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<span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading">read (system call)</h1> 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.ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-No_footnotes plainlinks metadata ambox ambox-style ambox-No_footnotes" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/40px-Text_document_with_red_question_mark.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/60px-Text_document_with_red_question_mark.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/80px-Text_document_with_red_question_mark.svg.png 2x" data-file-width="48" data-file-height="48" /></span></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article includes a <a href="/wiki/Wikipedia:Citing_sources" title="Wikipedia:Citing sources">list of references</a>, <a href="/wiki/Wikipedia:Further_reading" title="Wikipedia:Further reading">related reading</a>, or <a href="/wiki/Wikipedia:External_links" title="Wikipedia:External links">external links</a>, <b>but its sources remain unclear because it lacks <a href="/wiki/Wikipedia:Citing_sources#Inline_citations" title="Wikipedia:Citing sources">inline citations</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Wikipedia:WikiProject_Fact_and_Reference_Check" class="mw-redirect" title="Wikipedia:WikiProject Fact and Reference Check">improve</a> this article by <a href="/wiki/Wikipedia:When_to_cite" title="Wikipedia:When to cite">introducing</a> more precise citations.</span> <span class="date-container"><i>(<span class="date">February 2019</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>In modern <a href="/wiki/POSIX" title="POSIX">POSIX</a> compliant <a href="/wiki/Operating_systems" class="mw-redirect" title="Operating systems">operating systems</a>, a program that needs to access data from a <a href="/wiki/Computer_file" title="Computer file">file</a> stored in a <a href="/wiki/File_system" title="File system">file system</a> uses the <b>read system call</b>. The file is identified by a <a href="/wiki/File_descriptor" title="File descriptor">file descriptor</a> that is normally obtained from a previous call to <a href="/wiki/Open_(system_call)" title="Open (system call)">open</a>. This system call reads in data in <a href="/wiki/Byte" title="Byte">bytes</a>, the number of which is specified by the caller, from the file and stores then into a buffer supplied by the calling process. </p><p>The read system call takes three arguments: </p> <ol><li>The file descriptor of the file.</li> <li>The buffer where the read data is to be stored.</li> <li>The number of bytes to be read from the file.</li></ol> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="POSIX_usage">POSIX usage</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Read_(system_call)&action=edit&section=1" title="Edit section: POSIX usage"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The read system call interface is standardized by the POSIX specification. Data from a file is read by calling the read function: </p> <div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre><span></span><span class="kt">ssize_t</span><span class="w"> </span><span class="nf">read</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">fd</span><span class="p">,</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">buf</span><span class="p">,</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">count</span><span class="p">);</span> </pre></div> <p>The value returned is the number of bytes read (zero indicates <a href="/wiki/End_of_file" class="mw-redirect" title="End of file">end of file</a>) and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a <a href="/wiki/Pipe_(Unix)" class="mw-redirect" title="Pipe (Unix)">pipe</a>, or from a <a href="/wiki/Computer_terminal" title="Computer terminal">terminal</a>), or because the system call was interrupted by a <a href="/wiki/Signal_(computing)" class="mw-redirect" title="Signal (computing)">signal</a>. </p><p>Alternatively, -1 is returned when an error occurs, in such a case <a href="/wiki/Errno" class="mw-redirect" title="Errno">errno</a> is set appropriately and further it is left unspecified whether the file position (if any) changes. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Read_(system_call)&action=edit&section=2" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Write_(system_call)" title="Write (system call)">write (system call)</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Read_(system_call)&action=edit&section=3" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html">POSIX read</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Read_(system_call)&action=edit&section=4" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><span class="noviewer" typeof="mw:File"><a href="/wiki/File:Wikibooks-logo-en-noslogan.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/16px-Wikibooks-logo-en-noslogan.svg.png" decoding="async" width="16" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/24px-Wikibooks-logo-en-noslogan.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/32px-Wikibooks-logo-en-noslogan.svg.png 2x" data-file-width="400" data-file-height="400" /></a></span> <a href="https://en.wikibooks.org/wiki/C_Programming/C_Reference/stdio.h/fread" class="extiw" title="wikibooks:C Programming/C Reference/stdio.h/fread">C_Programming/C_Reference/stdio.h/fread</a> at Wikibooks</li></ul> <div class="navbox-styles"><style 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.navbox-odd{background-color:transparent}.mw-parser-output .navbox .hlist td dl,.mw-parser-output .navbox .hlist td ol,.mw-parser-output .navbox .hlist td ul,.mw-parser-output .navbox td.hlist dl,.mw-parser-output .navbox td.hlist ol,.mw-parser-output .navbox td.hlist ul{padding:0.125em 0}.mw-parser-output .navbox .navbar{display:block;font-size:100%}.mw-parser-output .navbox-title .navbar{float:left;text-align:left;margin-right:0.5em}body.skin--responsive .mw-parser-output .navbox-image img{max-width:none!important}@media print{body.ns-0 .mw-parser-output .navbox{display:none!important}}</style></div><div role="navigation" class="navbox" aria-labelledby="Computer_files" style="padding:3px"><table class="nowraplinks hlist mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style 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abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Computer_files" title="Template:Computer files"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Computer_files" title="Template talk:Computer files"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Computer_files" title="Special:EditPage/Template:Computer files"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Computer_files" style="font-size:114%;margin:0 4em"><a href="/wiki/Computer_file" title="Computer file">Computer files</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Types</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Binary_file" title="Binary file">Binary file</a> / <a href="/wiki/Text_file" title="Text file">text file</a></li> <li><a href="/wiki/Data_file" title="Data file">Data file</a></li> <li><a href="/wiki/File_format" title="File format">File format</a> <ul><li><a href="/wiki/List_of_file_formats" title="List of file formats">List of file formats</a></li> <li><a href="/wiki/List_of_file_signatures" title="List of file signatures">File signatures</a></li> <li><a href="/wiki/Magic_number_(programming)" title="Magic number (programming)">Magic number</a></li> <li><a href="/wiki/Open_file_format" title="Open file format">Open file formats</a></li> <li><a href="/wiki/Proprietary_file_format" title="Proprietary file format">Proprietary file formats</a></li></ul></li> <li><a href="/wiki/Metafile" class="mw-redirect" title="Metafile">Metafile</a></li> <li><a href="/wiki/Sidecar_file" title="Sidecar file">Sidecar file</a></li> <li><a href="/wiki/Sparse_file" title="Sparse file">Sparse file</a></li> <li><a href="/wiki/Swap_file" class="mw-redirect" title="Swap file">Swap file</a></li> <li><a href="/wiki/System_file" title="System file">System file</a></li> <li><a href="/wiki/Temporary_file" title="Temporary file">Temporary file</a></li> <li><a href="/wiki/Zero-byte_file" title="Zero-byte file">Zero-byte file</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Properties</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Filename" title="Filename">Filename</a> <ul><li><a href="/wiki/8.3_filename" title="8.3 filename">8.3 filename</a></li> <li><a href="/wiki/Long_filename" title="Long filename">Long filename</a></li> <li><a href="/wiki/Filename_mangling" title="Filename mangling">Filename mangling</a></li></ul></li> <li><a href="/wiki/Filename_extension" title="Filename extension">Filename extension</a> <ul><li><a href="/wiki/List_of_filename_extensions" title="List of filename extensions">List of filename extensions</a></li></ul></li> <li><a href="/wiki/File_attribute" title="File attribute">File attribute</a> <ul><li><a href="/wiki/Extended_file_attributes" title="Extended file attributes">Extended file attributes</a></li></ul></li> <li><a href="/wiki/File_size" title="File size">File size</a></li> <li><a href="/wiki/Hidden_file_and_hidden_directory" title="Hidden file and hidden directory">Hidden file / Hidden directory</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Organisation</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Directory_(computing)" title="Directory (computing)">Directory/folder</a> <ul><li><a href="/wiki/NTFS_links" title="NTFS links">NTFS links</a></li> <li><a href="/wiki/Temporary_folder" title="Temporary folder">Temporary folder</a></li></ul></li> <li><a href="/wiki/Directory_structure" title="Directory structure">Directory structure</a></li> <li><a href="/wiki/File_system" title="File system">File system</a> <ul><li><a href="/wiki/Filesystem_Hierarchy_Standard" title="Filesystem Hierarchy Standard">Filesystem Hierarchy Standard</a></li> <li><a href="/wiki/Grid_file_system" title="Grid file system">Grid file system</a></li> <li><a href="/wiki/Semantic_file_system" title="Semantic file system">Semantic file system</a></li></ul></li> <li><a href="/wiki/Path_(computing)" title="Path (computing)">Path</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/File_operation" class="mw-redirect" title="File operation">Operations</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Open_(system_call)" title="Open (system call)">Open</a></li> <li><a href="/wiki/Close_(system_call)" title="Close (system call)">Close</a></li> <li><a class="mw-selflink selflink">Read</a></li> <li><a href="/wiki/Write_(system_call)" title="Write (system call)">Write</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Linking</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/File_descriptor" title="File descriptor">File descriptor</a></li> <li><a href="/wiki/Hard_link" title="Hard link">Hard link</a></li> <li><a href="/wiki/Shortcut_(computing)" title="Shortcut (computing)">Shortcut</a> <ul><li><a href="/wiki/Alias_(Mac_OS)" title="Alias (Mac OS)">Alias</a></li> <li><a href="/wiki/Shadow_(OS/2)" title="Shadow (OS/2)">Shadow</a></li></ul></li> <li><a href="/wiki/Symbolic_link" title="Symbolic link">Symbolic link</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Management</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Backup" title="Backup">Backup</a></li> <li><a href="/wiki/File_comparison" title="File comparison">File comparison</a></li> <li><a href="/wiki/File_copying" title="File copying">File copying</a></li> <li><a href="/wiki/Data_compression" title="Data compression">Data compression</a></li> <li><a href="/wiki/File_manager" title="File manager">File manager</a> <ul><li><a href="/wiki/Comparison_of_file_managers" title="Comparison of file managers">Comparison of file managers</a></li></ul></li> <li><a href="/wiki/File_system_fragmentation" title="File system fragmentation">File system fragmentation</a></li> <li><a href="/wiki/File-system_permissions" title="File-system permissions">File-system permissions</a></li> <li><a href="/wiki/File_transfer" title="File transfer">File transfer</a> <ul><li><a href="/wiki/File_sharing" title="File sharing">File sharing</a></li> <li><a href="/wiki/File_synchronization" title="File synchronization">File synchronization</a></li></ul></li> <li><a href="/wiki/File_verification" title="File verification">File verification</a></li></ul> </div></td></tr></tbody></table></div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Read_(system_call)&oldid=1240691376">https://en.wikipedia.org/w/index.php?title=Read_(system_call)&oldid=1240691376</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" 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