[llvm-dev] How to create a sprintf call in IR

[Jeremy Lakeman via llvm-dev]

You know the C code you want to replicate. So ask llvm to generate the C++
api calls that would produce that output.

eg;
https://gist.github.com/alloy/d86b007b1b14607a112f


On Wed, Nov 25, 2015 at 5:34 AM, zhi chen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I created a global char array, char buffer[1024]. I want to call a
> function to append the string information to the buffer repeatedly. For
> example, I need to implement the following code, where length, a, b, c, are
> global variables.
>
> int length = 0;
> length += sprintf(buffer+length, "str%d", a);
> length += sprintf(buffer+length, "str%c", b);
> length += sprintf(buffer+length, "str%d", c);
>
> I did it in the following way.
> 1. Create the global array:
>     IntegerType CHARTYPE = IntegerType::get(llvm::getGlobalContext(), 8);
>     ArrayType* ty = ArrayType::get(CHARTYPE, 1024);
>
>     GlobalVariable* buffer = new GlobalVariable(mod, ty, false,
> GlobalValue::ExternalLinkage, 0, "buffer");
>
> 2. Create sprintf
>     std::vector<Type*> agrs(2, Type::getInt8PtrTy(ctxt));
>     FunctionType* sprintfFT = FunctionType::get(Type::getInt32Ty(ctxt),
> args, true);
>     Constant* sprintf = mod->getOrInsertFunction("sprintf", sprintfFT);
>
>     Now, I got stuck to create teh function call for sprintf.
>
>     I didn't it like the following:
>    Value* str = builder.CreateGlobalStringPtr("str", "");
>    Value* data  = builder.CreateLoad(a);  //load a, b, or c
>    Value* buf = builder.CreateLoad(buffer);
>    builder.CreateCall3((value*)sprintf, buf, str, data);
>
> I want to test the sprintf function before adding the offset, but it
> didn't work. The error information is "llvm.sys:PrintStackTrace(_IO_FILE)".
> Any help will be appreciated.
>
> Best,
> Zhi
>
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