add doctest for sys::mman::mprotect

Author: acfoltzer

src/sys/mman.rs

@@ -258,6 +258,23 @@ pub unsafe fn madvise(addr: *mut c_void, length: size_t, advise: MmapAdvise) ->
     Errno::result(libc::madvise(addr, length, advise as i32)).map(drop)
 }
 
+/// Calls to `mprotect` are inherently unsafe, as changes to memory protections can lead to
+/// SIGSEGVs.
+///
+/// ```
+/// use nix::sys::mman::{mmap, mprotect, MapFlags, ProtFlags};
+/// use std::ptr;
+///
+/// let mut slice: &mut [u8] = unsafe {
+///     let mem = mmap(ptr::null_mut(), 1024, ProtFlags::PROT_NONE,
+///                    MapFlags::MAP_ANONYMOUS | MapFlags::MAP_PRIVATE, -1, 0).unwrap();
+///     mprotect(mem, 1024, ProtFlags::PROT_READ | ProtFlags::PROT_WRITE).unwrap();
+///     std::slice::from_raw_parts_mut(mem as *mut u8, 1024)
+/// };
+/// assert_eq!(slice[0], 0x00);
+/// slice[0] = 0xFF;
+/// assert_eq!(slice[0], 0xFF);
+/// ```
 pub unsafe fn mprotect(addr: *mut c_void, length: size_t, prot: ProtFlags) -> Result<()> {
     Errno::result(libc::mprotect(addr, length, prot.bits())).map(drop)
 }