[libvirt] [PATCH Rust 3/4] fix pointer type of macro used to convert rs string to c string

[Martin Kletzander]

On Wed, Jul 17, 2019 at 08:59:32AM +0000, Sahid Orentino Ferdjaoui wrote:
>In this commit we also add todo and warning to avoid using them +
>remove them in future.
>
>Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui at canonical.com>
>---
> src/lib.rs | 23 +++++++++++++++++++++--
> 1 file changed, 21 insertions(+), 2 deletions(-)
>
>diff --git a/src/lib.rs b/src/lib.rs
>index 77bf4a9..64d49cd 100644
>--- a/src/lib.rs
>+++ b/src/lib.rs
>@@ -99,12 +99,31 @@ macro_rules! c_chars_to_string {
>
> }
>
>+// Those two macros are not completely safe and we should probably
>+// stop using them to avoid possibility of pointers dangling. The
>+// memory may be freed too early.
>+//
>+// To avoid that, the right pattern would be:
>+//
>+// let cstring = CString::new(rs_string).unwrap();
>+// unsafe {
>+//   some_c_function(cstring.as_ptr() as *const libc::c_char);
>+// }
>+//
>+// So we ensure the pointer passed to 'some_c_function()' will live
>+// until 'cstring' exists.
>+//
>+// TODO(sahid): fix code + remove macros.
>+

Just in case you won't get around to do that, could you put this to the
BiteSizedTasks[1] wiki?

Also while checking the usage numbers of these macros I found out you pushed
this already, so sorry for late review.

[1] https://wiki.libvirt.org/page/BiteSizedTasks

> macro_rules! string_to_c_chars {
>-    ($x:expr) => (::std::ffi::CString::new($x).unwrap().as_ptr())
>+    ($x:expr) => (
>+        ::std::ffi::CString::new($x).unwrap().as_ptr() as *const libc::c_char)
> }
>
> macro_rules! string_to_mut_c_chars {
>-    ($x:expr) => (::std::ffi::CString::new($x).unwrap().into_raw())
>+    ($x:expr) => (
>+        // Usage of this should ensure deallocation.
>+        ::std::ffi::CString::new($x).unwrap().into_raw() as *mut libc::c_char)
> }
>
> macro_rules! impl_from {
>-- 
>2.17.1
>
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