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# 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.
