CallingConvention

CallingConvention is a low-level contract between a caller and a callee that specifies how a function call is performed at the machine level. It governs how arguments are passed (which registers or stack slots, and in what order), how the stack is cleaned up after the call, how the return value is delivered, and how function names may be decorated for linkage. It also defines which registers must be preserved across calls and which may be overwritten by the callee. Because it fixes binary interfaces, the calling convention affects code compatibility across modules and languages.

In practice, a calling convention establishes several practical rules: whether arguments are passed in registers or on the stack, the sequence of argument evaluation, who is responsible for cleaning the stack, and how variadic arguments are handled. It may also indicate how the callee should present the return value and how non-volatile registers should be preserved by the callee. Name decoration or mangling is sometimes part of the convention to link functions correctly across translation units or language boundaries.

There are many conventions, often tied to architecture and toolchains. Common examples include cdecl, stdcall, fastcall, thiscall, and vectorcall, each with different rules for argument passing, stack cleanup, and variadic support. Platform-specific Application Binary Interfaces (ABIs) such as System V on Unix-like systems and Windows calling conventions also prescribe their own details for x86 and x64 targets. These conventions are critical for language interoperability, foreign function interfaces, and the correctness of compiled binaries, tracing issues in linking, debugging, and performance-sensitive interoperation.