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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/macros/json_no_thread_local/" rel="canonical"><link href=../json_no_io/ rel=prev><link href=../json_skip_library_version_check/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON_NO_THREAD_LOCAL - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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</code></pre></div> <p>When defined, the library does not use <code class=highlight><span class=k>thread_local</span></code> storage. This is relevant for the few environments whose toolchain does not support it.</p> <p>The copy constructor copies the first levels of a value by copying the containers, which copy their elements, and completes whatever is nested deeper than that without the call stack, so that copying a value cannot exhaust the stack however deeply it is nested. It counts the levels it has descended into in a <code class=highlight><span class=k>thread_local</span></code> variable, as a counter shared between threads would be raced.</p> <p>Without that counter, no descent can be bounded safely, so objects and arrays are copied without the call stack right away. Copying keeps working exactly as it does otherwise - the same values come out, and deeply nested values are copied just as safely - but copying is slower, because the containers no longer copy themselves. Copying the benchmark documents takes 9% (<code>canada.json</code>) to 34% (<code>twitter.json</code>) longer; values built mostly from objects are affected the most.</p> <h2 id=default-definition>Default definition<a class=headerlink href=#default-definition title="Permanent link">¶</a></h2> <p>By default, <code class=highlight><span class=n>JSON_NO_THREAD_LOCAL</span></code> is not defined.</p> <div class=highlight><pre><span></span><code><span class=cp>#undef JSON_NO_THREAD_LOCAL</span>
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</code></pre></div> <p>The library defines it by itself for Clang targeting MinGW, which does not survive the <code class=highlight><span class=k>thread_local</span></code> storage: copying a value segfaults there, with both old and current Clang versions, while <abbr title="GNU Compiler Collection">GCC</abbr> targeting MinGW is unaffected.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <details class=example> <summary>Example</summary> <p>The code below forces the library not to use <code class=highlight><span class=k>thread_local</span></code> storage.</p> <div class=highlight><pre><span></span><code><span class=cp>#define JSON_NO_THREAD_LOCAL 1</span>
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
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<span class=p>...</span>
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</code></pre></div> </details> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Added in version 3.12.1.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="September 24, 2026 15:01:19 UTC">September 24, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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# JSON_NO_THREAD_LOCAL
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```
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#define JSON_NO_THREAD_LOCAL
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```
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When defined, the library does not use `thread_local` storage. This is relevant for the few environments whose toolchain does not support it.
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The copy constructor copies the first levels of a value by copying the containers, which copy their elements, and completes whatever is nested deeper than that without the call stack, so that copying a value cannot exhaust the stack however deeply it is nested. It counts the levels it has descended into in a `thread_local` variable, as a counter shared between threads would be raced.
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Without that counter, no descent can be bounded safely, so objects and arrays are copied without the call stack right away. Copying keeps working exactly as it does otherwise - the same values come out, and deeply nested values are copied just as safely - but copying is slower, because the containers no longer copy themselves. Copying the benchmark documents takes 9% (`canada.json`) to 34% (`twitter.json`) longer; values built mostly from objects are affected the most.
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## Default definition
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By default, `JSON_NO_THREAD_LOCAL` is not defined.
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```
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#undef JSON_NO_THREAD_LOCAL
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```
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The library defines it by itself for Clang targeting MinGW, which does not survive the `thread_local` storage: copying a value segfaults there, with both old and current Clang versions, while GCC targeting MinGW is unaffected.
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## Examples
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Example
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The code below forces the library not to use `thread_local` storage.
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```
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#define JSON_NO_THREAD_LOCAL 1
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#include <nlohmann/json.hpp>
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...
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```
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## Version history
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- Added in version 3.12.1.
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