Function Call Conventions

callcon.md

Most Common Calling Conventions

Most commons are cdecl, stdcall, fastcall

In function calls, parameters are pushed onto the stack/registers from right to left.

Example Function Pseudo Code

int func(int x, int y, int z, int m, int k);
 
int a, b, c, d, e, ret;

ret = func(a, b, c, d, e);

cdecl

• C declaration
• uses stack frame
• The caller cleans up the stack

Code

push e
push d
push c
push b
push a
call func
add esp, 20  ; cleaning up the stack. 5 integer parameters --> 4-bytes x 5 = 20

stdcall

• Standart call
• used by Windows API
• uses stack frame
• callee cleans up the stack

Code

push e
push d
push c
push b
push a
call func
; callee cleans up the stack so no need to add esp, 20 

fastcall

• uses registers(for first few instructions) and stack frame
• callee cleans up the stack
• ecx, edx (32-bit)(Microsoft)
• rcx, rdx, r8, r9 (64-bit)(Microsoft, Intel)

Code

push e
push d
push c
mov edx, b
mov ecx, a
call func

; for x64

push e
mov r9, d
mov r8, c
mov rdx, b
mov rcx, a
call func