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 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P1759R0: Native handle from file streams</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1759R0<br>Native handle from file streams</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-06-17">2019-06-17</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P1759R0">https://wg21.link/P1759R0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="https://eliaskosunen.com">Elias Kosunen</a> <a class="u-email email" href="mailto:isocpp@eliaskosunen.com">isocpp@eliaskosunen.com</a>
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Audience:
     <dd>LEWGI
     <dt>Source:
     <dd><a href="https://github.com/eliaskosunen/wg21/blob/master/P1759R0.bs">github.com/eliaskosunen/wg21: P1759R0</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper proposes adding functionality to fstream and filebuf to retrieve the native file handle.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#history"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0"><span class="secno">1.1</span> <span class="content">Revision 0</span></a>
     </ol>
    <li><a href="#overview"><span class="secno">2</span> <span class="content">Overview</span></a>
    <li><a href="#motivation"><span class="secno">3</span> <span class="content">Motivation</span></a>
    <li><a href="#scope"><span class="secno">4</span> <span class="content">Scope</span></a>
    <li>
     <a href="#design"><span class="secno">5</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li><a href="#free-func"><span class="secno">5.1</span> <span class="content">Free function and namespace-scope typedef</span></a>
      <li><a href="#separate-handle-type"><span class="secno">5.2</span> <span class="content">Separate <code class="highlight"><c- n>native_handle_type</c-></code> from <code class="highlight"><c- kr>thread</c-></code> and Networking TS</span></a>
      <li><a href="#handle-type"><span class="secno">5.3</span> <span class="content">Type of <code class="highlight"><c- n>native_file_handle_type</c-></code></span></a>
      <li><a href="#naming"><span class="secno">5.4</span> <span class="content">Naming</span></a>
     </ol>
    <li>
     <a href="#design-alternatives"><span class="secno">6</span> <span class="content">Design Alternatives</span></a>
     <ol class="toc">
      <li><a href="#member-typedef"><span class="secno">6.1</span> <span class="content">Member typedef instead of namespace scope typedef</span></a>
      <li><a href="#member"><span class="secno">6.2</span> <span class="content">Member instead of namespace scope</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">7</span> <span class="content">Impact On the Standard and Existing Code</span></a>
    <li><a href="#implementation"><span class="secno">8</span> <span class="content">Implementation</span></a>
    <li>
     <a href="#prior-art"><span class="secno">9</span> <span class="content">Prior Art</span></a>
     <ol class="toc">
      <li><a href="#discussion"><span class="secno">9.1</span> <span class="content">Discussion</span></a>
     </ol>
    <li>
     <a href="#standardese"><span class="secno">10</span> <span class="content">Technical Specifications</span></a>
     <ol class="toc">
      <li><a href="#file.handle"><span class="secno"></span> <span class="content">29.9.* Function template <code class="highlight"><c- n>native_file_handle</c-></code> [<strong>file.handle</strong>]</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">11</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="history"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#history"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r0"><span class="secno">1.1. </span><span class="content">Revision 0</span><a class="self-link" href="#r0"></a></h3>
    Initial revision. 
   <h2 class="heading settled" data-level="2" id="overview"><span class="secno">2. </span><span class="content">Overview</span><a class="self-link" href="#overview"></a></h2>
   <p>This paper proposes adding a new free function: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>native_file_handle</c-></code>.
It would take a standard file stream or stream buffer, and return the native file handle corresponding to that stream.</p>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>CharT</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Traits</c-><c- o>></c->
<c- k>auto</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_fstream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>-></c-> <c- n>native_file_handle_type</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>CharT</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Traits</c-><c- o>></c->
<c- k>auto</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_ifstream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>-></c-> <c- n>native_file_handle_type</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>CharT</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Traits</c-><c- o>></c->
<c- k>auto</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_ofstream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>-></c-> <c- n>native_file_handle_type</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>CharT</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Traits</c-><c- o>></c->
<c- k>auto</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>-></c-> <c- n>native_file_handle_type</c-><c- p>;</c->
</pre>
   <p>The return type of this function is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>native_file_handle_type</c-></code>,
which is a typedef to whatever type the platform uses for its file descriptors: <code class="highlight"><c- b>int</c-></code> on POSIX, <code class="highlight"><c- n>HANDLE</c-></code> (<code class="highlight"><c- b>void</c-><c- o>*</c-></code>) on Windows, and something else on other platforms.
This type is a non-owning handle and is to be small, <code class="highlight"><c- n>Regular</c-></code>, and trivially copyable.</p>
   <h2 class="heading settled" data-level="3" id="motivation"><span class="secno">3. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>For some operations, using OS/platform-specific file APIs is necessary.
If this is the case, they are unable to use iostreams without reopening the file with the platform-specific APIs.</p>
   <p>For example, if someone wanted to query the time a file was last modified on POSIX, they’d use <code class="highlight"><c- o>::</c-><c- n>fstat</c-></code>, which takes a file descriptor:</p>
<pre class="language-cpp highlight"><c- b>int</c-> <c- n>fd</c-> <c- o>=</c-> <c- o>::</c-><c- n>open</c-><c- p>(</c-><c- s>"~/foo.txt"</c-><c- p>,</c-> <c- n>O_RDONLY</c-><c- p>);</c->
<c- o>::</c-><c- n>stat</c-> <c- n>s</c-><c- p>{};</c->
<c- b>int</c-> <c- n>err</c-> <c- o>=</c-> <c- o>::</c-><c- n>fstat</c-><c- p>(</c-><c- n>fd</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>s</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>sys_seconds</c-> <c- n>last_modified</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>seconds</c-><c- p>(</c-><c- n>s</c-><c- p>.</c-><c- n>st_mtime</c-><c- p>.</c-><c- n>tv_sec</c-><c- p>);</c->
</pre>
   <p class="note" role="note"><span>Note:</span> The Filesystem TS introduced the <code class="highlight"><c- n>file_status</c-></code> structure and <code class="highlight"><c- n>status</c-></code> function retuning one.
This doesn’t solve our problem, because <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>filesystem</c-><c- o>::</c-><c- n>status</c-></code> takes a path, not a native file descriptor
(using paths is potentially racy),
and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>filesystem</c-><c- o>::</c-><c- n>file_status</c-></code> only contains member functions <code class="highlight"><c- n>type</c-><c- p>()</c-></code> and <code class="highlight"><c- n>permissions</c-><c- p>()</c-></code>,
not one for last time of modification.
Extending this structure is out of scope for this proposal.</p>
   <p>If the user needs to do a single operation not supported by the standard library,
they have to make choice between only using OS APIs, or reopening the file every time necessary,
likely forgetting to close the file, or running into buffering or synchronization issues.</p>
<pre class="language-cpp highlight"><c- c1>// Writing the latest modification date to a file</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>sys_seconds</c-> <c- n>last_modified</c-><c- p>(</c-><c- b>int</c-> <c- n>fd</c-><c- p>)</c-> <c- p>{</c->
    <c- c1>// See above</c->
<c- p>}</c->

<c- c1>// Today’s code</c->
<c- p>{</c->
    <c- b>int</c-> <c- n>fd</c-> <c- o>=</c-> <c- o>::</c-><c- n>open</c-><c- p>(</c-><c- s>"~/foo.txt"</c-><c- p>,</c-> <c- n>O_RDONLY</c-><c- p>);</c-> <c- c1>// CreateFile on Windows</c->
    <c- k>auto</c-> <c- n>lm</c-> <c- o>=</c-> <c- n>last_modified</c-><c- p>(</c-><c- n>fd</c-><c- p>);</c->

    <c- c1>// Using iostreams</c->
    <c- k>if</c-> <c- p>(</c-><c- n>use_iostreams</c-><c- p>)</c-> <c- p>{</c->
        <c- o>::</c-><c- n>close</c-><c- p>(</c-><c- n>fd</c-><c- p>);</c-> <c- c1>// CloseFile on Windows</c->
        <c- c1>// Hope the path still points to the file!</c->
        <c- n>std</c-><c- o>::</c-><c- n>ofstream</c-> <c- n>of</c-><c- p>(</c-><c- s>"~/foo.txt"</c-><c- p>);</c->
        <c- n>of</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"%c"</c-><c- p>,</c-> <c- n>lm</c-><c- p>)</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
    <c- p>}</c->

    <c- c1>// Or using POSIX ::write()</c->
    <c- k>if</c-> <c- p>(</c-><c- n>use_posix</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// Using ::write() is clunky;</c->
        <c- c1>// skipping error handling for brevity</c->
        <c- k>auto</c-> <c- n>str</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"%c"</c-><c- p>,</c-> <c- n>lm</c-><c- p>);</c->
        <c- n>str</c-><c- p>.</c-><c- n>push_back</c-><c- p>(</c-><c- sc>'\n'</c-><c- p>);</c->
        <c- o>::</c-><c- n>write</c-><c- p>(</c-><c- n>fd</c-><c- p>,</c-> <c- n>str</c-><c- p>.</c-><c- n>data</c-><c- p>(),</c-> <c- n>str</c-><c- p>.</c-><c- n>size</c-><c- p>());</c->
        <c- c1>// Remember to close!</c->
        <c- o>::</c-><c- n>close</c-><c- p>(</c-><c- n>fd</c-><c- p>);</c->
    <c- p>}</c->
<c- p>}</c->

<c- c1>// This proposal</c->
<c- p>{</c->
    <c- c1>// No need to use platform-specific APIs to open the file</c->
    <c- n>std</c-><c- o>::</c-><c- n>ofstream</c-> <c- n>of</c-><c- p>(</c-><c- s>"~/foo.txt"</c-><c- p>);</c->
    <c- k>auto</c-> <c- n>lm</c-> <c- o>=</c-> <c- n>last_modified</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>native_file_handle</c-><c- p>(</c-><c- n>of</c-><c- p>));</c->
    <c- n>of</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"%c"</c-><c- p>,</c-> <c- n>lm</c-><c- p>)</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
    <c- c1>// RAII does ownership handling for us</c->
<c- p>}</c->
</pre>
   <p>The utility of getting a file descriptor (or other native file handle) is not limited to getting the last modification date.
Other examples include, but are definitely not limited to:</p>
   <ul>
    <li data-md>
     <p>file locking (<code class="highlight"><c- n>fcntl</c-><c- p>()</c-></code> + <code class="highlight"><c- n>F_SETLK</c-></code> on POSIX, <code class="highlight"><c- n>LockFile</c-></code> on Windows)</p>
    <li data-md>
     <p>getting file status flags (<code class="highlight"><c- n>fcntl</c-><c- p>()</c-></code> + <code class="highlight"><c- n>F_GETFL</c-></code> on POSIX, <code class="highlight"><c- n>GetFileInformationByHandle</c-></code> on Windows)</p>
    <li data-md>
     <p>non-blocking IO (<code class="highlight"><c- n>fcntl</c-><c- p>()</c-></code> + <code class="highlight"><c- n>O_NONBLOCK</c-></code>/<code class="highlight"><c- n>F_SETSIG</c-></code> on POSIX)</p>
   </ul>
   <p>Basically, this paper would make standard file streams interoperable with operating system interfaces,
making iostreams more useful in that regard.</p>
   <p>Facilities replacing iostreams, although desirable, are not going to be available in the standard in the near future.
The author, alongside many others, would thus find this functionality useful.
When an iostreams replacement eventually arrives, the overload set can then be extended for easy interop.</p>
   <h2 class="heading settled" data-level="4" id="scope"><span class="secno">4. </span><span class="content">Scope</span><a class="self-link" href="#scope"></a></h2>
   <p>This paper does <em>not</em> propose constructing a file stream or stream buffer from a native file handle.
The author is worried of ownership and implementation issues possibly associated with this design.</p>
<pre class="language-cpp highlight"><c- c1>// Not part of this proposal</c->
<c- c1>// POSIX example</c->
<c- cp>#include</c-> &lt;fstream>
<c- cp>#include</c-> &lt;fcntl.h>

<c- n>std</c-><c- o>::</c-><c- n>native_file_handle_type</c-> <c- n>fd</c-> <c- o>=</c-> <c- o>::</c-><c- n>open</c-><c- p>(</c-><c- d>/* ... */</c-><c- p>);</c->
<c- k>auto</c-> <c- n>f</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>fstream</c-><c- p>{</c-><c- n>fd</c-><c- p>};</c->
</pre>
   <p>This paper also does <em>not</em> propose getting the native handle from a C-style file stream <code class="highlight"><c- b>FILE</c-></code>.</p>
<pre class="language-cpp highlight"><c- c1>// Not part of this proposal</c->
<c- cp>#include</c-> &lt;cstdio>

<c- k>auto</c-> <c- n>fd</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>native_file_handle</c-><c- p>(</c-><c- n>stdout</c-><c- p>);</c-> <c- c1>// stdout is of type FILE*</c->
<c- c1>// This would essentially be a wrapper over POSIX fileno or Windows _fileno + _get_osfhandle</c->
</pre>
   <p>The author is open to extending the paper to cover one or all of these areas,
in this paper or some separate one (whichever is more appropriate),
should there be a desire and consensus for doing so.</p>
   <h2 class="heading settled" data-level="5" id="design"><span class="secno">5. </span><span class="content">Design Decisions</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="free-func"><span class="secno">5.1. </span><span class="content">Free function and namespace-scope typedef</span><a class="self-link" href="#free-func"></a></h3>
   <p>In this proposal, <code class="highlight"><c- n>native_file_handle</c-></code> is a namespace-scope free function
to avoid needing to add a new <code class="highlight"><c- k>virtual</c-></code> function, causing an ABI break.</p>
   <p>This paper opted for a namespace scope typedef for deliberately introducing inconsistency; <code class="highlight"><c- n>std</c-><c- o>::</c-><c- kr>thread</c-></code> has a member function <code class="highlight"><c- n>native_handle</c-></code>, and since we have a free function,
a namespace scope typedef is used to draw attention to the fact that the interface is different.</p>
   <h3 class="heading settled" data-level="5.2" id="separate-handle-type"><span class="secno">5.2. </span><span class="content">Separate <code class="highlight"><c- n>native_handle_type</c-></code> from <code class="highlight"><c- kr>thread</c-></code> and Networking TS</span><a class="self-link" href="#separate-handle-type"></a></h3>
   <p>C++11 thread support library includes functionality for getting a native handle out of a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- kr>thread</c-></code>.
Several types in there have members <code class="highlight"><c- n>native_handle_type</c-></code> and <code class="highlight"><c- n>native_handle</c-></code>.
The same case also applies for the Networking TS <a data-link-type="biblio" href="#biblio-n4734">[N4734]</a>.
The author feels like it’d be good design to keep <code class="highlight"><c- n>native_file_handle_type</c-></code> separate from threads for the sake of type safety,
even though they could be the same underlying type.</p>
   <h3 class="heading settled" data-level="5.3" id="handle-type"><span class="secno">5.3. </span><span class="content">Type of <code class="highlight"><c- n>native_file_handle_type</c-></code></span><a class="self-link" href="#handle-type"></a></h3>
   <p>This paper describes <code class="highlight"><c- n>native_file_handle_type</c-></code> as a typedef to the native file descriptor type.
Alternatives to this could be:</p>
   <ul>
    <li data-md>
     <p>making it an <code class="highlight"><c- k>enum</c-> <c- k>class</c-></code>, à la <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>byte</c-></code></p>
    <li data-md>
     <p>making it a standard layout <code class="highlight"><c- k>struct</c-></code> with a member function/variable returning the actual handle</p>
   </ul>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- kr>thread</c-></code> defines its <code class="highlight"><c- n>native_handle_type</c-></code> as an implementation-defined typedef, just like this paper does.</p>
   <h3 class="heading settled" data-level="5.4" id="naming"><span class="secno">5.4. </span><span class="content">Naming</span><a class="self-link" href="#naming"></a></h3>
   <p>The names <code class="highlight"><c- n>native_file_handle</c-></code> and <code class="highlight"><c- n>native_file_handle_type</c-></code> are subject to bikeshedding.
A non-exhaustive list of alternatives:</p>
   <table>
    <thead>
     <tr>
      <th>Function
      <th>Type
    <tbody>
     <tr>
      <td><code class="highlight"><c- n>get_native_file_handle</c-></code>
      <td><code class="highlight"><c- n>native_file_handle</c-></code>
     <tr>
      <td><code class="highlight"><c- n>native_handle</c-></code>
      <td><code class="highlight"><c- n>native_handle_type</c-></code>
     <tr>
      <td><code class="highlight"><c- n>get_native_handle</c-></code>
      <td><code class="highlight"><c- n>native_handle</c-></code>
     <tr>
      <td><code class="highlight"><c- n>get_native_handle</c-></code>
      <td><code class="highlight"><c- n>native_handle_type</c-></code>
     <tr>
      <td><code class="highlight"><c- n>file_descriptor</c-></code>
      <td><code class="highlight"><c- n>file_descriptor_type</c-></code>
     <tr>
      <td><code class="highlight"><c- n>get_file_descriptor</c-></code>
      <td><code class="highlight"><c- n>file_descriptor</c-></code>
     <tr>
      <td><code class="highlight"><c- n>native_file_descriptor</c-></code>
      <td><code class="highlight"><c- n>native_file_descriptor_type</c-></code>
     <tr>
      <td><code class="highlight"><c- n>get_native_file_descriptor</c-></code>
      <td><code class="highlight"><c- n>native_file_descriptor</c-></code>
   </table>
   <h2 class="heading settled" data-level="6" id="design-alternatives"><span class="secno">6. </span><span class="content">Design Alternatives</span><a class="self-link" href="#design-alternatives"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="member-typedef"><span class="secno">6.1. </span><span class="content">Member typedef instead of namespace scope typedef</span><a class="self-link" href="#member-typedef"></a></h3>
   <p>It could be a viable alternative to provide <code class="highlight"><c- n>native_file_handle_type</c-></code> as a member typedef inside <code class="highlight"><c- n>basic_filebuf</c-></code> and <code class="highlight"><c- n>basic_</c-><c- p>(</c-><c- n>i</c-><c- o>|</c-><c- n>o</c-><c- p>)</c-><c- n>fstream</c-></code>.</p>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-><c- o>></c->
<c- k>class</c-> <c- nc>basic_filebuf</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- k>using</c-> <c- n>native_handle_type</c-> <c- o>=</c-> <c- d>/* implementation-defined */</c->
    <c- c1>// ...</c->
<c- p>};</c->
<c- c1>// Same for file streams</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-><c- o>></c->
<c- k>auto</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>buf</c-><c- p>)</c-> <c- k>noexcept</c->
    <c- o>-></c-> <c- k>typename</c-> <c- k>decltype</c-><c- p>(</c-><c- n>buf</c-><c- p>)</c-><c- o>::</c-><c- n>native_handle_type</c-><c- p>;</c->
<c- c1>// Also overloads for file streams</c->
</pre>
   <h3 class="heading settled" data-level="6.2" id="member"><span class="secno">6.2. </span><span class="content">Member instead of namespace scope</span><a class="self-link" href="#member"></a></h3>
   <p>This paper proposes adding a free function and a typedef in namespace scope.
Should adding a member function be possible (having it not be <code class="highlight"><c- k>virtual</c-></code>), there is an alternative design:</p>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-><c- o>></c->
<c- k>class</c-> <c- nc>basic_filebuf</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- k>using</c-> <c- n>native_handle_type</c-> <c- o>=</c-> <c- d>/* implementation-defined */</c->
    <c- c1>// ...</c->
    <c- n>native_handle_type</c-> <c- n>native_handle</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>};</c->
<c- c1>// Same for file streams</c->
</pre>
   <p>The author would prefer this, if possible, due to consistency with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- kr>thread</c-></code> and the Networking TS.</p>
   <p>However, the author is doubtful about implementability and usability of this.
See <a href="#free-func">§5.1 Free function and namespace-scope typedef</a> for rationale.</p>
   <p>Should this alternative design be adopted, the names should be changed from <code class="highlight"><c- n>native_file_handle_type</c-></code> and <code class="highlight"><c- n>native_file_handle</c-></code> to <code class="highlight"><c- n>native_handle_type</c-></code> and <code class="highlight"><c- n>native_handle</c-></code>, respectively (dropping the <code class="highlight"><c- n>file</c-></code>), again for consistency with <code class="highlight"><c- kr>thread</c-></code>.</p>
   <h2 class="heading settled" data-level="7" id="impact"><span class="secno">7. </span><span class="content">Impact On the Standard and Existing Code</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal is a pure library extension, requiring no changes to the core language.
It would cause no existing conforming code to break.</p>
   <h2 class="heading settled" data-level="8" id="implementation"><span class="secno">8. </span><span class="content">Implementation</span><a class="self-link" href="#implementation"></a></h2>
   <p>Implementing this paper should be a relatively trivial task.
The only issue is, that a lot of the data is hidden behind a private interface,
so modification of the library internals would be required.
To go around this, the following reference implementations use an exposition-only
function <code class="highlight"><c- n>__get_cfile_handle</c-></code>, which returns an internal C stdio file handle
and could be implemented as a <code class="highlight"><c- k>friend</c-></code>.</p>
   <p>Although all implementations surveyed (libstdc++ and MSVC) use <code class="highlight"><c- b>FILE</c-><c- o>*</c-></code> instead of native file descriptors in their <code class="highlight"><c- n>basic_filebuf</c-></code> implementations,
these platforms provide facilites to get a native handle from a <code class="highlight"><c- b>FILE</c-><c- o>*</c-></code>; <code class="highlight"><c- n>fileno</c-></code> on POSIX, and <code class="highlight"><c- n>_fileno</c-></code> + <code class="highlight"><c- n>_get_osfhandle</c-></code> on Windows.
The following reference implementations use these.</p>
   <p>For libstdc++ on Linux:</p>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
    <c- k>using</c-> <c- n>native_file_handle_type</c-> <c- o>=</c-> <c- b>int</c-><c- p>;</c->

    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-><c- o>></c->
    <c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>buf</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// _M_file is a protected member variable of basic_filebuf,</c->
        <c- c1>// so using friend __get_cfile_handle instead</c->
        <c- k>const</c-> <c- n>__basic_file</c-><c- o>&lt;</c-><c- b>char</c-><c- o>>&amp;</c-> <c- n>file</c-> <c- o>=</c-> <c- n>__get_cfile_handle</c-><c- p>(</c-><c- n>buf</c-><c- p>);</c->
        <c- c1>// __basic_file&lt;char> has a member function for this purpose</c->
        <c- k>return</c-> <c- n>file</c-><c- p>.</c-><c- n>fd</c-><c- p>();</c->
        <c- c1>// ::fileno(file.file()) could also be used</c->
    <c- p>}</c->

    <c- c1>// Other overloads are trivial with rdbuf()</c->
<c- p>}</c->
</pre>
   <p>For MSVC:</p>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
    <c- k>using</c-> <c- n>native_file_handle_type</c-> <c- o>=</c-> <c- n>HANDLE</c-><c- p>;</c->

    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-><c- o>></c->
    <c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>buf</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// _Myfile is a private member of basic_filebuf,</c->
        <c- c1>// so using friend __get_cfile_handle instead</c->
        <c- k>auto</c-> <c- n>cfile</c-> <c- o>=</c-> <c- o>::</c-><c- n>_fileno</c-><c- p>(</c-><c- n>__get_cfile_handle</c-><c- p>(</c-><c- n>buf</c-><c- p>));</c->
        <c- k>return</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>HANDLE</c-><c- o>></c-><c- p>(</c-><c- o>::</c-><c- n>_get_osfhandle</c-><c- p>(</c-><c- n>cfile</c-><c- p>));</c->
    <c- p>}</c->

    <c- c1>// Other overloads are trivial with rdbuf()</c->
<c- p>}</c->
</pre>
   <h2 class="heading settled" data-level="9" id="prior-art"><span class="secno">9. </span><span class="content">Prior Art</span><a class="self-link" href="#prior-art"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-boostiostreams">[Boost.IOStreams]</a> provides <code class="highlight"><c- n>file_descriptor</c-></code>, <code class="highlight"><c- n>file_descriptor_source</c-></code>, and <code class="highlight"><c- n>file_descriptor_sink</c-></code>, which,
when used in conjunction with <code class="highlight"><c- n>stream_buffer</c-></code>, are <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>basic_streambuf</c-></code>s using a file descriptor.
These classes can be constructed from a path or a native handle (<code class="highlight"><c- b>int</c-></code> or <code class="highlight"><c- n>HANDLE</c-></code>) and can also return it with member function <code class="highlight"><c- n>handle</c-><c- p>()</c-></code>.</p>
   <p>Niall Douglas’s <a data-link-type="biblio" href="#biblio-p1031r1">[P1031R1]</a> also defined a structure <code class="highlight"><c- n>native_handle_type</c-></code> with an extensive interface and a member <code class="highlight"><c- k>union</c-></code> with an <code class="highlight"><c- b>int</c-></code> and a <code class="highlight"><c- n>HANDLE</c-></code>, with a constructor taking either one of these.</p>
   <h3 class="heading settled" data-level="9.1" id="discussion"><span class="secno">9.1. </span><span class="content">Discussion</span><a class="self-link" href="#discussion"></a></h3>
   <p>There has been some discussion over the years about various things relating to this issue,
but as far as the author is aware, no concrete proposal has ever been submitted.</p>
   <p>There have been a number of threads on std-discussion and std-proposals: <a data-link-type="biblio" href="#biblio-std-proposals-native-handle">[std-proposals-native-handle]</a>, <a data-link-type="biblio" href="#biblio-std-discussion-fd-io">[std-discussion-fd-io]</a>, <a data-link-type="biblio" href="#biblio-std-proposals-native-raw-io">[std-proposals-native-raw-io]</a>, <a data-link-type="biblio" href="#biblio-std-proposals-fd-access">[std-proposals-fd-access]</a>.
The last one of these lead to a draft paper, that was never submitted: <a data-link-type="biblio" href="#biblio-access-file-descriptors">[access-file-descriptors]</a>.</p>
   <p>The consensus that the author took from these discussions is, that native handle support for iostreams would be very much welcome.</p>
   <p>An objection was raised by Billy O’Neal to being able to retrieve a native file handle from a standard file stream:</p>
   <blockquote>
     [This] also would need to mandate that the C++ streams be implemented directly such that there was a 1:1 native handle relationship, which may not be the case.
For instance, a valid implementation of C++ iostreams would be on top of cstdio, which would not have any kind of native handle to expose. 
    <p>– Billy O’Neal: <a data-link-type="biblio" href="#biblio-std-proposals-billy-oneal">[std-proposals-billy-oneal]</a></p>
   </blockquote>
   <p>Every implementation surveyed did implement <code class="highlight"><c- n>basic_filebuf</c-></code> on top of C stdio, but these platforms also provide functionality for getting a file descriptor out of a <code class="highlight"><c- b>FILE</c-><c- o>*</c-></code>.
On every platform, file I/O is ultimately implemented on top of native APIs, so not providing access to a file descriptor from a <code class="highlight"><c- b>FILE</c-><c- o>*</c-></code> would be rather unfortunate.
Should such a platform exist, they probably don’t have a conforming C++ implementation anyway.
See <a href="#implementation">§8 Implementation</a> for more.</p>
   <h2 class="heading settled" data-level="10" id="standardese"><span class="secno">10. </span><span class="content">Technical Specifications</span><a class="self-link" href="#standardese"></a></h2>
   <p>The proposed wording is likely to be incomplet and/or incorrekt.</p>
   <p>Add the following into <i>Header <code class="highlight"><c- o>&lt;</c-><c- n>iosfwd</c-><c- o>></c-></code> synopsis</i> [<strong>iosfwd.syn</strong>]:</p>
   <blockquote>
<pre class="language-cpp highlight"><c- k>using</c-> <c- n>native_file_handle_type</c-> <c- o>=</c-> <c- d>/* implementation-defined */</c-><c- p>;</c->
</pre>
   </blockquote>
   <p>Add the following two paragraphs between § 5 and § 6 of <i>Overview</i> [<strong>iostream.forward.overview</strong>]:</p>
   <blockquote>
     The type <code class="highlight"><c- n>native_file_handle_type</c-></code> serves as a type representing a platform-specific handle to a file.
It satisfies the requirements of <code class="highlight"><c- n>Regular</c-></code> and is trivially copyable. 
    <p>[<i>Note:</i> For operating systems based on POSIX, <code class="highlight"><c- n>native_file_handle_type</c-></code> should be <code class="highlight"><c- b>int</c-></code>.
For Windows-based operating systems, it should be <code class="highlight"><c- n>HANDLE</c-></code>.]</p>
   </blockquote>
   <p>Add the following into <i>Header <code class="highlight"><c- o>&lt;</c-><c- n>fstream</c-><c- o>></c-></code> synopsis</i> [<strong>fstream.syn</strong>]:</p>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>buf</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_ifstream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_ofstream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_fstream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   </blockquote>
   <p>Modify <i>Class template <code class="highlight"><c- n>basic_filebuf</c-></code></i> [<strong>filebuf</strong>] § 1</p>
   <blockquote>
     The class <code class="highlight"><c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>></c-></code> associates both the input sequence and the output sequence with a file. 
    <ins> The file has an associated <code class="highlight"><c- n>native_file_handle_type</c-></code>. </ins>
   </blockquote>
   <p>Add the following to the appropriate section of <i>File-based streams</i> [<strong>file.streams</strong>].
Replace the * in 29.9.* with the appropriate number.</p>
   <blockquote>
    <h3 class="no-num heading settled" id="file.handle"><span class="content">29.9.* Function template <code class="highlight"><c- n>native_file_handle</c-></code> [<strong>file.handle</strong>]</span><a class="self-link" href="#file.handle"></a></h3>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_filebuf</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>buf</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p><em>Returns:</em> The <code class="highlight"><c- n>native_file_handle_type</c-></code> associated with the underlying file of <code class="highlight"><c- n>buf</c-></code>.</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_ifstream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_ofstream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-><c- o>></c->
<c- n>native_file_handle_type</c-> <c- n>native_file_handle</c-><c- p>(</c-><c- k>const</c-> <c- n>basic_fstream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p><em>Returns:</em> <code class="highlight"><c- n>native_file_handle</c-><c- p>(</c-><c- o>*</c-><c- n>stream</c-><c- p>.</c-><c- n>rdbuf</c-><c- p>());</c-></code></p>
   </blockquote>
   <h2 class="heading settled" data-level="11" id="acknowledgements"><span class="secno">11. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to the rest of the co-authors of <a data-link-type="biblio" href="#biblio-p1750r0">[P1750R0]</a> for the idea after cutting this functionality out.</p>
   <p>A special thanks to Jeff Garland for providing the heads-up about ABI that I totally would’ve missed.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-access-file-descriptors">[ACCESS-FILE-DESCRIPTORS]
   <dd>Bruce S. O. Adams. <a href="https://docs.google.com/viewer?a=v&amp;pid=forums&amp;srcid=MTEwODAzNzI2MjM1OTc0MjE3MjkBMDY0OTY1OTUzMjAwNzY0MTA0MjkBakhWMHBFLUNGd0FKATAuMQFpc29jcHAub3JnAXYy&amp;authuser=0">file streams and access to the file descriptor</a>. URL: <a href="https://docs.google.com/viewer?a=v&amp;pid=forums&amp;srcid=MTEwODAzNzI2MjM1OTc0MjE3MjkBMDY0OTY1OTUzMjAwNzY0MTA0MjkBakhWMHBFLUNGd0FKATAuMQFpc29jcHAub3JnAXYy&amp;authuser=0">https://docs.google.com/viewer?a=v&amp;pid=forums&amp;srcid=MTEwODAzNzI2MjM1OTc0MjE3MjkBMDY0OTY1OTUzMjAwNzY0MTA0MjkBakhWMHBFLUNGd0FKATAuMQFpc29jcHAub3JnAXYy&amp;authuser=0</a>
   <dt id="biblio-boostiostreams">[Boost.IOStreams]
   <dd>Jonathan Turkanis. <a href="https://www.boost.org/doc/libs/1_70_0/libs/iostreams/doc/index.html">Boost.IOStreams</a>. URL: <a href="https://www.boost.org/doc/libs/1_70_0/libs/iostreams/doc/index.html">https://www.boost.org/doc/libs/1_70_0/libs/iostreams/doc/index.html</a>
   <dt id="biblio-n4734">[N4734]
   <dd>Jonathan Wakely. <a href="https://wg21.link/n4734">Working Draft, C ++ Extensions for Networking</a>. 4 April 2018. URL: <a href="https://wg21.link/n4734">https://wg21.link/n4734</a>
   <dt id="biblio-p1031r1">[P1031R1]
   <dd>Niall Douglas. <a href="https://wg21.link/p1031r1">Low level file i/o library</a>. URL: <a href="https://wg21.link/p1031r1">https://wg21.link/p1031r1</a>
   <dt id="biblio-p1750r0">[P1750R0]
   <dd>Klemens Morgenstern, Jeff Garland, Elias Kosunen, Fatih Bakir. <a href="https://wg21.link/p1750r0">A Proposal to Add Process Management to the C++ Standard Library</a>. URL: <a href="https://wg21.link/p1750r0">https://wg21.link/p1750r0</a>
   <dt id="biblio-std-discussion-fd-io">[STD-DISCUSSION-FD-IO]
   <dd><a href="https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/macDvhFDrjU">File descriptor-backed I/O stream?</a>. URL: <a href="https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/macDvhFDrjU">https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/macDvhFDrjU</a>
   <dt id="biblio-std-proposals-billy-oneal">[STD-PROPOSALS-BILLY-ONEAL]
   <dd>Billy O'Neal. <a href="https://groups.google.com/a/isocpp.org/d/msg/std-proposals/oCEErQbI9sM/rMkAMOkxFvMJ">Comment on 'native_handle for basic_filebuf'</a>. URL: <a href="https://groups.google.com/a/isocpp.org/d/msg/std-proposals/oCEErQbI9sM/rMkAMOkxFvMJ">https://groups.google.com/a/isocpp.org/d/msg/std-proposals/oCEErQbI9sM/rMkAMOkxFvMJ</a>
   <dt id="biblio-std-proposals-fd-access">[STD-PROPOSALS-FD-ACCESS]
   <dd><a href="https://groups.google.com/a/isocpp.org/d/topic/std-proposals/XcQ4FZJKDbM/discussion">file streams and access to the file descriptor</a>. URL: <a href="https://groups.google.com/a/isocpp.org/d/topic/std-proposals/XcQ4FZJKDbM/discussion">https://groups.google.com/a/isocpp.org/d/topic/std-proposals/XcQ4FZJKDbM/discussion</a>
   <dt id="biblio-std-proposals-native-handle">[STD-PROPOSALS-NATIVE-HANDLE]
   <dd><a href="https://groups.google.com/a/isocpp.org/d/topic/std-proposals/oCEErQbI9sM/discussion">native_handle for basic_filebuf</a>. URL: <a href="https://groups.google.com/a/isocpp.org/d/topic/std-proposals/oCEErQbI9sM/discussion">https://groups.google.com/a/isocpp.org/d/topic/std-proposals/oCEErQbI9sM/discussion</a>
   <dt id="biblio-std-proposals-native-raw-io">[STD-PROPOSALS-NATIVE-RAW-IO]
   <dd><a href="https://groups.google.com/a/isocpp.org/d/topic/std-proposals/Q4RdFSZggSE/discussion">Native raw IO and FILE* wrappers?</a>. URL: <a href="https://groups.google.com/a/isocpp.org/d/topic/std-proposals/Q4RdFSZggSE/discussion">https://groups.google.com/a/isocpp.org/d/topic/std-proposals/Q4RdFSZggSE/discussion</a>
  </dl>