How Do Calling Conventions Shape Function Argument Passing in C/C ?

Understanding Calling Conventions in C/C

Calling conventions define how functions pass and receive arguments and their values. C/C offers various calling conventions, each serving a specific purpose.

Number of Calling Conventions

There are several calling conventions available in C/C , including:

• cdecl
• syscall
• pascal
• stdcall
• fastcall
• vectorcall
• safecall
• Microsoft X64 Calling Convention

Meaning of Each Convention

cdecl:

• Arguments passed on the stack.
• Registers are designated for specific purposes (EAX for return value, EAX, ECX, EDX for caller-saved).

syscall:

• Similar to cdecl but does not preserve specific registers.

pascal:

• Parameters passed on the stack in left-to-right order.
• Callee responsible for stack cleanup.

stdcall:

• Callee cleans up the stack.
• Parameters pushed in right-to-left order, like cdecl.
• Registers designated for internal function use (EAX, ECX, EDX).

fastcall:

• First two arguments passed in registers (ECX and EDX).
• Remaining arguments pushed on the stack right-to-left.

vectorcall:

• Supports passing vector arguments using SIMD registers.
• Up to six registers allocated for vector arguments.

safecall (Delphi/Free Pascal):

• Encapsulates COM error handling.
• Exceptions returned in EAX as an HRESULT, while the result is passed by reference.

Microsoft X64 Calling Convention:

• Used in Windows and pre-boot UEFI.
• First four integer/pointer arguments passed in registers (RCX, RDX, R8, R9).

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