How to create function prototype that recognizes arguments passed by reference in IDA Pro

Question

I have a function prototype generated by IDA that looks like this:

However, it looks like there is a struct being referenced at the instruction lea edi, [esp+290h+var_240]. I would like IDA to reference this passed address as an argument in its function prototype.

I have tried the approach below, but it is clear that it does not work.

What I would like to see is something like this

Any help or suggestions would be greatly appreciated!

Answer 1

My guess is that the function signature should be something like:

int __usercall Call_HTTP@<eax>(int x, int y, void* http_object@<edi>);

• __usercall means the calling convention for the function is not a standard one (like stdcall, cdecl, etc.) as the function passes two arguments on stack and one in edi.
• @<eax> : function returns a value in eax register.
• int x and int y are passed on the stack.
• void* http_object@<edi> : http_object is a void* passed through the edi register.

You can change the type of the http_object by adding a proper structure for this object to the known IDA structures and pass the real type instead of void*.

For more information on function signature, see the IDA help on Set function/item type.

Edit

The right signature should be:

int __userpurge Call_HTTP@<eax>(int x, int y, void* http_object@<edi>);

As stated by @itsbriany:

• for __stdcall and __userpurge calling conventions, the callee cleans up the stack.
• in __cdecl and __usercall conventions, the caller cleans up the stack.