String buffers, part 2d: basic string buffer methods.

Sponsored by fmad.io.
author: Mike Pall <mike> 2021-06-01 05:16:32 +0200
committer: Mike Pall <mike> 2021-06-01 05:16:32 +0200
commit: a119497becdf2894fb7fa737f106464309dd7947 (patch)
tree: 95c7f88198ba3f12eff52fd5646207ffb31da2d5 /doc
parent: edd5cbadc5cdc7b5b66d5340ee97c5abe5a3892a (diff)
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1 files changed, 404 insertions, 53 deletions
diff --git a/doc/ext_buffer.html b/doc/ext_buffer.html
index 455c298d..94af757d 100644
--- a/doc/ext_buffer.html
+++ b/doc/ext_buffer.html
@@ -1,19 +1,30 @@
 <!DOCTYPE html>
 <html>
 <head>
-<title>String Buffers</title>
+<title>String Buffer Library</title>
 <meta charset="utf-8">
 <meta name="Copyright" content="Copyright (C) 2005-2021">
 <meta name="Language" content="en">
 <link rel="stylesheet" type="text/css" href="bluequad.css" media="screen">
 <link rel="stylesheet" type="text/css" href="bluequad-print.css" media="print">
+<style type="text/css">
+.lib {
+  vertical-align: middle;
+  margin-left: 5px;
+  padding: 0 5px;
+  font-size: 60%;
+  border-radius: 5px;
+  background: #c5d5ff;
+  color: #000;
+}
+</style>
 </head>
 <body>
 <div id="site">
 <a href="https://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
 </div>
 <div id="head">
-<h1>String Buffers</h1>
+<h1>String Buffer Library</h1>
 </div>
 <div id="nav">
 <ul><li>
@@ -57,31 +68,35 @@
 </div>
 <div id="main">
 <p>
 The string buffer library allows <b>high-performance manipulation of
 string-like data</b>.
 </p>
 <p>
 Unlike Lua strings, which are constants, string buffers are
 <b>mutable</b> sequences of 8-bit (binary-transparent) characters. Data
 can be stored, formatted and encoded into a string buffer and later
-converted, decoded or extracted.
+converted, extracted or decoded.
 </p>
 <p>
 The convenient string buffer API simplifies common string manipulation
 tasks, that would otherwise require creating many intermediate strings.
 String buffers improve performance by eliminating redundant memory
 copies, object creation, string interning and garbage collection
 overhead. In conjunction with the FFI library, they allow zero-copy
 operations.
+</p>
+<p>
+The string buffer libary also includes a high-performance
+<a href="serialize">serializer</a> for Lua objects.
+</p>
+<h2 id="wip" style="color:#ff0000">Work in Progress</h2>
+<p>
+<b style="color:#ff0000">This library is a work in progress. More
+functionality will be added soon.</b>
 </p>
-<h2 id="load">Using the String Buffer Library</h2>
+<h2 id="use">Using the String Buffer Library</h2>
 <p>
 The string buffer library is built into LuaJIT by default, but it's not
 loaded by default. Add this to the start of every Lua file that needs
@@ -90,137 +105,406 @@ one of its functions:
 <pre class="code">
 local buffer = require("string.buffer")
 </pre>
+<p>
+The convention for the syntax shown on this page is that <tt>buffer</tt>
+refers to the buffer library and <tt>buf</tt> refers to an individual
+buffer object.
+</p>
+<p>
+Please note the difference between a Lua function call, e.g.
+<tt>buffer.new()</tt> (with a dot) and a Lua method call, e.g.
+<tt>buf:reset()</tt> (with a colon).
+</p>
-<h2 id="wip" style="color:#ff0000">Work in Progress</h2>
+<h3 id="buffer_object">Buffer Objects</h3>
+<p>
+A buffer object is a garbage-collected Lua object. After creation with
+<tt>buffer.new()</tt>, it can (and should) be reused for many operations.
+When the last reference to a buffer object is gone, it will eventually
+be freed by the garbage collector, along with the allocated buffer
+space.
+</p>
+<p>
+Buffers operate like a FIFO (first-in first-out) data structure. Data
+can be appended (written) to the end of the buffer and consumed (read)
+from the front of the buffer. These operations can be freely mixed.
+</p>
+<p>
+The buffer space that holds the characters is managed automatically
+&mdash; it grows as needed and already consumed space is recycled. Use
+<tt>buffer.new(size)</tt> and <tt>buf:free()</tt>, if you need more
+control.
+</p>
+<p>
+The maximum size of a single buffer is the same as the maximum size of a
+Lua string, which is slightly below two gigabytes. For huge data sizes,
+neither strings nor buffers are the right data structure &mdash; use the
+FFI library to directly map memory or files up to the virtual memory
+limit of your OS.
+</p>
+<h3 id="buffer_overview">Buffer Method Overview</h3>
+<ul>
+<li>
+The <tt>buf:put*()</tt>-like methods append (write) characters to the
+end of the buffer.
+</li>
+<li>
+The <tt>buf:get*()</tt>-like methods consume (read) characters from the
+front of the buffer.
+</li>
+<li>
+Other methods, like <tt>buf:tostring()</tt> only read the buffer
+contents, but don't change the buffer.
+</li>
+<li>
+The <tt>buf:set()</tt> method allows zero-copy consumption of a string
+or an FFI cdata object as a buffer.
+</li>
+<li>
+The FFI-specific methods allow zero-copy read/write-style operations or
+modifying the buffer contents in-place. Please check the
+<a href="#ffi_caveats">FFI caveats</a> below, too.
+</li>
+<li>
+Methods that don't need to return anything specific, return the buffer
+object itself as a convenience. This allows method chaining, e.g.:
+<tt>buf:reset():encode(obj)</tt> or <tt>buf:skip(len):get()</tt>
+</li>
+</ul>
+<h2 id="create">Buffer Creation and Management</h2>
+<h3 id="buffer_new"><tt>local buf = buffer.new([size])</tt></h3>
+<p>
+Creates a new buffer object.
+</p>
 <p>
+The optional <tt>size</tt> argument ensures a minimum initial buffer
+size. This is strictly an optimization for cases where the required
+buffer size is known beforehand.
+</p>
-<b style="color:#ff0000">This library is a work in progress. More
+<h3 id="buffer_reset"><tt>buf = buf:reset()</tt></h3>
-functions will be added soon.</b>
+<p>
+Reset (empty) the buffer. The allocated buffer space is not freed and
+may be reused.
+</p>
+<h3 id="buffer_free"><tt>buf = buf:free()</tt></h3>
+<p>
+The buffer space of the buffer object is freed. The object itself
+remains intact, empty and it may be reused.
+</p>
+<p>
+Note: you normally don't need to use this method. The garbage collector
+automatically frees the buffer space, when the buffer object is
+collected. Use this method, if you need to free the associated memory
+immediately.
 </p>
-<h2 id="serialize">Serialization of Lua Objects</h2>
+<h2 id="write">Buffer Writers</h2>
+<h3 id="buffer_put"><tt>buf = buf:put([str|num|obj] [, ...])</tt></h3>
+<p>
+Appends a string <tt>str</tt>, a number <tt>num</tt> or any object
+<tt>obj</tt> with a <tt>__tostring</tt> metamethod to the buffer.
+Multiple arguments are appended in the given order.
+</p>
+<p>
+Appending a buffer to a buffer is possible and short-circuited
+internally. But it still involves a copy. Better combine the buffer
+writes to use a single buffer.
+</p>
+<h3 id="buffer_putf"><tt>buf = buf:putf(format, ...)</tt></h3>
+<p>
+Appends the formatted arguments to the buffer. The <tt>format</tt>
+string supports the same options as <tt>string.format()</tt>.
+</p>
+<h3 id="buffer_putcdata"><tt>buf = buf:putcdata(cdata, len)</tt><span class="lib">FFI</span></h3>
 <p>
+Appends the given <tt>len</tt> number of bytes from the memory pointed
+to by the FFI <tt>cdata</tt> object to the buffer. The object needs to
+be convertible to a (constant) pointer.
+</p>
+<h3 id="buffer_set"><tt>buf = buf:set(str)<br>
+buf = buf:set(cdata, len)</tt><span class="lib">FFI</span></h3>
+<p>
+This method allows zero-copy consumption of a string or an FFI cdata
+object as a buffer. It stores a reference to the passed string
+<tt>str</tt> or the FFI <tt>cdata</tt> object in the buffer. Any buffer
+space originally allocated is freed. This is <i>not</i> an append
+operation, unlike the <tt>buf:put*()</tt> methods.
+</p>
+<p>
+After calling this method, the buffer behaves as if
+<tt>buf:free():put(str)</tt> or <tt>buf:free():put(cdata,&nbsp;len)</tt>
+had been called. However, the data is only referenced and not copied, as
+long as the buffer is only consumed.
+</p>
+<p>
+In case the buffer is written to later on, the referenced data is copied
+and the object reference is removed (copy-on-write semantics).
+</p>
+<p>
+The stored reference is an anchor for the garbage collector and keeps the
+originally passed string or FFI cdata object alive.
+</p>
+<h3 id="buffer_reserve"><tt>ptr, len = buf:reserve(size)</tt><span class="lib">FFI</span><br>
+<tt>buf = buf:commit(used)</tt><span class="lib">FFI</span></h3>
+<p>
+The <tt>reserve</tt> method reserves at least <tt>size</tt> bytes of
+write space in the buffer. It returns an <tt>uint8_t&nbsp;*</tt> FFI
+cdata pointer <tt>ptr</tt> that points to this space.
+</p>
+<p>
+The available length in bytes is returned in <tt>len</tt>. This is at
+least <tt>size</tt> bytes, but may be more to facilitate efficient
+buffer growth. You can either make use of the additional space or ignore
+<tt>len</tt> and only use <tt>size</tt> bytes.
+</p>
+<p>
+The <tt>commit</tt> method appends the <tt>used</tt> bytes of the
+previously returned write space to the buffer data.
+</p>
+<p>
+This pair of methods allows zero-copy use of C read-style APIs:
+</p>
+<pre class="code">
+local MIN_SIZE = 65536
+repeat
+  local ptr, len = buf:reserve(MIN_SIZE)
+  local n = C.read(fd, ptr, len)
+  if n == 0 then break end -- EOF.
+  if n &lt; 0 then error("read error") end
+  buf:commit(n)
+until false
+</pre>
+<p>
+The reserved write space is <i>not</i> initialized. At least the
+<tt>used</tt> bytes <b>must</b> be written to before calling the
+<tt>commit</tt> method. There's no need to call the <tt>commit</tt>
+method, if nothing is added to the buffer (e.g. on error).
+</p>
+<h2 id="read">Buffer Readers</h2>
+<h3 id="buffer_length"><tt>len = #buf</tt></h3>
+<p>
+Returns the current length of the buffer data in bytes.
+</p>
+<h3 id="buffer_concat"><tt>res = str|num|buf .. str|num|buf [...]</tt></h3>
+<p>
+The Lua concatenation operator <tt>..</tt> also accepts buffers, just
+like strings or numbers. It always returns a string and not a buffer.
+</p>
+<p>
+Note that although this is supported for convenience, this thwarts one
+of the main reasons to use buffers, which is to avoid string
+allocations. Rewrite it with <tt>buf:put()</tt> and <tt>buf:get()</tt>.
+</p>
+<p>
+Mixing this with unrelated objects that have a <tt>__concat</tt>
+metamethod may not work, since these probably only expect strings.
+</p>
+<h3 id="buffer_skip"><tt>buf = buf:skip(len)</tt></h3>
+<p>
+Skips (consumes) <tt>len</tt> bytes from the buffer up to the current
+length of the buffer data.
+</p>
+<h3 id="buffer_get"><tt>str, ... = buf:get([len|nil] [,...])</tt></h3>
+<p>
+Consumes the buffer data and returns one or more strings. If called
+without arguments, the whole buffer data is consumed. If called with a
+number, up to <tt>len</tt> bytes are consumed. A <tt>nil</tt> argument
+consumes the remaining buffer space (this only makes sense as the last
+argument). Multiple arguments consume the buffer data in the given
+order.
+</p>
+<p>
+Note: a zero length or no remaining buffer data returns an empty string
+and not <tt>nil</tt>.
+</p>
+<h3 id="buffer_tostring"><tt>str = buf:tostring()<br>
+str = tostring(buf)</tt></h3>
+<p>
+Creates a string from the buffer data, but doesn't consume it. The
+buffer remains unchanged.
+</p>
+<p>
+Buffer objects also define a <tt>__tostring</tt> metamethod. This means
+buffers can be passed to the global <tt>tostring()</tt> function and
+many other functions that accept this in place of strings. The important
+internal uses in functions like <tt>io.write()</tt> are short-circuited
+to avoid the creation of an intermediate string object.
+</p>
+<h3 id="buffer_ref"><tt>ptr, len = buf:ref()</tt><span class="lib">FFI</span></h3>
+<p>
+Returns an <tt>uint8_t&nbsp;*</tt> FFI cdata pointer <tt>ptr</tt> that
+points to the buffer data. The length of the buffer data in bytes is
+returned in <tt>len</tt>.
+</p>
+<p>
+The returned pointer can be directly passed to C functions that expect a
+buffer and a length. You can also do bytewise reads
+(<tt>local&nbsp;x&nbsp;=&nbsp;ptr[i]</tt>) or writes
+(<tt>ptr[i]&nbsp;=&nbsp;0x40</tt>) of the buffer data.
+</p>
+<p>
+In conjunction with the <tt>skip</tt> method, this allows zero-copy use
+of C write-style APIs:
+</p>
+<pre class="code">
+repeat
+  local ptr, len = buf:ref()
+  if len == 0 then break end
+  local n = C.write(fd, ptr, len)
+  if n &lt; 0 then error("write error") end
+  buf:skip(n)
+until n >= len
+</pre>
+<p>
+Unlike Lua strings, buffer data is <i>not</i> implicitly
+zero-terminated. It's not safe to pass <tt>ptr</tt> to C functions that
+expect zero-terminated strings. If you're not using <tt>len</tt>, then
+you're doing something wrong.
+</p>
+<h2 id="serialize">Serialization of Lua Objects</h2>
+<p>
 The following functions and methods allow <b>high-speed serialization</b>
 (encoding) of a Lua object into a string and decoding it back to a Lua
 object. This allows convenient storage and transport of <b>structured
 data</b>.
 </p>
 <p>
 The encoded data is in an <a href="#serialize_format">internal binary
 format</a>. The data can be stored in files, binary-transparent
 databases or transmitted to other LuaJIT instances across threads,
 processes or networks.
 </p>
 <p>
 Encoding speed can reach up to 1 Gigabyte/second on a modern desktop- or
 server-class system, even when serializing many small objects. Decoding
 speed is mostly constrained by object creation cost.
 </p>
 <p>
 The serializer handles most Lua types, common FFI number types and
 nested structures. Functions, thread objects, other FFI cdata, full
 userdata and associated metatables cannot be serialized (yet).
 </p>
 <p>
 The encoder serializes nested structures as trees. Multiple references
 to a single object will be stored separately and create distinct objects
 after decoding. Circular references cause an error.
 </p>
-<h3 id="buffer_encode"><tt>str = buffer.encode(obj)</tt></h3>
+<h3 id="serialize_methods">Serialization Functions and Methods</h3>
-<p>
-Serializes (encodes) the Lua object <tt>obj</tt> into the string
-<tt>str</tt>.
+<h3 id="buffer_encode"><tt>str = buffer.encode(obj)<br>
+buf = buf:encode(obj)</tt></h3>
+<p>
+Serializes (encodes) the Lua object <tt>obj</tt>. The stand-alone
+function returns a string <tt>str</tt>. The buffer method appends the
+encoding to the buffer.
 </p>
 <p>
 <tt>obj</tt> can be any of the supported Lua types &mdash; it doesn't
 need to be a Lua table.
 </p>
 <p>
 This function may throw an error when attempting to serialize
 unsupported object types, circular references or deeply nested tables.
 </p>
-<h3 id="buffer_decode"><tt>obj = buffer.decode(str)</tt></h3>
+<h3 id="buffer_decode"><tt>obj = buffer.decode(str)<br>
+obj = buf:decode()</tt></h3>
 <p>
+The stand-alone function de-serializes (decodes) the string
-De-serializes (decodes) the string <tt>str</tt> into the Lua object
+<tt>str</tt>, the buffer method de-serializes one object from the
-<tt>obj</tt>.
+buffer. Both return a Lua object <tt>obj</tt>.
 </p>
 <p>
 The returned object may be any of the supported Lua types &mdash;
 even <tt>nil</tt>.
 </p>
 <p>
 This function may throw an error when fed with malformed or incomplete
-encoded data. The standalone function throws when there's left-over data
+encoded data. The stand-alone function throws when there's left-over
-after decoding a single top-level object.
+data after decoding a single top-level object. The buffer method leaves
+any left-over data in the buffer.
 </p>
-<h2 id="serialize_format">Serialization Format Specification</h2>
+<h3 id="serialize_stream">Streaming Serialization</h3>
+<p>
+In some contexts, it's desirable to do piecewise serialization of large
+datasets, also known as <i>streaming</i>.
+</p>
+<p>
+This serialization format can be safely concatenated and supports streaming.
+Multiple encodings can simply be appended to a buffer and later decoded
+individually:
+</p>
+<pre class="code">
+local buf = buffer.new()
+buf:encode(obj1)
+buf:encode(obj2)
+local copy1 = buf:decode()
+local copy2 = buf:decode()
+</pre>
 <p>
+Here's how to iterate over a stream:
+</p>
+<pre class="code">
+while #buf ~= 0 do
+  local obj = buf:decode()
+  -- Do something with obj.
+end
+</pre>
+<p>
+Since the serialization format doesn't prepend a length to its encoding,
+network applications may need to transmit the length, too.
+</p>
+<h3 id="serialize_format">Serialization Format Specification</h3>
+<p>
 This serialization format is designed for <b>internal use</b> by LuaJIT
 applications. Serialized data is upwards-compatible and portable across
 all supported LuaJIT platforms.
 </p>
 <p>
 It's an <b>8-bit binary format</b> and not human-readable. It uses e.g.
 embedded zeroes and stores embedded Lua string objects unmodified, which
 are 8-bit-clean, too. Encoded data can be safely concatenated for
 streaming and later decoded one top-level object at a time.
 </p>
 <p>
 The encoding is reasonably compact, but tuned for maximum performance,
 not for minimum space usage. It compresses well with any of the common
 byte-oriented data compression algorithms.
 </p>
 <p>
 Although documented here for reference, this format is explicitly
 <b>not</b> intended to be a 'public standard' for structured data
 interchange across computer languages (like JSON or MessagePack). Please
 do not use it as such.
 </p>
 <p>
 The specification is given below as a context-free grammar with a
 top-level <tt>object</tt> as the starting point. Alternatives are
 separated by the <tt>|</tt> symbol and <tt>*</tt> indicates repeats.
 Grouping is implicit or indicated by <tt>{…}</tt>. Terminals are
 either plain hex numbers, encoded as bytes, or have a <tt>.format</tt>
 suffix.
 </p>
 <pre>
 object    → nil | false | true
@@ -261,6 +545,73 @@ string    → (0x20+len).U len*char.B
     0xe0..0x1fdf → (0xe0|(((n-0xe0)>>8)&0x1f)).B ((n-0xe0)&0xff).B
   0x1fe0..       → 0xff n.I
 </pre>
+<h2 id="error">Error handling</h2>
+<p>
+Many of the buffer methods can throw an error. Out-of-memory or usage
+errors are best caught with an outer wrapper for larger parts of code.
+There's not much one can do after that, anyway.
+</p>
+<p>
+OTOH you may want to catch some errors individually. Buffer methods need
+to receive the buffer object as the first argument. The Lua colon-syntax
+<tt>obj:method()</tt> does that implicitly. But to wrap a method with
+<tt>pcall()</tt>, the arguments need to be passed like this:
+</p>
+<pre class="code">
+local ok, err = pcall(buf.encode, buf, obj)
+if not ok then
+  -- Handle error in err.
+end
+</pre>
+<h2 id="ffi_caveats">FFI caveats</h2>
+<p>
+The string buffer library has been designed to work well together with
+the FFI library. But due to the low-level nature of the FFI library,
+some care needs to be taken:
+</p>
+<p>
+First, please remember that FFI pointers are zero-indexed. The space
+returned by <tt>buf:reserve()</tt> and <tt>buf:ref()</tt> starts at the
+returned pointer and ends before <tt>len</tt> bytes after that.
+</p>
+<p>
+I.e. the first valid index is <tt>ptr[0]</tt> and the last valid index
+is <tt>ptr[len-1]</tt>. If the returned length is zero, there's no valid
+index at all. The returned pointer may even be <tt>NULL</tt>.
+</p>
+<p>
+The space pointed to by the returned pointer is only valid as long as
+the buffer is not modified in any way (neither append, nor consume, nor
+reset, etc.). The pointer is also not a GC anchor for the buffer object
+itself.
+</p>
+<p>
+Buffer data is only guaranteed to be byte-aligned. Casting the returned
+pointer to a data type with higher alignment may cause unaligned
+accesses. It depends on the CPU architecture whether this is allowed or
+not (it's always OK on x86/x64 and mostly OK on other modern
+architectures).
+</p>
+<p>
+FFI pointers or references do not count as GC anchors for an underlying
+object. E.g. an <tt>array</tt> allocated with <tt>ffi.new()</tt> is
+anchored by <tt>buf:set(array,&nbsp;len)</tt>, but not by
+<tt>buf:set(array+offset,&nbsp;len)</tt>. The addition of the offset
+creates a new pointer, even when the offset is zero. In this case, you
+need to make sure there's still a reference to the original array as
+long as its contents are in use by the buffer.
+</p>
+<p>
+Even though each LuaJIT VM instance is single-threaded (but you can
+create multiple VMs), FFI data structures can be accessed concurrently.
+Be careful when reading/writing FFI cdata from/to buffers to avoid
+concurrent accesses or modifications. In particular, the memory
+referenced by <tt>buf:set(cdata,&nbsp;len)</tt> must not be modified
+while buffer readers are working on it. Shared, but read-only memory
+mappings of files are OK, but only if the file does not change.
+</p>
 <br class="flush">
 </div>
 <div id="foot">
author	Mike Pall <mike>	2021-06-01 05:16:32 +0200
committer	Mike Pall <mike>	2021-06-01 05:16:32 +0200
commit	a119497becdf2894fb7fa737f106464309dd7947 (patch)
tree	95c7f88198ba3f12eff52fd5646207ffb31da2d5 /doc
parent	edd5cbadc5cdc7b5b66d5340ee97c5abe5a3892a (diff)
download	luajit-a119497becdf2894fb7fa737f106464309dd7947.tar.gz luajit-a119497becdf2894fb7fa737f106464309dd7947.tar.bz2 luajit-a119497becdf2894fb7fa737f106464309dd7947.zip