Add support for 'posix_fallocate'

CHANGELOG.md

@@ -19,6 +19,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#825](https://github.com/nix-rust/nix/pull/825))
 - Exposed `MSG_CMSG_CLOEXEC` on *BSD.
   ([#825](https://github.com/nix-rust/nix/pull/825))
+- Added `nix::sys::posix_fallocate` for Android, FreeBSD and Linux.
+  ([#779](https:://github.com/nix-rust/nix/pull/779))
 
 ### Changed
 - Display and Debug for SysControlAddr now includes all fields.

src/fcntl.rs

@@ -383,3 +383,20 @@ pub fn fallocate(fd: RawFd, mode: FallocateFlags, offset: libc::off_t, len: libc
     let res = unsafe { libc::fallocate(fd, mode.bits(), offset, len) };
     Errno::result(res)
 }
+
+/// Allocate file space.
+///
+/// Ensure disk space is allocated for the file referred to by the file
+/// descriptor `fd` for the bytes in the range starting at `offset` and
+/// continuing for `len` bytes.
+///
+/// # References
+///
+/// [`posix_fallocate`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_fallocate.html)
+#[cfg(any(target_os = "android", target_os = "freebsd", target_os = "linux"))]
+#[inline]
+pub fn posix_fallocate(fd: RawFd, offset: libc::off_t,
+                       len: libc::off_t) -> Result<c_int> {
+    let res = unsafe { libc::posix_fallocate(fd, offset, len) };
+    Errno::result(res)
+}

test/test_fcntl.rs

@@ -5,6 +5,7 @@ use tempdir::TempDir;
 use tempfile::NamedTempFile;
 use std::io::prelude::*;
 use std::os::unix::fs;
+use std::os::unix::io::AsRawFd;
 
 #[test]
 fn test_openat() {
@@ -143,3 +144,23 @@ mod linux_android {
         assert_eq!(100, read(fd, &mut buf).unwrap());
     }
 }
+
+#[cfg(any(target_os = "android", target_os = "freebsd", target_os = "linux"))]
+#[test]
+fn test_posix_fallocate() {
+    use nix::fcntl::posix_fallocate;
+    let tmp = NamedTempFile::new().unwrap();
+
+    let fd = tmp.as_raw_fd();
+    let ret = posix_fallocate(fd, 0, 100);
+    if ret.is_ok() {
+        // The test will fail if /tmp's filesystem doesn't support
+        // posix_fallocate. ZFS for example does not. POSIX requires
+        // that posix_fallocate return EINVAL in that case. Skip the
+        // test in this scenario.
+        //
+        // Check if we read exactly 100 bytes
+        let mut buf = [0u8; 200];
+        assert_eq!(100, read(fd, &mut buf).unwrap());
+    }
+}