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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>BufferWriter — AMC Traffic Server Documentation</title>

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HAS_SOURCE: true,

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</head>

<body>

<h3>Navigation</h3>

<ul>

<a href="genindex.html" title="General Index"

accesskey="I">index</a></li>

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accesskey="N">next</a> |</li>

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</a>

<h3><a href="index.html">Table Of Contents</a></h3>

<ul>

<li><a class="reference internal" href="#">BufferWriter</a><ul>

<li><a class="reference internal" href="#synopsis">Synopsis</a></li>

<li><a class="reference internal" href="#description">Description</a></li>

<li><a class="reference internal" href="#usage">Usage</a><ul>

<li><a class="reference internal" href="#writing">Writing</a></li>

<li><a class="reference internal" href="#reading">Reading</a></li>

<li><a class="reference internal" href="#advanced">Advanced</a></li>

</ul>

</li>

<li><a class="reference internal" href="#examples">Examples</a></li>

<li><a class="reference internal" href="#formatted-output">Formatted Output</a><ul>

<li><a class="reference internal" href="#usage-examples">Usage Examples</a></li>

</ul>

</li>

<li><a class="reference internal" href="#user-defined-formatting">User Defined Formatting</a><ul>

<li><a class="reference internal" href="#enum-example">Enum Example</a></li>

<li><a class="reference internal" href="#argument-forwarding">Argument Forwarding</a></li>

<li><a class="reference internal" href="#specialized-types">Specialized Types</a></li>

</ul>

</li>

<li><a class="reference internal" href="#global-names">Global Names</a></li>

<li><a class="reference internal" href="#working-with-standard-i-o">Working with standard I/O</a></li>

<li><a class="reference internal" href="#reference">Reference</a></li>

</ul>

</li>

</ul>

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title="previous chapter">Traffic Server Projects</a>

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title="next chapter">TSConfig / Lua</a>

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rel="nofollow">Show Source</a></li>

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<h3>Quick search</h3>

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</div>

<h1>BufferWriter<a class="headerlink" href="#bufferwriter" title="Permalink to this headline">¶</a></h1>

<h2>Synopsis<a class="headerlink" href="#synopsis" title="Permalink to this headline">¶</a></h2>

<div class="highlight-cpp"><div class="highlight"><pre>#include <ts/BufferWriterForward.h> // Custom formatter support only.

#include <ts/BufferWriter.h> // General usage.

</pre></div>

</div>

<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> is intended to increase code reliability and reduce complexity in the common

circumstance of generating formatted output strings in fixed buffers. Current usage is a mixture of

<code class="code docutils literal">snprintf</code> and <code class="code docutils literal">memcpy</code> which provides a large scope for errors and verbose code to

check for buffer overruns. The goal is to provide a wrapper over buffer size tracking to make such

code simpler and less vulnerable to implementation error.

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> itself is an abstract class to describe the base interface to wrappers for

various types of output buffers. As a common example, <a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter"><code class="xref cpp cpp-class docutils literal">FixedBufferWriter</code></a> is a subclass

designed to wrap a fixed size buffer. <a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter"><code class="xref cpp cpp-class docutils literal">FixedBufferWriter</code></a> is constructed by passing it a

buffer and a size, which it then tracks as data is written. Writing past the end of the buffer is

clipped to prevent overruns.

Consider current code that looks like this.

<div class="highlight-cpp"><div class="highlight"><pre>char buff[1024];

char * ptr = buff;

size_t len = sizeof(buff);

//...

if (len > 0) {

auto n = std::min(len, thing1_len);

memcpy(ptr, thing1, n);

len -= n;

}

if (len > 0) {

memcpy(ptr, thing2, n);

len -= n;

}

if (len > 0) {

memcpy(ptr, thing3, n);

len -= n;

}

</pre></div>

</div>

This is changed to

ts::FixedBufferWriter bw(buff, sizeof(buff));

//...

bw.write(thing1, thing1_len);

bw.write(thing2, thing2_len);

bw.write(thing3, thing3_len);

</pre></div>

</div>

The remaining length is updated every time and checked every time. A series of checks, calls to

<code class="code docutils literal">memcpy</code>, and size updates become a simple series of calls to <a class="reference internal" href="#_CPPv2N12BufferWriter5writeEPv6size_t" title="BufferWriter::write"><code class="xref cpp cpp-func docutils literal">BufferWriter::write()</code></a>.

For other types of interaction, <a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter"><code class="xref cpp cpp-class docutils literal">FixedBufferWriter</code></a> provides access to the unused buffer via

<a class="reference internal" href="#_CPPv2NK12BufferWriter9auxBufferEv" title="BufferWriter::auxBuffer"><code class="xref cpp cpp-func docutils literal">BufferWriter::auxBuffer()</code></a> and <a class="reference internal" href="#_CPPv2NK12BufferWriter9remainingEv" title="BufferWriter::remaining"><code class="xref cpp cpp-func docutils literal">BufferWriter::remaining()</code></a>. This makes it possible to easily

use <code class="code docutils literal">snprintf</code>, given that <code class="code docutils literal">snprint</code> returns the number of bytes written.

<a class="reference internal" href="#_CPPv2N12BufferWriter4fillE6size_t" title="BufferWriter::fill"><code class="xref cpp cpp-func docutils literal">BufferWriter::fill()</code></a> is used to indicate how much of the unused buffer was used. Therefore

something like (riffing off the previous example):

<div class="highlight-cpp"><div class="highlight"><pre>if (len > 0) {

len -= snprintf(ptr, len, "format string", args...);

}

</pre></div>

</div>

becomes:

<div class="highlight-cpp"><div class="highlight"><pre>bw.fill(snprintf(bw.auxBuffer(), bw.remaining(),

"format string", args...));

</pre></div>

</div>

By hiding the length tracking and checking, the result is a simple linear sequence of output chunks,

making the logic much eaier to follow.

</div>

<h2>Usage<a class="headerlink" href="#usage" title="Permalink to this headline">¶</a></h2>

The header files are divided in to two variants. <a class="reference external" href="https://github.com/apache/trafficserver/blob/master/lib/ts/BufferWriter.h">BufferWriter.h</a> provides the basic

capabilities of buffer output control. <a class="reference external" href="https://github.com/apache/trafficserver/blob/master/lib/ts/BufferWriterFormat.h">BufferWriterFormat.h</a> provides the basic

<a class="reference internal" href="#bw-formatting">formatted output mechanisms</a>, primarily the implementation and ancillary

classes for <a class="reference internal" href="#_CPPv27BWFSpec" title="BWFSpec"><code class="xref cpp cpp-class docutils literal">BWFSpec</code></a> which is used to build formatters.

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> is an abstract base class, in the style of <code class="code docutils literal">std::ostream</code>. There are

several subclasses for various use cases. When passing around this is the common type.

<a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter"><code class="xref cpp cpp-class docutils literal">FixedBufferWriter</code></a> writes to an externally provided buffer of a fixed length. The buffer must

be provided to the constructor. This will generally be used in a function where the target buffer is

external to the function or already exists.

<a class="reference internal" href="#_CPPv2I_6size_tE17LocalBufferWriter" title="LocalBufferWriter"><code class="xref cpp cpp-class docutils literal">LocalBufferWriter</code></a> is a templated class whose template argument is the size of an internal

buffer. This is useful when the buffer is local to a function and the results will be transferred

from the buffer to other storage after the output is assembled. Rather than having code like:

</pre></div>

</div>

it can be written more compactly as:

<div class="highlight-cpp"><div class="highlight"><pre>ts::LocalBufferWriter<1024> bw;

</pre></div>

</div>

In many cases, when using <a class="reference internal" href="#_CPPv2I_6size_tE17LocalBufferWriter" title="LocalBufferWriter"><code class="xref cpp cpp-class docutils literal">LocalBufferWriter</code></a> this is the only place the size of the buffer

needs to be specified and therefore can simply be a constant without the overhead of defining a size

to maintain consistency. The choice between <a class="reference internal" href="#_CPPv2I_6size_tE17LocalBufferWriter" title="LocalBufferWriter"><code class="xref cpp cpp-class docutils literal">LocalBufferWriter</code></a> and <a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter"><code class="xref cpp cpp-class docutils literal">FixedBufferWriter</code></a>

comes down to the owner of the buffer - the former has its own buffer while the latter operates on

a buffer owned by some other object. Therefore if the buffer is declared locally, use

<a class="reference internal" href="#_CPPv2I_6size_tE17LocalBufferWriter" title="LocalBufferWriter"><code class="xref cpp cpp-class docutils literal">LocalBufferWriter</code></a> and if the buffer is recevied from an external source (such as via a

function parameter) use <a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter"><code class="xref cpp cpp-class docutils literal">FixedBufferWriter</code></a>.

<h3>Writing<a class="headerlink" href="#writing" title="Permalink to this headline">¶</a></h3>

The basic mechanism for writing to a <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> is <a class="reference internal" href="#_CPPv2N12BufferWriter5writeEPv6size_t" title="BufferWriter::write"><code class="xref cpp cpp-func docutils literal">BufferWriter::write()</code></a>.

This is an overloaded method for a character (<code class="code docutils literal">char</code>), a buffer (<code class="code docutils literal">void *, size_t</code>)

and a string view (<code class="xref cpp cpp-class docutils literal">string_view</code>). Because there is a constructor for <code class="xref cpp cpp-class docutils literal">string_view</code>

that takes a <code class="code docutils literal">const char*</code> as a C string, passing a literal string works as expected.

There are also stream operators in the style of C++ stream I/O. The basic template is

<div class="highlight-cpp"><div class="highlight"><pre>template < typename T > ts::BufferWriter& operator << (ts::BufferWriter& w, T const& t);

</pre></div>

</div>

The stream operators are provided as a convenience, the primary mechanism for formatted output is

via overloading the <a class="reference internal" href="#_CPPv2I0E8bwformatR12BufferWriterR7BWFSpecR1V" title="bwformat"><code class="xref cpp cpp-func docutils literal">bwformat()</code></a> function. Except for a limited set of cases the stream operators

are implemented by calling <a class="reference internal" href="#_CPPv2I0E8bwformatR12BufferWriterR7BWFSpecR1V" title="bwformat"><code class="xref cpp cpp-func docutils literal">bwformat()</code></a> with the Buffer Writer, the argument, and a default

format specification.

</div>

<h3>Reading<a class="headerlink" href="#reading" title="Permalink to this headline">¶</a></h3>

Data in the buffer can be extracted using <a class="reference internal" href="#_CPPv2NK12BufferWriter4dataEv" title="BufferWriter::data"><code class="xref cpp cpp-func docutils literal">BufferWriter::data()</code></a>. This and

<a class="reference internal" href="#_CPPv2NK12BufferWriter4sizeEv" title="BufferWriter::size"><code class="xref cpp cpp-func docutils literal">BufferWriter::size()</code></a> return a pointer to the start of the buffer and the amount of data

written to the buffer. This is effectively the same as <a class="reference internal" href="#_CPPv2NK12BufferWriter4viewEv" title="BufferWriter::view"><code class="xref cpp cpp-func docutils literal">BufferWriter::view()</code></a> which returns a

<code class="xref cpp cpp-class docutils literal">string_view</code> which covers the output data. Calling <a class="reference internal" href="#_CPPv2NK12BufferWriter5errorEv" title="BufferWriter::error"><code class="xref cpp cpp-func docutils literal">BufferWriter::error()</code></a> will indicate

if more data than space available was written (i.e. the buffer would have been overrun).

<a class="reference internal" href="#_CPPv2NK12BufferWriter6extentEv" title="BufferWriter::extent"><code class="xref cpp cpp-func docutils literal">BufferWriter::extent()</code></a> returns the amount of data written to the <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a>. This

can be used in a two pass style with a null / size 0 buffer to determine the buffer size required

for the full output.

</div>

<h3>Advanced<a class="headerlink" href="#advanced" title="Permalink to this headline">¶</a></h3>

The <a class="reference internal" href="#_CPPv2N12BufferWriter4clipE6size_t" title="BufferWriter::clip"><code class="xref cpp cpp-func docutils literal">BufferWriter::clip()</code></a> and <a class="reference internal" href="#_CPPv2N12BufferWriter6extendE6size_t" title="BufferWriter::extend"><code class="xref cpp cpp-func docutils literal">BufferWriter::extend()</code></a> methods can be used to reserve space

in the buffer. A common use case for this is to guarantee matching delimiters in output if buffer

space is exhausted. <a class="reference internal" href="#_CPPv2N12BufferWriter4clipE6size_t" title="BufferWriter::clip"><code class="xref cpp cpp-func docutils literal">BufferWriter::clip()</code></a> can be used to temporarily reduce the buffer size by

an amount large enough to hold the terminal delimiter. After writing the contained output,

<a class="reference internal" href="#_CPPv2N12BufferWriter6extendE6size_t" title="BufferWriter::extend"><code class="xref cpp cpp-func docutils literal">BufferWriter::extend()</code></a> can be used to restore the capacity and then output the terminal

delimiter.

Warning

Never call <a class="reference internal" href="#_CPPv2N12BufferWriter6extendE6size_t" title="BufferWriter::extend"><code class="xref cpp cpp-func docutils literal">BufferWriter::extend()</code></a> without previously calling <a class="reference internal" href="#_CPPv2N12BufferWriter4clipE6size_t" title="BufferWriter::clip"><code class="xref cpp cpp-func docutils literal">BufferWriter::clip()</code></a> and always pass the same argument value.

</div>

<a class="reference internal" href="#_CPPv2NK12BufferWriter9remainingEv" title="BufferWriter::remaining"><code class="xref cpp cpp-func docutils literal">BufferWriter::remaining()</code></a> returns the amount of buffer space not yet consumed.

<a class="reference internal" href="#_CPPv2NK12BufferWriter9auxBufferEv" title="BufferWriter::auxBuffer"><code class="xref cpp cpp-func docutils literal">BufferWriter::auxBuffer()</code></a> returns a pointer to the first byte of the buffer not yet used. This

is useful to do speculative output, or do bounded output in a manner similar to using

<a class="reference internal" href="#_CPPv2N12BufferWriter4clipE6size_t" title="BufferWriter::clip"><code class="xref cpp cpp-func docutils literal">BufferWriter::clip()</code></a> and <a class="reference internal" href="#_CPPv2N12BufferWriter6extendE6size_t" title="BufferWriter::extend"><code class="xref cpp cpp-func docutils literal">BufferWriter::extend()</code></a>. A new <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> instance

can be constructed with

<div class="highlight-cpp"><div class="highlight"><pre>ts::FixedBufferWriter subw(w.auxBuffer(), w.remaining());

</pre></div>

</div>

or as a convenience

<div class="highlight-cpp"><div class="highlight"><pre>ts::FixedBuffer subw{w.auxBuffer()};

</pre></div>

</div>

Output can be written to subw. If successful, then <code class="code docutils literal">w.fill(subw.size())</code> will add that

output to the main buffer. Depending on the purpose, <code class="code docutils literal">w.fill(subw.extent())</code> can be used -

this will track the attempted output if sizing is important. Note that space for any terminal

markers can be reserved by bumping down the size from <a class="reference internal" href="#_CPPv2NK12BufferWriter9remainingEv" title="BufferWriter::remaining"><code class="xref cpp cpp-func docutils literal">BufferWriter::remaining()</code></a>. Be careful of

underrun as the argument is an unsigned type.

If there is an error then subw can be ignored and some suitable error output written to

w instead. A common use case is to verify there is sufficient space in the buffer and create

a “not enough space” message if not. E.g.

<div class="highlight-cpp"><div class="highlight"><pre>ts::FixedBufferWriter subw{w.auxWriter()};

this->write_some_output(subw);

if (!subw.error()) w.fill(subw.size());

else w.write("Insufficient space"_sv);

</pre></div>

</div>

<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>

For example, error prone code that looks like

<div class="highlight-cpp"><div class="highlight"><pre>char new_via_string[1024]; // 512-bytes for hostname+via string, 512-bytes for the debug info

char * via_string = new_via_string;

char * via_limit = via_string + sizeof(new_via_string);

// ...

* via_string++ = ' ';

* via_string++ = '[';

// incoming_via can be max MAX_VIA_INDICES+1 long (i.e. around 25 or so)

if (s->txn_conf->insert_request_via_string > 2) { // Highest verbosity

via_string += nstrcpy(via_string, incoming_via);

} else {

memcpy(via_string, incoming_via + VIA_CLIENT, VIA_SERVER - VIA_CLIENT);

via_string += VIA_SERVER - VIA_CLIENT;

}

*via_string++ = ']';

</pre></div>

</div>

becomes

// ...

w.write(" ["_sv);

w.write(incoming_via);

} else {

w.write(ts::string_view{incoming_via + VIA_CLIENT, VIA_SERVER - VIA_CLIENT});

}

w.write(']');

</pre></div>

</div>

There will be no overrun on the memory buffer in w, in strong contrast to the original code.

This can be done better, as

<div class="highlight-cpp"><div class="highlight"><pre>if (w.remaining() >= 3) {

w.clip(1).write(" ["_sv);

w.write(incoming_via);

} else {

}

w.extend(1).write(']');

}

</pre></div>

</div>

This has the result that the terminal bracket will always be present which is very much appreciated

by code that parses the resulting log file.

</div>

<h2>Formatted Output<a class="headerlink" href="#formatted-output" title="Permalink to this headline">¶</a></h2>

The base <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> was made to provide memory safety for formatted output. Support for

formmatted output was made to provide type safety. The implementation deduces the types of the

arguments to be formatted and handles them in a type specific and safe way.

The formatting style is of the “prefix” or “printf” style - the format is specified first and then

all the arguments. This contrasts to the “infix” or “streaming” style where formatting, literals,

and argument are intermixed in the order of output. There are various arguments for both styles but

conversations within the Traffic Server community indicated a clear preference for the prefix style. Therefore

formatted out consists of a format string, containing formats, which are replaced during output

with the values of arguments to the print function.

The primary use case for formatting is formatted output to fixed buffers. This is by far the

dominant style of output in Traffic Server and during the design phase I was told any performance loss must be

minimal. While work has and will be done to extend <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> to operate on non-fixed

buffers, such use is secondary to operating directly on memory.

Important

The overriding design goal is to provide the type specific formatting and flexibility of C++

stream operators with the performance of <code class="code docutils literal">snprintf</code> and <code class="code docutils literal">memcpy</code>.

</div>

This will preserve the general style of output in Traffic Server while still reaping the benefits of type safe

formatting with little to no performance cost.

Type safe formatting has two major benefits -

<li>No mismatch between the format specifier and the argument. Although some modern compilers do

better at catching this at run time, there is still risk (especially with non-constant format

strings) and divergence between operating systems such that there is no <a class="reference external" href="https://github.com/apache/trafficserver/pull/3476/files">universally correct

choice</a>. In addition the number of

arguments can be verified to be correct which is often useful.</li>

<li>Formatting can be customized per type or even per partial type (e.g. <code class="code docutils literal">T*</code> for generic

<code class="code docutils literal">T</code>). This enables embedding common formatting work in the format system once, rather than

duplicating it in many places (e.g. converting enum values to names). This makes it easier for

developers to make useful error messages. See <a class="reference internal" href="#bwf-http-debug-name-example">this example</a>

for more detail.</li>

</ul>

As a result of these benefits there has been other work on similar projects, to replace

<code class="code docutils literal">printf</code> a better mechanism. Unfortunately most of these are rather project specific and don’t

suit the use case in Traffic Server. The two best options, <a class="reference external" href="https://www.boost.org/doc/libs/1_64_0/libs/format/">Boost.Format</a> and <a class="reference external" href="https://github.com/fmtlib/fmt">fmt</a>,

while good, are also not quite close enough to outweight the benefits of a version specifically

tuned for Traffic Server. <code class="docutils literal">Boost.Format</code> is not acceptable because of the Boost footprint. <code class="docutils literal">fmt</code> has the

problem of depending on C++ stream operators and therefore not having the required level of

performance or memory characteristics. Its main benefit, of reusing stream operators, doesn’t apply

to Traffic Server because of the nigh non-existence of such operators. The possibility of using C++ stream

operators was investigated but changing those to use pre-existing buffers not allocated internally

was very difficult, judged worse than building a relatively simple implementation from scratch. The

actual core implementation of formatted output for <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> is not very large - most of

the overall work will be writing formatters, work which would need to be done in any case but in

contrast to current practice, only done once.

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> supports formatting output in a style similar to Python formatting via

<a class="reference internal" href="#_CPPv2N12BufferWriter5printE8TextViewz" title="BufferWriter::print"><code class="xref cpp cpp-func docutils literal">BufferWriter::print()</code></a>. Looking at the other versions of work in this area, almost all of them

have gone with this style. Boost.Format also takes basically this same approach, just using

different paired delimiters. Traffic Server contains increasing amounts of native Python code which means many

Traffic Server developers will already be familiar (or should become familiar) with this style of formatting.

While not exactly the same at the Python version, BWF (<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> Formatting) tries to

be as similar as language and internal needs allow.

As noted previously and in the Python and even <code class="code docutils literal">printf</code> way, a format string consists of

literal text in which formats are embedded. Each format marks a place where formatted data of

an argument will be placed, along with argument specific formatting. The format is divided in to

three parts, separated by colons.

While this seems a bit complex, all of it is optional. If default output is acceptable, then BWF

will work with just the format <code class="docutils literal">{}</code>. In a sense, <code class="docutils literal">{}</code> serves the same function for output as

<code class="code docutils literal">auto</code> does for programming - the compiler knows the type, it should be able to do something

reasonable without the programmer needing to be explicit.

<pre>

format ::= "{" [name] [":" [specifier] [":" extension]] "}"

name ::= index | ICHAR+

index ::= non-negative integer

extension ::= ICHAR*

ICHAR ::= a printable ASCII character except for '{', '}', ':'

</pre>

<dd>The <a class="reference internal" href="#grammar-token-name"><code class="xref std std-token docutils literal">name</code></a> of the argument to use. This can be a non-negative integer in which case it is

the zero based index of the argument to the method call. E.g. <code class="docutils literal">{0}</code> means the first argument

and <code class="docutils literal">{2}</code> is the third argument after the format.

<div><code class="docutils literal">bw.print("{0} {1}", 'a', 'b')</code> => <code class="docutils literal">a b</code>

<code class="docutils literal">bw.print("{1} {0}", 'a', 'b')</code> => <code class="docutils literal">b a</code>

</div></blockquote>

The <a class="reference internal" href="#grammar-token-name"><code class="xref std std-token docutils literal">name</code></a> can be omitted in which case it is treated as an index in parallel to the

position in the format string. Only the position in the format string matters, not what names

other format elements may have used.

<div><code class="docutils literal">bw.print("{0} {2} {}", 'a', 'b', 'c')</code> => <code class="docutils literal">a c c</code>

<code class="docutils literal">bw.print("{0} {2} {2}", 'a', 'b', 'c')</code> => <code class="docutils literal">a c c</code>

</div></blockquote>

Note that an argument can be printed more than once if the name is used more than once.

<div><code class="docutils literal">bw.print("{0} {} {0}", 'a', 'b')</code> => <code class="docutils literal">a b a</code>

<code class="docutils literal">bw.print("{0} {1} {0}", 'a', 'b')</code> => <code class="docutils literal">a b a</code>

</div></blockquote>

Alphanumeric names refer to values in a global table. These will be described in more detail

someday. Such names, however, do not count in terms of default argument indexing.

</dd>

<dt><a class="reference internal" href="#grammar-token-specifier"><code class="xref std std-token docutils literal">specifier</code></a></dt>

<dd>Basic formatting control.

<pre>

specifier ::= [[fill]align][sign]["#"]["0"][[min][.precision][,max][type]]

fill ::= fill-char | URI-char

URI-char ::= "%" hex-digit hex-digit

fill-char ::= printable character except "{", "}", ":", "%"

align ::= "<" | ">" | "=" | "^"

sign ::= "+" | "-" | " "

min ::= non-negative integer

precision ::= positive integer

max ::= non-negative integer

type ::= type: "g" | "s" | "S" | "x" | "X" | "d" | "o" | "b" | "B" | "p" | "P"

hex-digit ::= "0" .. "9" | "a" .. "f" | "A" .. "F"

</pre>

The output is placed in a field that is at least <a class="reference internal" href="#grammar-token-min"><code class="xref std std-token docutils literal">min</code></a> wide and no more than <a class="reference internal" href="#grammar-token-max"><code class="xref std std-token docutils literal">max</code></a> wide. If

the output is less than <a class="reference internal" href="#grammar-token-min"><code class="xref std std-token docutils literal">min</code></a> then

<li>The <a class="reference internal" href="#grammar-token-fill"><code class="xref std std-token docutils literal">fill</code></a> character is used for the extra space required. This can be an explicit

character or a URI encoded one (to allow otherwise reserved characters).

</li>

<li>The output is shifted according to the <a class="reference internal" href="#grammar-token-align"><code class="xref std std-token docutils literal">align</code></a>.

<dd>Align to the left, fill to the right.

</dd>

</dl>

<dd>Align to the right, fill to the left.

</dd>

</dl>

<dd>Align in the middle, fill to left and right.

</dd>

</dl>

<dd>Numerically align, putting the fill between the sign character and the value.

</dd>

</dl>

</li>

</ul>

</div></blockquote>

The output is clipped by <a class="reference internal" href="#grammar-token-max"><code class="xref std std-token docutils literal">max</code></a> width characters and by the end of the buffer.

<a class="reference internal" href="#grammar-token-precision"><code class="xref std std-token docutils literal">precision</code></a> is used by floating point values to specify the number of places of precision.

<a class="reference internal" href="#grammar-token-type"><code class="xref std std-token docutils literal">type</code></a> is used to indicate type specific formatting. For integers it indicates the output

radix and if <code class="docutils literal">#</code> is present the radix is prefix is generated (one of <code class="docutils literal">0xb</code>, <code class="docutils literal">0</code>, <code class="docutils literal">0x</code>).

Format types of the same letter are equivalent, varying only in the character case used for

output. Most commonly ‘x’ prints values in lower cased hexadecimal (<code class="code docutils literal">0x1337beef</code>) while ‘X’

prints in upper case hexadecimal (<code class="code docutils literal">0X1337BEEF</code>). Note there is no upper case decimal or

octal type because case is irrelevant for those.

</colgroup>

<td>generic, default.</td>

</tr>

<td>binary</td>

</tr>

<td>Binary</td>

</tr>

<td>decimal</td>

</tr>

<td>octal</td>

</tr>

<td>hexadecimal</td>

</tr>

<td>Hexadecimal</td>

</tr>

<td>pointer (hexadecimal address)</td>

</tr>

<td>Pointer (Hexadecimal address)</td>

</tr>

<td>string</td>

</tr>

<td>String (upper case)</td>

</tr>

</tbody>

</table>

</div></blockquote>

For several specializations the hexadecimal format is taken to indicate printing the value as if

it were a hexidecimal value, in effect providing a hex dump of the value. This is the case for

<code class="xref cpp cpp-class docutils literal">string_view</code> and therefore a hex dump of an object can be done by creating a

<code class="xref cpp cpp-class docutils literal">string_view</code> covering the data and then printing it with <code class="code docutils literal">{:x}</code>.

The string type (‘s’ or ‘S’) is generally used to cause alphanumeric output for a value that would

normally use numeric output. For instance, a <code class="code docutils literal">bool</code> is normally <code class="docutils literal">0</code> or <code class="docutils literal">1</code>. Using the

type ‘s’ yields <code class="docutils literal">true` or ``false</code>. The upper case form, ‘S’, applies only in these cases where the

formatter generates the text, it does not apply to normally text based values unless specifically noted.

</dd>

<dt><a class="reference internal" href="#grammar-token-extension"><code class="xref std std-token docutils literal">extension</code></a></dt>

<dd>Text (excluding braces) that is passed to the type specific formatter function. This can be used

to provide extensions for specific argument types (e.g., IP addresses). The base logic ignores it

but passes it on to the formatting function which can then behave different based on the

extension.</dd>

</dl>

<h3>Usage Examples<a class="headerlink" href="#usage-examples" title="Permalink to this headline">¶</a></h3>

Some examples, comparing <code class="code docutils literal">snprintf</code> and <a class="reference internal" href="#_CPPv2N12BufferWriter5printE8TextViewz" title="BufferWriter::print"><code class="xref cpp cpp-func docutils literal">BufferWriter::print()</code></a>.

auto n = snprintf(buff, len, "count %d", count);

len -= n;

buff += n;

}

bw.print("count {}", count);

// --

if (len > 0) {

auto n = snprintf(buff, len, "Size %" PRId64 " bytes", sizeof(thing));

len -= n;

buff += n;

}

bw.print("Size {} bytes", sizeof(thing));

// --

if (len > 0) {

auto n = snprintf(buff, len, "Number of items %ld", thing->count());

len -= n;

buff += n;

}

bw.print("Number of items {}", thing->count());

</pre></div>

</div>

Enumerations become easier. Note in this case argument indices are used in order to print both a

name and a value for the enumeration. A key benefit here is the lack of need for a developer to know

the specific free function or method needed to do the name lookup. In this case,

<code class="code docutils literal">HttpDebugNuames::get_server_state_name</code>. Rather than every developer having to memorize the

assocation between the type and the name lookup function, or grub through the code hoping for an

example, the compiler is told once and henceforth does the lookup. The internal implementation of

this is <a class="reference internal" href="#bwf-http-debug-name-example">here</a>

auto n = snprintf(buff, len, "Unexpected event %d in state %s[%d] for %.*s",

event,

HttpDebugNames::get_server_state_name(t_state.current.state),

t_state.current.state,

static_cast<int>(host_len), host);

buff += n;

len -= n;

}

bw.print("Unexpected event {0} in state {1}[{1:d}] for {2}",

event, t_state.current.state, string_view{host, host_len});

</pre></div>

</div>

Using <code class="code docutils literal">std::string</code>, which illustrates the advantage of a formatter overloading knowing how to

get the size from the object and not having to deal with restrictions on the numeric type (e.g.,

that <code class="code docutils literal">%.*s</code> requires an <code class="code docutils literal">int</code>, not a <code class="code docutils literal">size_t</code>).

len -= snprintf(buff, len, "%.*s", static_cast<int>(s.size()), s.data);

}

bw.print("{}", s);

</pre></div>

</div>

IP addresses are much easier. There are two big advantages here. One is not having to know the

conversion function name. The other is the lack of having to declare local variables and having to

remember what the appropriate size is. Beyond there this code is more performant because the output

is rendered directly in the output buffer, not rendered to a temporary and then copied over. This

lack of local variables can be particularly nice in the context of a <code class="code docutils literal">switch</code> statement where

local variables for a <code class="code docutils literal">case</code> mean having to add extra braces, or declare the temporaries at an

outer scope.

<div class="highlight-cpp"><div class="highlight"><pre>char ip_buff1[INET6_ADDRPORTSTRLEN];

char ip_buff2[INET6_ADDRPORTSTRLEN];

ats_ip_nptop(ip_buff1, sizeof(ip_buff1), addr1);

ats_ip_nptop(ip_buff2, sizeof(ip_buff2), add2);

if (len > 0) {

snprintf(buff, len, "Connecting to %s from %s", ip_buff1, ip_buff2);

}

bw.print("Connecting to {} from {}", addr1, addr2);

</pre></div>

</div>

<h2>User Defined Formatting<a class="headerlink" href="#user-defined-formatting" title="Permalink to this headline">¶</a></h2>

To get the full benefit of type safe formatting it is necessary to provide type specific formatting

functions which are called when a value of that type is formatted. This is how type specific

knowledge such as the names of enumeration values are encoded in a single location. Additional type

specific formatting can be provided via the <a class="reference internal" href="#grammar-token-extension"><code class="xref std std-token docutils literal">extension</code></a> field. Without this, special formatting

requires extra functions and additional work at the call site, rather than a single consolidated

formatting function.

To provide a formatter for a type <code class="code docutils literal">V</code> the function <code class="code docutils literal">bwformat</code> is overloaded. The signature

would look like this:

<div class="highlight-cpp"><div class="highlight"><pre>BufferWriter& ts::bwformat(BufferWriter& w, BWFSpec const& spec, V const& v)

</pre></div>

</div>

w is the output and spec the parsed specifier, including the extension (if any). The

calling framework will handle basic alignment as per spec therfore the overload does not need

to unless the alignment requirements are more detailed (e.g. integer alignment operations) or

performance is critical. In the latter case the formatter should make sure to use at least the

minimum width in order to disable any additional alignment operation.

It is important to note that a formatter can call another formatter. For example, the formatter for

pointers looks like:

<div class="highlight-cpp"><div class="highlight"><pre>// Pointers that are not specialized.

inline BufferWriter &

bwformat(BufferWriter &w, BWFSpec const &spec, const void * ptr)

{

BWFSpec ptr_spec{spec};

ptr_spec._radix_lead_p = true;

if (ptr_spec._type == BWFSpec::DEFAULT_TYPE || ptr_spec._type == 'p') {

// if default or specifically 'p', switch to lower case hex.

ptr_spec._type = 'x';

} else if (ptr_spec._type == 'P') {

// Incoming 'P' means upper case hex.

ptr_spec._type = 'X';

}

return bw_fmt::Format_Integer(w, ptr_spec,

reinterpret_cast<intptr_t>(ptr), false);

}

</pre></div>

</div>

The code checks if the type <code class="docutils literal">p</code> or <code class="docutils literal">P</code> was used in order to select the appropriate case, then

delegates the actual rendering to the integer formatter with a type of <code class="docutils literal">x</code> or <code class="docutils literal">X</code> as

appropriate. In turn other formatters, if given the type <code class="docutils literal">p</code> or <code class="docutils literal">P</code> can cast the value to

<code class="code docutils literal">const void*</code> and call <code class="code docutils literal">bwformat</code> on that to output the value as a pointer.

To help reduce duplication, the output stream operator <code class="code docutils literal">operator<<</code> is defined to call this

function with a default constructed <code class="code docutils literal">BWFSpec</code> instance so that absent a specific overload

a BWF formatter will also provide a C++ stream output operator.

<h3>Enum Example<a class="headerlink" href="#enum-example" title="Permalink to this headline">¶</a></h3>

For a specific example of using BufferWriter formatting to make debug messages easier, consider the

case of <code class="code docutils literal">HttpDebugNames</code>. This is a class that serves as a namespace to provide various

methods that convert state machine related data into descriptive strings. Currently this is

undocumented (and even uncommented) and is therefore used infrequently, as that requires either

blind cut and paste, or tracing through header files to understand the code. This can be greatly

simplified by adding formatters to <a class="reference external" href="https://github.com/apache/trafficserver/blob/master/proxy/http/HttpDebugNames.h">HttpDebugNames.h</a>

<div class="highlight-cpp"><div class="highlight"><pre>inline ts::BufferWriter &

bwformat(ts::BufferWriter &w, ts::BWFSpec const &spec, HttpTransact::ServerState_t state)

{

if (spec.has_numeric_type()) {

// allow the user to force numeric output with '{:d}' or other numeric type.

return bwformat(w, spec, static_cast<uintmax_t>(state));

} else {

return bwformat(w, spec, HttpDebugNames::get_server_state_name(state));

}

</pre></div>

</div>

With this in place, any one wanting to print the name of the server state enumeration can do

<div class="highlight-cpp"><div class="highlight"><pre>bw.print("state {}", t_state.current_state);

</pre></div>

</div>

There is no need to remember names like <code class="code docutils literal">HttpDebugNames</code> nor which method in it does the

conversion. The developer making the <code class="code docutils literal">HttpDebugNames</code> class or equivalent can take care of

that in the same header file that provides the type.

Note

In actual practice, due to this method being so obscure it’s not actually used as far as I

can determine.

</div>

<h3>Argument Forwarding<a class="headerlink" href="#argument-forwarding" title="Permalink to this headline">¶</a></h3>

It will frequently be useful for other libraries to allow local formatting (such as <code class="code docutils literal">Errata</code>).

For such cases the class methods will need to take variable arguments and then forward them on to

the formatter. <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> provides the <a class="reference internal" href="#_CPPv2IDpEN12BufferWriter6printvE8TextViewRRNSt5tupleIDp4ArgsEE" title="BufferWriter::printv"><code class="xref cpp cpp-func docutils literal">BufferWriter::printv()</code></a> overload for this

purpose. Instead of taking variable arguments, these overloads take a <code class="code docutils literal">std::tuple</code> of

arguments. Such as tuple is easily created with <a class="reference external" href="http://en.cppreference.com/w/cpp/utility/tuple/forward_as_tuple">std::forward_as_tuple</a>. A standard implementation that

uses the <code class="code docutils literal">std::string</code> overload for <a class="reference internal" href="#_CPPv2IDpE7bwprintRNSt6stringE11string_viewDpRR4Args" title="bwprint"><code class="xref cpp cpp-func docutils literal">bwprint()</code></a> would look like

std::string message(string_view fmt, Args &&... args) {

std::string zret;

return ts::bwprint(zret, fmt, std::forward_as_tuple(args...));

}

</pre></div>

</div>

This gathers the argument (generally references to the arguments) in to a single tuple which is then

passed by reference, to avoid restacking the arguments for every nested function call.

</div>

<h3>Specialized Types<a class="headerlink" href="#specialized-types" title="Permalink to this headline">¶</a></h3>

These are types for which there exists a type specific BWF formatter.

<dt><code class="xref cpp cpp-class docutils literal">string_view</code></dt>

<dd>Generally the contents of the view.

<dd>A hexadecimal dump of the contents of the view in lower (‘x’) or upper (‘X’) case.</dd>

<dd>The pointer and length value of the view in lower (‘p’) or upper (‘P’) case.</dd>

</dl>

The <a class="reference internal" href="#grammar-token-precision"><code class="xref std std-token docutils literal">precision</code></a> is interpreted specially for this type to mean “skip <a class="reference internal" href="#grammar-token-precision"><code class="xref std std-token docutils literal">precision</code></a>

initial characters”. When combined with <a class="reference internal" href="#grammar-token-max"><code class="xref std std-token docutils literal">max</code></a> this allows a mechanism for printing

substrings of the <code class="xref cpp cpp-class docutils literal">string_view</code>. For instance, to print the 10th through 20th characters

the format <code class="docutils literal">{:.10,20}</code> would suffice. Given the method <code class="code docutils literal">substr</code> for <code class="xref cpp cpp-class docutils literal">string_view</code>

is cheap, it’s unclear how useful this is.

</dd>

<dt><code class="code docutils literal">sockaddr const*</code></dt>

<dd>The IP address is printed. Fill is used to fill in address segments if provided, not to the

minimum width if specified. <code class="xref cpp cpp-class docutils literal">IpEndpoint</code> and <code class="xref cpp cpp-class docutils literal">IpAddr</code> are supported with the same

formatting. The formatting support in this case is extensive because of the commonality and

importance of IP address data.

Type overrides

<dd>The pointer address is printed as hexadecimal lower (‘p’) or upper (‘P’) case.</dd>

</dl>

</div></blockquote>

The extension can be used to control which parts of the address are printed. These can be in any order,

the output is always address, port, family. The default is the equivalent of “ap”. In addition, the

character ‘=’ (“numeric align”) can be used to internally right justify the elements.

<dd>The address.</dd>

<dd>The port (host order).</dd>

<dd>The IP address family.</dd>

<dd>Internally justify the numeric values. This must be the first or second character. If it is the second

the first character is treated as the internal fill character. If omitted ‘0’ (zero) is used.</dd>

</dl>

E.g.

<div class="last highlight-cpp"><div class="highlight"><pre>void func(sockaddr const* addr) {

bw.print("To {}", addr); // -> "To 172.19.3.105:4951"

bw.print("To {0::a} on port {0::p}", addr); // -> "To 172.19.3.105 on port 4951"

bw.print("To {::=}", addr); // -> "To 127.019.003.105:04951"

bw.print("Using address family {::f}", addr);

bw.print("{::a}",addr); // -> "172.19.3.105"

bw.print("{::=a}",addr); // -> "172.019.003.105"

bw.print("{::0=a}",addr); // -> "172.019.003.105"

bw.print("{:: =a}",addr); // -> "172. 19. 3.105"

bw.print("{:>20:a}",addr); // -> " 172.19.3.105"

bw.print("{:>20:=a}",addr); // -> " 172.019.003.105"

bw.print("{:>20: =a}",addr); // -> " 172. 19. 3.105"

}

</pre></div>

</div>

</dd>

</dl>

</div>

<h2>Global Names<a class="headerlink" href="#global-names" title="Permalink to this headline">¶</a></h2>

As a convenience, there are a few predefined global names that can be used to generate output. These

do not take any arguments to <a class="reference internal" href="#_CPPv2N12BufferWriter5printE8TextViewz" title="BufferWriter::print"><code class="xref cpp cpp-func docutils literal">BufferWriter::print()</code></a>, the data needed for output is either

process or thread global and is retrieved directly. They also are not counted for automatic indexing.

<dd>The epoch time in seconds.</dd>

<dd>The high resolution clock tick.</dd>

<dt>timestamp</dt>

<dd>UTC time in the format “Year Month Date Hour:Minute:Second”, e.g. “2018 Apr 17 14:23:47”.</dd>

<dt>thread-id</dt>

<dd>The id of the current thread.</dd>

<dt>thread-name</dt>

<dd>The name of the current thread.</dd>

<dt>ts-thread</dt>

<dd>A pointer to the Traffic Server <code class="xref cpp cpp-class docutils literal">Thread</code> object for the current thread. This is useful for comparisons.</dd>

<dt>ts-ethread</dt>

<dd>A pointer to the Traffic Server <code class="xref cpp cpp-class docutils literal">EThread</code> object for the current thread. This is useful for comparisons

or to indicate if the thread is an <code class="xref cpp cpp-class docutils literal">EThread</code> (if not, the value will be <code class="code docutils literal">nullptr</code>).</dd>

</dl>

For example, to have the same output as the normal diagnostic messages with a timestamp and the current thread:

<div class="highlight-cpp"><div class="highlight"><pre>bw.print("{timestamp} {ts-thread} Counter is {}", counter);

</pre></div>

</div>

Note that even though no argument is provided the global names do not count as part of the argument

indexing, therefore the preceeding example could be written as:

<div class="highlight-cpp"><div class="highlight"><pre>bw.print("{timestamp} {ts-thread} Counter is {0}", counter);

</pre></div>

</div>

<h2>Working with standard I/O<a class="headerlink" href="#working-with-standard-i-o" title="Permalink to this headline">¶</a></h2>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> can be used with some of the basic I/O functionality of a C++ environment. At the lowest

level the output stream operator can be used with a file descriptor or a <code class="code docutils literal">std::ostream</code>. For these

examples assume <code class="code docutils literal">bw</code> is an instance of <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> with data in it.

<div class="highlight-cpp"><div class="highlight"><pre>int fd = open("some_file", O_RDWR);

bw >> fd; // Write to file.

bw >> std::cout; // write to standard out.

</pre></div>

</div>

For convenience a stream operator for <code class="code docutils literal">std::stream</code> is provided to make the use more natural.

std::cout << bw.view(); // identical effect as the previous line.

</pre></div>

</div>

Using a <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> with <code class="code docutils literal">printf</code> is straight forward by use of the sized string

format code.

<div class="highlight-cpp"><div class="highlight"><pre>ts::LocalBufferWriter<256> bw;

bw.print("Failed to connect to {}", addr1);

printf("%.*s\n", static_cast<int>(bw.size()), bw.data());

</pre></div>

</div>

Alternatively the output can be null terminated in the formatting to avoid having to pass the size.

<div class="highlight-cpp"><div class="highlight"><pre>ts::LocalBufferWriter<256> bw;

printf("%s\n", bw.print("Failed to connect to {}\0", addr1).data());

</pre></div>

</div>

When using C++ stream I/O, writing to a stream can be done without any local variables at all.

<div class="highlight-cpp"><div class="highlight"><pre>std::cout << ts::LocalBufferWriter<256>().print("Failed to connect to {}\n", addr1);

</pre></div>

</div>

This is handy for temporary debugging messages as it avoids having to clean up local variable

declarations later, particularly when the types involved themselves require additional local

declarations (such as in this example, an IP address which would normally require a local text

buffer for conversion before printing). As noted previously this is particularly useful inside a

<code class="code docutils literal">case</code> where local variables are more annoying to set up.

</div>

<h2>Reference<a class="headerlink" href="#reference" title="Permalink to this headline">¶</a></h2>

class <code class="descclassname"></code><code class="descname">BufferWriter</code><a class="headerlink" href="#_CPPv212BufferWriter" title="Permalink to this definition">¶</a> </dt>

<dd><a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> is the abstract base class which defines the basic client interface. This

is intended to be the reference type used when passing concrete instances rather than having to

support the distinct types.

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">write</code>(void *data, <a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> length)<a class="headerlink" href="#_CPPv2N12BufferWriter5writeEPv6size_t" title="Permalink to this definition">¶</a> </dt>

<dd>Write to the buffer starting at data for at most length bytes. If there is not

enough room to fit all the data, none is written.

</dd></dl>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">write</code>(string_view str)<a class="headerlink" href="#_CPPv2N12BufferWriter5writeE11string_view" title="Permalink to this definition">¶</a> </dt>

<dd>Write the string str to the buffer. If there is not enough room to write the string no

data is written.

</dd></dl>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">write</code>(char c)<a class="headerlink" href="#_CPPv2N12BufferWriter5writeEc" title="Permalink to this definition">¶</a> </dt>

<dd>Write the character c to the buffer. If there is no space in the buffer the character

is not written.

</dd></dl>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">fill</code>(<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> n)<a class="headerlink" href="#_CPPv2N12BufferWriter4fillE6size_t" title="Permalink to this definition">¶</a> </dt>

<dd>Increase the output size by n without changing the buffer contents. This is used in

conjuction with <a class="reference internal" href="#_CPPv2NK12BufferWriter9auxBufferEv" title="BufferWriter::auxBuffer"><code class="xref cpp cpp-func docutils literal">BufferWriter::auxBuffer()</code></a> after writing output to the buffer returned by

that method. If this method is not called then such output will not be counted by

</dd></dl>

char *<code class="descclassname"></code><code class="descname">data</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter4dataEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return a pointer to start of the buffer.

</dd></dl>

<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> <code class="descclassname"></code><code class="descname">size</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter4sizeEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return the number of valid (written) bytes in the buffer.

</dd></dl>

string_view <code class="descclassname"></code><code class="descname">view</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter4viewEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return a <code class="xref cpp cpp-class docutils literal">string_view</code> that covers the valid data in the buffer.

</dd></dl>

<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> <code class="descclassname"></code><code class="descname">remaining</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter9remainingEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return the number of available remaining bytes that could be written to the buffer.

</dd></dl>

<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> <code class="descclassname"></code><code class="descname">capacity</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter8capacityEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return the number of bytes in the buffer.

</dd></dl>

char *<code class="descclassname"></code><code class="descname">auxBuffer</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter9auxBufferEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return a pointer to the first byte in the buffer not yet consumed.

</dd></dl>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">clip</code>(<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> n)<a class="headerlink" href="#_CPPv2N12BufferWriter4clipE6size_t" title="Permalink to this definition">¶</a> </dt>

<dd>Reduce the available space by n bytes.

</dd></dl>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">extend</code>(<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> n)<a class="headerlink" href="#_CPPv2N12BufferWriter6extendE6size_t" title="Permalink to this definition">¶</a> </dt>

<dd>Increase the available space by n bytes. Extreme care must be used with this method as

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> will trust the argument, having no way to verify it. In general this

should only be used after calling <a class="reference internal" href="#_CPPv2N12BufferWriter4clipE6size_t" title="BufferWriter::clip"><code class="xref cpp cpp-func docutils literal">BufferWriter::clip()</code></a> and passing the same value.

Together these allow the buffer to be temporarily reduced to reserve space for the trailing

element of a required pair of output strings, e.g. making sure a closing quote can be written

even if part of the string is not.

</dd></dl>

bool <code class="descclassname"></code><code class="descname">error</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter5errorEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return <code class="code docutils literal">true</code> if the buffer has overflowed from writing, <code class="code docutils literal">false</code> if not.

</dd></dl>

<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> <code class="descclassname"></code><code class="descname">extent</code>() const<a class="headerlink" href="#_CPPv2NK12BufferWriter6extentEv" title="Permalink to this definition">¶</a> </dt>

<dd>Return the total number of bytes in all attempted writes to this buffer. This value allows a

successful retry in case of overflow, presuming the output data doesn’t change. This works

well with the standard “try before you buy” approach of attempting to write output, counting

the characters needed, then allocating a sufficiently sized buffer and actually writing.

</dd></dl>

<a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">print</code>(TextView fmt, ...)<a class="headerlink" href="#_CPPv2N12BufferWriter5printE8TextViewz" title="Permalink to this definition">¶</a> </dt>

<dd>Print the arguments according to the format. See <a class="reference internal" href="#bw-formatting">bw-formatting</a>.

</dd></dl>

template<typename ...<code class="descname">Args</code>> <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a> &<code class="descclassname"></code><code class="descname">printv</code>(TextView fmt, <a class="reference internal" href="external-types.en.html#_CPPv2St" title="std">std</a>::tuple<<a class="reference internal" href="#_CPPv2IDpEN12BufferWriter6printvE8TextViewRRNSt5tupleIDp4ArgsEE" title="BufferWriter::printv::Args">Args</a>...> &&args)<a class="headerlink" href="#_CPPv2IDpEN12BufferWriter6printvE8TextViewRRNSt5tupleIDp4ArgsEE" title="Permalink to this definition">¶</a> </dt>

<dd>Print the arguments in the tuple args according to the format. See <a class="reference internal" href="#bw-formatting">bw-formatting</a>.

</dd></dl>

<a class="reference internal" href="external-types.en.html#_CPPv2St" title="std">std</a>::ostream &<code class="descclassname"></code><code class="descname">operator>></code>(<a class="reference internal" href="external-types.en.html#_CPPv2St" title="std">std</a>::ostream &stream) const<a class="headerlink" href="#_CPPv2NK12BufferWriterrsERNSt7ostreamE" title="Permalink to this definition">¶</a> </dt>

<dd>Write the contents of the buffer to stream and return stream.

</dd></dl>

ssize_t <code class="descclassname"></code><code class="descname">operator>></code>(int fd)<a class="headerlink" href="#_CPPv2N12BufferWriterrsEi" title="Permalink to this definition">¶</a> </dt>

<dd>Write the contents of the buffer to file descriptor fd and return the number of bytes

write (the results of the call to file <code class="code docutils literal">write()</code>).

</dd></dl>

class <code class="descclassname"></code><code class="descname">FixedBufferWriter</code> : public <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter">BufferWriter</a><a class="headerlink" href="#_CPPv217FixedBufferWriter" title="Permalink to this definition">¶</a> </dt>

<dd>This is a class that implements <a class="reference internal" href="#_CPPv212BufferWriter" title="BufferWriter"><code class="xref cpp cpp-class docutils literal">BufferWriter</code></a> on a fixed buffer, passed in to the constructor.

<code class="descclassname"></code><code class="descname">FixedBufferWriter</code>(void *buffer, <a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> length)<a class="headerlink" href="#_CPPv2N17FixedBufferWriter17FixedBufferWriterEPv6size_t" title="Permalink to this definition">¶</a> </dt>

<dd>Construct an instance that will write to buffer at most length bytes. If more

data is written, all data past the maximum size is discarded.

</dd></dl>

<a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter">FixedBufferWriter</a> &<code class="descclassname"></code><code class="descname">reduce</code>(<a class="reference internal" href="external-types.en.html#_CPPv26size_t" title="size_t">size_t</a> n)<a class="headerlink" href="#_CPPv2N17FixedBufferWriter6reduceE6size_t" title="Permalink to this definition">¶</a> </dt>

<dd>Roll back the output to have n valid (used) bytes.

</dd></dl>

<a class="reference internal" href="#_CPPv217FixedBufferWriter" title="FixedBufferWriter">FixedBufferWriter</a> &<code class="descclassname"></code><code class="descname">reset</code>()<a class="headerlink" href="#_CPPv2N17FixedBufferWriter5resetEv" title="Permalink to this definition">¶</a> </dt>

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