How to use the LLVMBuildGEP function in the LLVM C API - c

Question

I'm trying to make an LLVM function that reads a field from a struct i pass, I've googled some stuff about getelementptr instruction and i guess that's what I'll need here, but how is it exactly used in the C API.
The struct is defined in C but I'm trying to access its fields via LLVM since they're in the same runtime I'm guessing it will work? (Correct me if I'm wrong)
Here's the code so far.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <llvm-c/Core.h>
#include <llvm-c/Analysis.h>
#include <llvm-c/ExecutionEngine.h>
typedef struct {
char* name;
int age;
} user;
int main() {
LLVMTypeRef type = LLVMStructCreateNamed(LLVMGetGlobalContext(), "struct.user");
LLVMModuleRef module = LLVMModuleCreateWithName("test");
LLVMBuilderRef builder = LLVMCreateBuilder();
LLVMTypeRef ret = LLVMFunctionType(LLVMPointerType(LLVMInt32Type(), 0),
(LLVMTypeRef[]){ LLVMPointerType(type, 0) }, 1, false);
LLVMValueRef fn = LLVMAddFunction(module, "stuff", ret);
LLVMBasicBlockRef entry = LLVMAppendBasicBlock(fn, "entry");
LLVMPositionBuilderAtEnd(builder, entry);
LLVMValueRef gep = LLVMBuildGEP(builder, LLVMGetParam(fn, 0), (LLVMValueRef[]){ LLVMConstInt(LLVMInt32Type(), 0, false),
LLVMConstInt(LLVMInt32Type(), 1, false) }, 2, "gep");
LLVMBuildRet(builder, gep);
char *error = NULL;
LLVMVerifyModule(module, LLVMAbortProcessAction, &error);
LLVMDisposeMessage(error);
LLVMExecutionEngineRef engine;
error = NULL;
LLVMLinkInMCJIT();
LLVMInitializeNativeTarget();
LLVMInitializeNativeAsmPrinter();
LLVMInitializeNativeAsmParser();
if(LLVMCreateExecutionEngineForModule(&engine, module, &error) != 0) {
fprintf(stderr, "failed to create execution engine\n");
abort();
}
if(error) {
fprintf(stderr, "error: %s\n", error);
LLVMDisposeMessage(error);
exit(EXIT_FAILURE);
}
user m;
m.name = "John";
m.age = 17;
int* (*stuff)(user*) = (int* (*)(user*)) LLVMGetFunctionAddress(engine, "stuff");
printf("%d\n", *stuff(&m));
LLVMDisposeBuilder(builder);
LLVMDisposeExecutionEngine(engine);
return 0;
}
Basically the function I'm trying to build is equal to something like this in C
int* stuff(user* u) {
return &u->age;
}
So what happens is it crashes with a segmentation fault, i've debugged it using GDB and it seems like it happens in the line where i do the BuildGEP call with the following error from the LLVM shared library
Program received signal SIGSEGV, Segmentation fault.
0xb533baec in llvm::PointerType::get(llvm::Type*, unsigned int) ()
Now at this point i have no idea what i'm doing.
The main 2 questions i need answer for:
How do you use the LLVMBuildGEP from the C API?
Am i thinking this completely wrong?

Answer 1

LLVMStructCreateNamed creates an opaque struct type (like struct user; would in C), meaning LLVM doesn't know what the elements inside are. You'll need to populate that struct with a definition, likely using LLVMStructSetBody. So something like:
LLVMTypeRef struct_user_members[] = {
LLVMPointerType(LLVMInt8Type(), 0),
LLVMInt32Type()
};
LLVMStructSetBody(type, struct_user_members, 2, false);
After that, LLVMBuildGEP should be able to dereference the members, which it looks to me like you've done appropriately.