Make preadv take immutable slice of IoVecs, fixes #913

Author: farnoy

CHANGELOG.md

@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
 ### Changed
+- Made `preadv` take immutable slice of IoVec. ([issue #913](https://github.com/nix-rust/nix/issues/913))
 
 ### Fixed
 

src/sys/uio.rs

@@ -51,7 +51,7 @@ pub fn pwritev(fd: RawFd, iov: &[IoVec<&[u8]>],
           target_os = "linux",
           target_os = "netbsd",
           target_os = "openbsd"))]
-pub fn preadv(fd: RawFd, iov: &mut [IoVec<&mut [u8]>],
+pub fn preadv(fd: RawFd, iov: &[IoVec<&mut [u8]>],
               offset: off_t) -> Result<usize> {
     let res = unsafe {
         libc::preadv(fd, iov.as_ptr() as *const libc::iovec, iov.len() as c_int, offset)

test/sys/test_uio.rs

@@ -182,9 +182,9 @@ fn test_preadv() {
 
     {
         // Borrow the buffers into IoVecs and preadv into them
-        let mut iovecs: Vec<_> = buffers.iter_mut().map(
+        let iovecs: Vec<_> = buffers.iter_mut().map(
             |buf| IoVec::from_mut_slice(&mut buf[..])).collect();
-        assert_eq!(Ok(100), preadv(file.as_raw_fd(), &mut iovecs, 100));
+        assert_eq!(Ok(100), preadv(file.as_raw_fd(), &iovecs, 100));
     }
 
     let all = buffers.concat();