Calling Conventions
Compilers handle the function calling processes differently, and we have several conventions.
Adapted to how programmers use the languages (number of arguments).
Adapted to several registers and other architectural details.
These dictate:
How arguments are passed to the callee.
How return codes are passed to the caller.
Who allocates the stack?
Who stores important registers such as the Program Counter.
cdecl
Created by Microsoft compilers, widely used in x86, including GCC.
The standard method for most code in x86 environments.
Arguments: passed in the stack, in inverted order (right to left).
First argument is pushed last.
Registers: Mixed
Caller saves RIP, A, C, D.
Callee saves BP, and others and restores RIP
stdcall
Official call convention for the Win32API (32 bits).
Arguments: passed in the stack from right to left.
Additional arguments are passed in the stack.
Registers: Callee saves.
Except EAX, ECX and EDX which can be freely used.
Stack Red Zone: Leaf functions have a 128-byte area kept safe which doesn’t need to be allocated.
Can be used for local variables, and avoids the use of two operations (sub rsp, add rsp).
Leaf functions are functions that do not call others.
fastcall
Official call convention for Win32API 64bits.
Arguments: left to right, first as registers.
Additional arguments are passed in the stack.
Registers: Caller saves.
Stack Shadow Zone: Leaf functions have a 32 byte area kept safe which doesn’t need to be allocated.
Can be used for local variables, and avoids the use of two operations (sub rsp, add rsp).
Leaf functions are functions that do not call others.
Fastcall for 64bits (Windows)
Official convention for x86_64 architectures with MSVC (Windows).
Mandatory if compiling for x86_64 in Windows.
Arguments: passed as RDX, RCX, R8, R9.
Additional arguments are passed in the stack (right to left).
Registers: Mixed.
Caller save: RAX, RCX, RDX, R8, R9, R10, R11.
Callee save: RBX, RBP, RDI, RSI, RSP, R12, R13, R14, and R15.
Stack Red Zone: Leaf functions have a 32 byte area kept safe, allocated by the callee.
Can be used to store RDX, RCX, R8, R9.
(Leaf functions are functions that do not call others).
System V AMD64 ABI
Official convention for x64 architectures using Linux, BSD, Unix, and Windows.
Arguments: passed as RDI, RSI, RDX, RCX, R8, R9.
Additional arguments are passed in the stack.
Registers: Caller saves.
Except for RBX, RSP, RBP, R12-R15 which callee must save if they are used.
Stack Red Zone: Leaf functions have a 128-byte area kept safe which doesn’t need to be allocated.
Can be used for local variables, and avoids the use of two operations (sub rsp, add rsp).
Leaf functions are functions that do not call others.
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