Hi. VPtr<> is supposed to be equivalent to void *. Even with a c void *
though, you can't (in standard c) use it as an array of bytes. If you need
it to be an array of bytes, you need to use VPtr<uint8_t>. There are some
facilities to cast VPtrs of different types, but I don't remember how
extensive that support is, or if VPtr<> is part of it. Also VPtrs in
general can't be used as arrays, just as pointers. You can approximate
using them as arrays by using pointer (like) arithmetic on them and
increment them with ++, or I think add an integer offset to them, etc. I
intend to give them that capability eventually, and then replace BufferArg
with VPtr so there's only one system. In the mean time, if you really need
array like behavior, you might want to use BufferArg, and manage its
contents manually with copyIn and copyOut. I would suggest using a
VPtr<uint8_t> though, if what you're manipulating is an array of bytes and
you're accessing those bytes individually.
In the arguments for a system call, you should still use VPtr<> for a bare
pointer/address even if you access the data with BufferArg, since then the
system call mechanism will know that those arguments should be pointer
sized for whatever the simulated system is. VPtr<> can be cast to Addr
which will turn it into the underlying address which was passed in as an
argument. Something like this (not tested):
void
sayHello(SyscallDesc *desc, ThreadContext *tc, VPtr<> hello_ptr, size_t
size)
{
BufferArg hello_buf(hello_ptr, size);
hello_buf.copyIn(tc->getVirtProxy());
const char *hello_str = (const char *)hello_buf.bufferPtr();
std::cout << "Hello " << hello_str << "!\n";
hello_str[0] = '\0';
hello_buf.copyOut(tc->getVirtProxy());
}
Alternatively you could do mostly the same thing with VPtr<char> (also
untested):
void
sayHello(SyscallDesc *desc, ThreadContext *tc, VPtr<char> hello_ptr, size_t
size)
{
std::cout << "Hello ";
while (size--)
std::cout << *hello_ptr++;
std::cout << "!\n";
}
Gabe
On Mon, May 3, 2021 at 2:24 PM Harsh via gem5-users <gem5-users@gem5.org>
wrote:
> I'm trying to implement a few of the system calls. The system call 318
> includes copying random bytes to a void* few. However, in the
> src/sim/syscall_emul.cc file, I could not find any use of void* there are
> uses of VPtr<>. I'm not sure whether they are the same thing.
>
> Also, I looked into the implementation of VPtr<>
> http://pages.cs.wisc.edu/~swilson/gem5-docs/vptr_8hh_source.html#l00083
> and there is no functionality of inserting elements into the underlying
> buffer attribute. I tried creating the BufferArg object and then calling
> its bufferPtr() method, however, I'm not sure if this is the best method.
> Some of the examples make use of std::memcpy on the bufferPtr, but I do not
> have any other buffer object to copy from.
>
> Can someone please confirm whether VPtr<> is the alternative of void* and
> whether there is any other way to implement
> for (int i = 0; i < size ; i++) {buf[i] = some random byte;} using VPtr<>
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