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+# fd-slicer
+
+[![Build Status](https://travis-ci.org/andrewrk/node-fd-slicer.svg?branch=master)](https://travis-ci.org/andrewrk/node-fd-slicer)
+
+Safe `fs.ReadStream` and `fs.WriteStream` using the same fd.
+
+Let's say that you want to perform a parallel upload of a file to a remote
+server. To do this, we want to create multiple read streams. The first thing
+you might think of is to use the `{start: 0, end: 0}` API of
+`fs.createReadStream`. This gives you two choices:
+
+ 0. Use the same file descriptor for all `fs.ReadStream` objects.
+ 0. Open the file multiple times, resulting in a separate file descriptor
+    for each read stream.
+
+Neither of these are acceptable options. The first one is a severe bug,
+because the API docs for `fs.write` state:
+
+> Note that it is unsafe to use `fs.write` multiple times on the same file
+> without waiting for the callback. For this scenario, `fs.createWriteStream`
+> is strongly recommended.
+
+`fs.createWriteStream` will solve the problem if you only create one of them
+for the file descriptor, but it will exhibit this unsafety if you create
+multiple write streams per file descriptor.
+
+The second option suffers from a race condition. For each additional time the
+file is opened after the first, it is possible that the file is modified. So
+in our parallel uploading example, we might upload a corrupt file that never
+existed on the client's computer.
+
+This module solves this problem by providing `createReadStream` and
+`createWriteStream` that operate on a shared file descriptor and provides
+the convenient stream API while still allowing slicing and dicing.
+
+This module also gives you some additional power that the builtin
+`fs.createWriteStream` do not give you. These features are:
+
+ * Emitting a 'progress' event on write.
+ * Ability to set a maximum size and emit an error if this size is exceeded.
+ * Ability to create an `FdSlicer` instance from a `Buffer`. This enables you
+   to provide API for handling files as well as buffers using the same API.
+
+## Usage
+
+```js
+var fdSlicer = require('fd-slicer');
+var fs = require('fs');
+
+fs.open("file.txt", 'r', function(err, fd) {
+  if (err) throw err;
+  var slicer = fdSlicer.createFromFd(fd);
+  var firstPart = slicer.createReadStream({start: 0, end: 100});
+  var secondPart = slicer.createReadStream({start: 100});
+  var firstOut = fs.createWriteStream("first.txt");
+  var secondOut = fs.createWriteStream("second.txt");
+  firstPart.pipe(firstOut);
+  secondPart.pipe(secondOut);
+});
+```
+
+You can also create from a buffer:
+
+```js
+var fdSlicer = require('fd-slicer');
+var slicer = FdSlicer.createFromBuffer(someBuffer);
+var firstPart = slicer.createReadStream({start: 0, end: 100});
+var secondPart = slicer.createReadStream({start: 100});
+var firstOut = fs.createWriteStream("first.txt");
+var secondOut = fs.createWriteStream("second.txt");
+firstPart.pipe(firstOut);
+secondPart.pipe(secondOut);
+```
+
+## API Documentation
+
+### fdSlicer.createFromFd(fd, [options])
+
+```js
+var fdSlicer = require('fd-slicer');
+fs.open("file.txt", 'r', function(err, fd) {
+  if (err) throw err;
+  var slicer = fdSlicer.createFromFd(fd);
+  // ...
+});
+```
+
+Make sure `fd` is a properly initialized file descriptor. If you want to
+use `createReadStream` make sure you open it for reading and if you want
+to use `createWriteStream` make sure you open it for writing.
+
+`options` is an optional object which can contain:
+
+ * `autoClose` - if set to `true`, the file descriptor will be automatically
+   closed once the last stream that references it is closed. Defaults to
+   `false`. `ref()` and `unref()` can be used to increase or decrease the
+   reference count, respectively.
+
+### fdSlicer.createFromBuffer(buffer, [options])
+
+```js
+var fdSlicer = require('fd-slicer');
+var slicer = fdSlicer.createFromBuffer(someBuffer);
+// ...
+```
+
+`options` is an optional object which can contain:
+
+ * `maxChunkSize` - A `Number` of bytes. see `createReadStream()`.
+   If falsey, defaults to unlimited.
+
+#### Properties
+
+##### fd
+
+The file descriptor passed in. `undefined` if created from a buffer.
+
+#### Methods
+
+##### createReadStream(options)
+
+Available `options`:
+
+ * `start` - Number. The offset into the file to start reading from. Defaults
+   to 0.
+ * `end` - Number. Exclusive upper bound offset into the file to stop reading
+   from.
+ * `highWaterMark` - Number. The maximum number of bytes to store in the
+   internal buffer before ceasing to read from the underlying resource.
+   Defaults to 16 KB.
+ * `encoding` - String. If specified, then buffers will be decoded to strings
+   using the specified encoding. Defaults to `null`.
+
+The ReadableStream that this returns has these additional methods:
+
+ * `destroy(err)` - stop streaming. `err` is optional and is the error that
+   will be emitted in order to cause the streaming to stop. Defaults to
+   `new Error("stream destroyed")`.
+
+If `maxChunkSize` was specified (see `createFromBuffer()`), the read stream
+will provide chunks of at most that size. Normally, the read stream provides
+the entire range requested in a single chunk, but this can cause performance
+problems in some circumstances.
+See [thejoshwolfe/yauzl#87](https://github.com/thejoshwolfe/yauzl/issues/87).
+
+##### createWriteStream(options)
+
+Available `options`:
+
+ * `start` - Number. The offset into the file to start writing to. Defaults to
+   0.
+ * `end` - Number. Exclusive upper bound offset into the file. If this offset
+   is reached, the write stream will emit an 'error' event and stop functioning.
+   In this situation, `err.code === 'ETOOBIG'`. Defaults to `Infinity`.
+ * `highWaterMark` - Number. Buffer level when `write()` starts returning
+   false. Defaults to 16KB.
+ * `decodeStrings` - Boolean. Whether or not to decode strings into Buffers
+   before passing them to` _write()`. Defaults to `true`.
+
+The WritableStream that this returns has these additional methods:
+
+ * `destroy()` - stop streaming
+
+And these additional properties:
+
+ * `bytesWritten` - number of bytes written to the stream
+
+And these additional events:
+
+ * 'progress' - emitted when `bytesWritten` changes.
+
+##### read(buffer, offset, length, position, callback)
+
+Equivalent to `fs.read`, but with concurrency protection.
+`callback` must be defined.
+
+##### write(buffer, offset, length, position, callback)
+
+Equivalent to `fs.write`, but with concurrency protection.
+`callback` must be defined.
+
+##### ref()
+
+Increase the `autoClose` reference count by 1.
+
+##### unref()
+
+Decrease the `autoClose` reference count by 1.
+
+#### Events
+
+##### 'error'
+
+Emitted if `fs.close` returns an error when auto closing.
+
+##### 'close'
+
+Emitted when fd-slicer closes the file descriptor due to `autoClose`. Never
+emitted if created from a buffer.