
- Only source code syntax
- The compiler version number
- Binary compatibility of executables
- Class naming style
Show answer

Binary compatibility of executables — ABI defines binary details like calling convention, layout and mangling.

Complete the extern C declaration

extern "C" void ____();

Show answer

extern "C" void greet();

Match each component to its role

calling convention

Parameter passing
struct layout

Member arrangement
name mangling

Symbol naming
vtable

Virtual function table

Next lesson: Name mangling

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies