Extract assembly from an object file or assembly file, optionally for a specific function

Prerequisites

The input must be a compiled object file (.o or .obj) or assembly file (.s or .asm). If you have source code, compile it first (e.g. via CMake build or standalone clang -S).

Arguments

$1 = input file (.o, .obj, .s, or .asm)
$2 = function name (optional — if omitted, disassemble the entire file)

Examples:

trees.c.o compress_block — single function
trees.c.o — entire file
trees.s my_function
trees.obj compress_block

Steps

1. Disassemble object files (`.o` / `.obj`)

Use the appropriate tool for the platform:

Platform	Tool	Notes
macOS	`llvm-objdump`	Ships with Xcode
Linux	`objdump`	GNU binutils 2.32+
Windows	`llvm-objdump`	Ships with VS LLVM/clang workload or standalone LLVM

Single function ($2 provided):

Platform	Command
macOS	`llvm-objdump --disassemble-symbols=_<name> --no-show-raw-insn <file>`
Linux	`objdump --disassemble=<name> --no-show-raw-insn <file>`
Windows	`llvm-objdump --disassemble-symbols=<name> --no-show-raw-insn <file>`

Entire file ($2 omitted):

Platform	Command
macOS	`llvm-objdump -d --no-show-raw-insn <file>`
Linux	`objdump -d --no-show-raw-insn <file>`
Windows	`llvm-objdump -d --no-show-raw-insn <file>`

Windows dumpbin fallback — if llvm-objdump is not available:

VSPATH=$(vswhere -latest -property installationPath)
cmd.exe /c "call \"${VSPATH}\VC\Auxiliary\Build\vcvarsall.bat\" amd64 >nul 2>&1 && dumpbin /disasm <file>.obj"

Then manually extract the function: find <function_name>: label, collect until the next label or summary line.

2. Extract from assembly files (`.s` / `.asm`)

Read the file directly. If $2 is provided, extract the named function:

Mach-O: Find _<function_name>:, collect until next ^_[a-zA-Z] label (excluding _Ltmp, _LCFI, _LBB prefixes).
ELF/other: Find <function_name>:, collect until the next function label.

3. Clean up and output

Filter out assembler directives (. prefix), comments (#, ;, //, @), and blank lines
Keep only instruction lines
Output the assembly to stdout with a summary line showing the instruction count

Symbol Names

Symbol names differ by platform — use this when passing to --disassemble-symbols or searching in .s files:

Platform	C function `foo`	C++ function `foo`
macOS (Mach-O)	`_foo`	`_Z3foov` (Itanium mangled, `_` prefixed)
Linux (ELF)	`foo`	`_Z3foov` (Itanium mangled)
Windows (PE/COFF)	`foo`	`?foo@@YAXXZ` (MSVC decorated)

Architecture Notes

Architecture	Key details
x86-64	`offset(%reg)` memory syntax. `leaq (%rip)` is PC-relative, not a load.
AArch64	`[reg, #offset]` memory syntax. Loads: `ldr`, `ldur`, `ldp`, `ldrb`, `ldrh`, `ldrsw`. Stores: `str`, `stur`, `stp`, `strb`, `strh`. Apple Silicon compiles natively.
x86-64 (MSVC)	Intel syntax (dest first): `mov rax, [rcx+168]`. Args in `rcx`, `rdx`, `r8`, `r9`. C++ names may be decorated (`?name@@...`).

CMake Build Paths

Unix: build/CMakeFiles/<target>.dir/<source>.o
Windows (Ninja): build/CMakeFiles/<target>.dir/<source>.obj
Windows (VS): build/CMakeFiles/<target>.dir/<config>/<source>.obj

extract-asm