diff --git a/Cargo.lock b/Cargo.lock
index 06be72e..2f59e2f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -19,6 +19,7 @@ checksum = "1482821306169ec4d07f6aca392a4681f66c75c9918aa49641a2595db64053cb"
 name = "mapcat"
 version = "0.1.0"
 dependencies = [
+ "libc",
  "memmap",
  "num_cpus",
  "rustc_version",
diff --git a/Cargo.toml b/Cargo.toml
index 69759a1..1ada125 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -8,7 +8,7 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-mapcat = { path = "." }
+libc = "0.2.81"
 memmap = "0.7.0"
 num_cpus = "1.13.0"
 smallvec = "1.5.1"
diff --git a/src/main.rs b/src/main.rs
index c7b5f29..2231a76 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -26,17 +26,18 @@ mod map;
 mod job;
 mod work;
 mod arg;
+mod open;
 
 mod consts;
 
-
-
 fn work(arg::Operation{output, inputs}: arg::Operation) -> Result<(), Box<dyn std::error::Error>>
 {
     // todo:
     //  - create and open output file
+    let output = open::output(output)?;
     //  - open and stat all input files in order (`job::create_from_file`).
     //  - `fallocate` the output file fd to the sum of all input file sizes
+    
     //  - `mmap` the output file as writable
     //
     //  - spawn the task thread pool
diff --git a/src/map.rs b/src/map.rs
index 0edd4ee..7ef2007 100644
--- a/src/map.rs
+++ b/src/map.rs
@@ -46,6 +46,17 @@ impl MemoryMapMut
     {
 	self.as_ref()
     }
+    pub fn map(file: fs::File) -> io::Result<Self>
+    {
+	Ok(Self{
+	    map: unsafe{ MmapMut::map_mut(&file)? },
+	    file,
+	})
+    }
+    #[inline] pub fn len(&self) -> usize
+    {
+	self.map.len()
+    }
     pub fn open(from: impl AsRef<Path>) -> io::Result<Self>
     {
 	let file = fs::OpenOptions::new()
diff --git a/src/open.rs b/src/open.rs
new file mode 100644
index 0000000..f5a47f3
--- /dev/null
+++ b/src/open.rs
@@ -0,0 +1,101 @@
+use super::*;
+use std::{
+    fs,io,
+    path::Path,
+    fmt, error,
+};
+use fs::File;
+use std::borrow::Cow;
+use std::os::unix::io::AsRawFd as _;
+
+fn fallocate(file: &mut File, to: usize) -> Result<(), OutputError>
+{
+    let fd = file.as_raw_fd();
+    unsafe {
+	if libc::fallocate(fd, 0, 0, to as libc::off_t) == 0 {
+	    Ok(())
+	} else {
+	    let errno = *libc::__errno_location();
+	    Err(OutputError::Allocate(errno))
+	}
+    }
+}
+
+/// A reserved output file.
+#[derive(Debug)]
+pub struct Output(File);
+
+impl Output
+{
+    /// Allocate this reserved output file `to` bytes, the map it and return.
+    pub fn complete(self, to: usize) -> Result<map::MemoryMapMut, OutputError>
+    {
+	let mut file = self.0;
+	fallocate(&mut file, to)?;
+	map::MemoryMapMut::map(file)
+	    .map_err(OutputError::Map)
+	    .and_then(|map| if map.len() != to {
+		Err(OutputError::Size{expected: to, got: map.len()})
+	    } else {
+		Ok(map)
+	    })
+    }
+}
+
+/// Create a reserved output fd
+#[inline] pub fn output(path: impl AsRef<Path>) -> Result<Output, OutputError>
+{
+    fs::OpenOptions::new()
+	.create(true)
+	.write(true)
+	.open(path)
+	.map(Output)
+	.map_err(OutputError::Open)
+}
+
+fn errno_str<'a>(errno: libc::c_int) -> Cow<'a, str>
+{
+    unsafe {
+	let ptr = libc::strerror(errno) as *const libc::c_char;
+	if ptr.is_null() {
+	    return Cow::Borrowed("Unknown");
+	}
+	let cstr = std::ffi::CStr::from_ptr(ptr);
+	cstr.to_string_lossy()
+    }
+}
+
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum OutputError {
+    Open(io::Error),
+    Allocate(libc::c_int),
+    Map(io::Error),
+    Size{expected: usize, got: usize},
+}
+impl error::Error for OutputError
+{
+    fn source(&self) -> Option<&(dyn error::Error + 'static)>
+    {
+	match &self {
+	    Self::Open(io)
+		| Self::Map(io)
+		=> Some(io),
+	    _ => None,
+	}
+    }
+}
+impl fmt::Display for OutputError
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
+    {
+	match self {
+	    Self::Open(io) => write!(f, "Failed to open file: {}", io),
+	    Self::Allocate(errno) => write!(f, "fallocate() failed with error {}: {}", errno, errno_str(*errno)),
+	    Self::Map(io) => write!(f, "mmap() failed: {}", io),
+	    Self::Size{expected, got} => write!(f, "mapped file size mismatch: expected {}, got {}", expected, got),
+	}
+    }
+}
+
+