Added documentation to Whence.

Changed SeekData and SeekHole hardcoded values to libc ones and
added availability only to linux freebsd and dragonflybsd.

Removed impl Whence and assigned the libc values directly in the enum definition.

Author: ncaracci

src/unistd.rs

@@ -737,36 +737,40 @@ pub fn write(fd: RawFd, buf: &[u8]) -> Result<usize> {
     Errno::result(res).map(|r| r as usize)
 }
 
+/// Directive that tells [`lseek`] and [`lseek64`] what the offset is relative to.
+/// [`lseek`]: ./fn.lseek.html
+/// [`lseek64`]: ./fn.lseek64.html
+#[repr(i32)]
 pub enum Whence {
-    SeekSet,
-    SeekCur,
-    SeekEnd,
-    SeekData,
-    SeekHole
-}
-
-impl Whence {
-    fn to_libc_type(&self) -> c_int {
-        match self {
-            &Whence::SeekSet => libc::SEEK_SET,
-            &Whence::SeekCur => libc::SEEK_CUR,
-            &Whence::SeekEnd => libc::SEEK_END,
-            &Whence::SeekData => 3,
-            &Whence::SeekHole => 4
-        }
-    }
-
+    /// Specify an offset relative to the start of the file.
+    SeekSet = libc::SEEK_SET,
+    /// Specify an offset relative to the current file location.
+    SeekCur = libc::SEEK_CUR,
+    /// Specify an offset relative to the end of the file.
+    SeekEnd = libc::SEEK_END,
+    /// Specify an offset relative to the next location in the file greater than or 
+    /// equal to offset that contains some data. If offset points to
+    /// some data, then the file offset is set to offset.
+    #[cfg(any(target_os = "dragonfly", target_os = "freebsd", target_os = "linux"))]
+    SeekData = libc::SEEK_DATA,
+    /// Specify an offset relative to the next hole in the file greater than
+    /// or equal to offset. If offset points into the middle of a hole, then 
+    /// the file offset should be set to offset. If there is no hole past offset,
+    /// then the file offset should be adjusted to the end of the file (i.e., there
+    /// is an implicit hole at the end of any file).
+    #[cfg(any(target_os = "dragonfly", target_os = "freebsd", target_os = "linux"))]
+    SeekHole = libc::SEEK_HOLE
 }
 
 pub fn lseek(fd: RawFd, offset: libc::off_t, whence: Whence) -> Result<libc::off_t> {
-    let res = unsafe { libc::lseek(fd, offset, whence.to_libc_type()) };
+    let res = unsafe { libc::lseek(fd, offset, whence as i32) };
 
     Errno::result(res).map(|r| r as libc::off_t)
 }
 
 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub fn lseek64(fd: RawFd, offset: libc::off64_t, whence: Whence) -> Result<libc::off64_t> {
-    let res = unsafe { libc::lseek64(fd, offset, whence.to_libc_type()) };
+    let res = unsafe { libc::lseek64(fd, offset, whence as i32) };
 
     Errno::result(res).map(|r| r as libc::off64_t)
 }