Calling Conventions: cdecl, stdcall, and System V
Compare the major x86 calling conventions so your assembly and C code agree on where arguments go, who cleans the stack, and which registers are preserved.
Calling Conventions: cdecl, stdcall, and System V is a free Assembly Language & x86 Low-Level Systems Programming lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Assembly Language & x86 Low-Level Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Conventions Matter
For assembly and C to interoperate, both sides must agree on a calling convention: how arguments are passed, who cleans the stack, and which registers must be preserved.
Argument Passing
Conventions differ on where arguments go: on the stack, in registers, or a mix. Getting this wrong corrupts data silently.
cdecl
cdecl (32-bit, the C default) pushes arguments on the stack right to left, and the caller cleans them up after the call.
push arg2
push arg1
call func
add esp, 8 ; caller cleansstdcall
stdcall also passes on the stack right to left, but the callee cleans the stack via ret n. The Win32 API uses it.
ret 8 ; callee cleans 8 bytesCaller vs Callee Cleanup
The key contrast:
- cdecl: caller cleans, supports variadic functions
- stdcall: callee cleans, smaller call sites
System V AMD64
The 64-bit Linux/macOS convention passes the first six integer args in registers: RDI, RSI, RDX, RCX, R8, R9; extras go on the stack.
mov rdi, arg1
mov rsi, arg2
call funcMicrosoft x64
64-bit Windows uses a different register set: RCX, RDX, R8, R9 for the first four args, plus 32 bytes of shadow space.
Return Values
Integer and pointer results come back in EAX (32-bit) or RAX (64-bit); floating point uses XMM0 in 64-bit conventions.
Preserved vs Scratch Registers
Each convention designates callee-saved registers (the callee must restore them) and caller-saved ones (free to clobber). Violating this corrupts the caller.
Stack Alignment
System V requires the stack to be 16-byte aligned at a call. Forgetting this crashes code that uses SSE instructions.
Matching the Compiler
Always declare functions on the C side with the matching convention attribute (e.g. __stdcall) so both halves agree.
Quick Check
Test your understanding of calling conventions.
Recap
You learned calling conventions:
- cdecl: stack args, caller cleans, supports varargs
- stdcall: stack args, callee cleans
- System V x64: args in RDI,RSI,RDX,RCX,R8,R9
- Return in EAX/RAX; respect callee-saved registers and alignment
Frequently asked questions
Is the “Calling Conventions: cdecl, stdcall, and System V” lesson free?
Yes — the full text of “Calling Conventions: cdecl, stdcall, and System V” is free to read here on the web, and the Assembly Language & x86 Low-Level Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Assembly Language & x86 Low-Level Systems Programming course, upgrade to CoddyKit PRO.
What will I learn in “Calling Conventions: cdecl, stdcall, and System V”?
Compare the major x86 calling conventions so your assembly and C code agree on where arguments go, who cleans the stack, and which registers are preserved. You practise Assembly Language & x86 Low-Level Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Assembly Language & x86 Low-Level Systems Programming?
No prior experience is required. Assembly Language & x86 Low-Level Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Calling Conventions: cdecl, stdcall, and System V” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Assembly Language & x86 Low-Level Systems Programming lesson?
Yes. Every Assembly Language & x86 Low-Level Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Calling Assembly from C
- Calling C from Assembly
- Mixed-Language Programming Techniques
- Calling Conventions: cdecl, stdcall, and System V