New c_str/ffi makes error buffer operations harder than necessary

[viraptor]

After the c_str/ffi interface changed in #20444 operations which return strings into preallocated buffer are harder to handle.

Specifically calls like prctl can take form prctl(int, char[16]). This could be handled by:

let mut name = [0 as u8; 17];
let res = prctl(PrctlOption::PR_GET_NAME as c_int, name.as_mut_ptr() as c_ulong, 0, 0, 0);
(res, CString::from_slice(name.as_slice()))

But the name is likely to contain zeros and there's no simple "construct CString until the first zero" function. This shouldn't be a big issue in case of a simple CString type, but since this one is meant specifically for FFI, I think such constructor should be supported.

(rustc 1.0.0-nightly (44a287e 2015-01-08 17:03:40 -0800))

[shepmaster]

Related to #20475?

[viraptor]

I think it's close. The same function could probably satisfy both issues.
The operation could construct the cstring from a slice/vec up to the first
zero, or until max_length - whichever happens first.
On 12 Jan 2015 11:38 am, "Jake Goulding" [email protected] wrote:

Related to #20475 #20475?

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#20928 (comment).

[alexcrichton]

It sounds like you don't necessarily want to return a CString but rather a Vec<u8>. are you later passing the string back into other C functions? I'd be thinking of something like:

let mut name = [0 as u8; 17];
let res = prctl(PrctlOption::PR_GET_NAME as c_int, name.as_mut_ptr() as c_ulong, 0, 0, 0);
(res, ffi::c_str_to_bytes(name.as_ptr()).to_vec())

[viraptor]

I think it's better for the interface if I return a cstring, since that's
exactly what I get back. Vec would suggest some binary data. I can't make
it a proper string since the caller will know the encoding.
I think the interface would need more comments about the nulls, length,
etc. in my library, instead of just providing cstring.
Am I mistaken here?
On 13 Jan 2015 5:59 am, "Alex Crichton" [email protected] wrote:

It sounds like you don't necessarily want to return a CString but rather
a Vec. are you later passing the string back into other C functions?
I'd be thinking of something like:

let mut name = [0 as u8; 17];let res = prctl(PrctlOption::PR_GET_NAME as c_int, name.as_mut_ptr() as c_ulong, 0, 0, 0);
(res, ffi::c_str_to_bytes(name.as_ptr()).to_vec())

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#20928 (comment).

[alexcrichton]

Vec would suggest some binary data.

To rust, however, C strings are just binary data. The CString type is basically just a type that derefs to [u8]. The only added benefit of CString over Vec<u8> is a static guarantee that there are no interior nul bytes (which might be useful if you're handing the data right back to C).

[alexcrichton]

Looking back at this, I'm going to close this as a dupe of #20475 as a from_slice_truncate method looks to be what'd fit the ticket here.