Thank you for an early response. It's really helpful.
Harsh
On Tue, May 4, 2021 at 6:31 AM Gabe Black <gabe.bl...@gmail.com> wrote:
> 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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