Conversion/assign value into char array in structure

[Musharraf]

Hi,

I want to convert below C code into RUST.

struct Msg
{
	unsigned short m_hashedPasswordLength;
	unsigned short m_reserved1;
	unsigned __int64 m_requestId1;
	unsigned __int64 m_requestId2;
	char m_loginId[MAX_EMAILADDRESS_SIZE - 1];
	unsigned char m_hashedPassword[MAX_PASSWORDHASH_SIZE];
	unsigned char m_executorMask;
	unsigned char m_reserved2;
	unsigned short m_reserved3;
};

I facing issue with char array type, how can we assign value into [m_loginId, m_hashedPassword] variables when I make mutable variable of that structure.

Thanks

[H2CO3]

In Rust, arrays aren't special when it comes to assignment. In fact, no type really is – assignment works for all types (of which the size is statically known). If you have an array in a struct, you can assign to it using the_struct.the_array_field = another_array;.

One thing to keep in mind is that an array of characters is not how you usually represent strings in idiomatic Rust. You should probably use the String type instead.

[Musharraf]

Hi H2CO3,

Its a legacy code that I will have to use this structure to send in message, I define a mut variable of struct and assigning value array so facing problem.

//structure in RUST
struct TReqMobileBinLoginIdLong{

    m_length:u16,
    m_type:u16,
    m_hashedpasswordlength:u16,
	m_reserved1:u16,
	m_requestId1:u64,
	m_requestId2:u64,
	m_loginid:[u8;20],
	m_hashedpassword:[u8;20],
	m_executormask:c_uchar,
	m_reserved2:c_uchar,
	m_reserved3:c_uchar
}

//mutable variable
let mut loginMsg = TReqMobileBinLoginIdLong{

                        m_length:64,
                        m_type:63125,
                        m_hashedpasswordlength:u16,
                        m_reserved1:1,
                        m_requestId1:1,
                        m_requestId2:1,
                        m_loginid: //facing issue here
                        m_hashedpassword: // facing issue here..
                        m_executorMask:0,
                        m_reserved2:0,
                        m_reserved3:0
                    };

Thanks

[H2CO3]

Are you asking how to create an array literal? I don't understand what other "issue you might be facing".

[Musharraf]

Yes,

when trying to assign like below.

m_loginid:("test").as_bytes()

getting error

Thanks

[H2CO3]

You should read the documentation or at the very minimum the signature of the functions you are using. str::as_bytes() returns a reference to a dynamically-sized slice, and not a value of type array.

If you want a literal array of known size, you can initialize it using array literal syntax, listing each element in order:

let login_msg = TReqMobileBinLoginIdLong {
    ...
    loginid: [1, 2, 3],
    ...
};

or in the case of arrays of bytes, the special b"..." literal syntax is also available, which returns a reference to a 'static array-of-u8 of the appropriate length:

let login_msg = TReqMobileBinLoginIdLong {
    ...
    loginid: *b"foo@mail.com",
    ...
};

By the way, drop the m_ prefix from your field names. They serve no purpose in Rust.

[H2CO3]

Mind you, from the C declaration (MAX_EMAILADDRESS_SIZE) it looks like the array is expected to be initialized with at most that many characters, in which case a b"byte string literal" won't work (unless you manually pad it to the correct size, presumably with \x00 if this is expected to be consumed by a C API).

In this case, you can just initialize it with all zeroes using the repeating array literal construct, [0; MAX_EMAIL_ADDRESS_SIZE + 1], and then use <[u8]>::copy_from_slice() on the appropriate subrange in order to copy the contents of another slice, e.g.:

login_msg.login_id[..email.len()].copy_from_slice(&email);

or something along those lines.