[LLVMdev] Disassembly arbitrary machine-code byte arrays

Kevin Enderby enderby at apple.com
Mon Dec 19 12:14:52 PST 2011

Hi Aiden,

The 'C' based interface you could use in is llvm/include/llvm-c/Disassembler.h, which in there is:

 * Disassemble a single instruction using the disassembler context specified in
 * the parameter DC.  The bytes of the instruction are specified in the
 * parameter Bytes, and contains at least BytesSize number of bytes.  The
 * instruction is at the address specified by the PC parameter.  If a valid
 * instruction can be disassembled, its string is returned indirectly in
 * OutString whose size is specified in the parameter OutStringSize.  This
 * function returns the number of bytes in the instruction or zero if there was
 * no valid instruction.
size_t LLVMDisasmInstruction(LLVMDisasmContextRef DC, uint8_t *Bytes,
                             uint64_t BytesSize, uint64_t PC,
                             char *OutString, size_t OutStringSize);

This is used in darwin's otool(1) which is an objdump(1) like tool.  It ends up in the libLTO shared library.


On Dec 19, 2011, at 1:23 AM, James Molloy wrote:

> Hi Aiden,
> The easiest thing I can do is to point you to the source of the "llvm-mc" tool, which does exactly what you ask in its "-disassemble" mode. The code is rather small, so it should be easy to work out.
> tools/llvm-mc
> Cheers,
> James
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Aidan Steele
> Sent: 19 December 2011 04:30
> To: llvmdev at cs.uiuc.edu
> Subject: [LLVMdev] Disassembly arbitrary machine-code byte arrays
> Hi,
> My apologies if this appears to be a very trivial question -- I have
> tried to solve this on my own and I am stuck. Any assistance that
> could be provided would be immensely appreciated.
> What is the absolute bare minimum that I need to do to disassemble an
> array of, say, ARM machine code bytes? Or an array of Thumb machine
> code bytes? For example, I might have an array of unsigned chars --
> how could I go about decoding these into MCInst objects? Does such a
> decoding process take place in one fell swoop or do I parse the stream
> one instruction at a time? Can I ask it to "decode the next 10 bytes"?
> What follows is my (feeble) attempt at getting started. It probably
> doesn't help that I am only familiar with C and Objective-C and find
> C++ syntax absolutely bewildering.
> Kind regards,
> Aidan Steele
> int main (int argc, const char *argv[])
> {
> LLVMInitializeARMTargetInfo();
> LLVMInitializeARMTargetMC();
> LLVMInitializeARMAsmParser();
> LLVMInitializeARMDisassembler();
> const llvm::Target Target;
> llvm::OwningPtr<const llvm::MCSubtargetInfo>
> STI(Target.createMCSubtargetInfo("", "", ""));
> llvm::OwningPtr<const llvm::MCDisassembler>
> disassembler(Target.createMCDisassembler(*STI));
> llvm::OwningPtr<llvm::MemoryBuffer> Buffer;
> llvm::MemoryBuffer::getFile(llvm::StringRef("/path/to/file.bin"), Buffer);
> llvm::MCInst Inst;
> uint64_t Size = 0;
> disassembler->getInstruction(Inst, Size, *Buffer.take(), 0,
> llvm::nulls(), llvm::nulls());
> //  llvm::StringRef TheArchString("arm-apple-darwin");
> //  std::string normalized = llvm::Triple::normalize(TheArchString);
> //
> //  llvm::Triple TheTriple;
> //  TheTriple.setArch(llvm::Triple::arm);
> //  TheTriple.setOS(llvm::Triple::Darwin);
> //  TheTriple.setVendor(llvm::Triple::Apple);
> //  llvm::Target *TheTarget = NULL;
> return 0;
> }
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