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 </code></pre></div> <p>When defined to <code>1</code>, parsing from a <code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> reads every character with <code>std::fgetc</code>, as in version 3.12.0 and earlier, instead of locking the file once and reading it with the unlocked stdio functions. This also turns off the scan of the <code>FILE</code>'s buffer in bulk (see below), which <a href=../json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> does not affect.</p> <p>It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below).</p> <p>Builds with MemorySanitizer use <code>std::fgetc</code> automatically, because the inline <code>getc_unlocked</code> of glibc reads the stdio buffer directly, which glibc (not instrumented) filled, and MemorySanitizer would report it as uninitialized.</p> <h2 id=default-definition>Default definition<a class=headerlink href=#default-definition title="Permanent link">&para;</a></h2> <p>The default value is <code>0</code>: the file is locked once and read with the unlocked stdio functions where they are available.</p> <div class=highlight><pre><span></span><code><span class=cp>#define JSON_DISABLE_UNLOCKED_FILE_IO 0</span>
 </code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition note"> <p class=admonition-title>Background</p> <p>The C standard library makes every <code>std::fgetc</code> call thread-safe by locking and unlocking the <code>FILE</code> object. When parsing from a <code>FILE*</code>, the parser reads one character at a time, so this happens for every character of the input.</p> <p>Instead, the parser locks the file once when parsing starts and unlocks it when parsing ends - also when an exception is thrown - and reads the characters in between without locking:</p> <table> <thead> <tr> <th>C library</th> <th>functions</th> </tr> </thead> <tbody> <tr> <td>glibc (Linux), Apple (macOS, iOS), Android, FreeBSD, NetBSD, OpenBSD, DragonFly BSD</td> <td><code>flockfile</code>, <code>getc_unlocked</code>, <code>funlockfile</code> (POSIX)</td> </tr> <tr> <td>Microsoft (<abbr title="Microsoft Visual C++">MSVC</abbr>, clang-cl)</td> <td><code>_lock_file</code>, <code>_fgetc_nolock</code>, <code>_unlock_file</code></td> </tr> <tr> <td>others (e.g., musl, <abbr title="Minimalist GNU for Windows">MinGW</abbr>, newlib)</td> <td><code>std::fgetc</code>, as before</td> </tr> </tbody> </table> <p>With glibc and the Apple C library, the parser also scans the characters that the <code>FILE</code> already holds in its buffer in bulk, as it scans the get area of a stream (see <a href=../json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a>): it reads the same fields of the <code>FILE</code> that the C library's own <code>getc_unlocked</code> reads, and it consumes exactly the characters that <code>getc_unlocked</code> would have returned, so the file is left where it was left before.</p> <p>Locking the file once makes parsing from a <code>FILE*</code> 3 to 4 times as fast with the Apple C library and 1.1 to 1.3 times as fast with glibc, whose locking is cheaper. Scanning the buffer in bulk makes <code>parse()</code> another 1.7 to 2.6 times as fast (and <code>accept()</code> 2.9 to 4.4 times), about as fast as parsing from a <code>std::ifstream</code>. The binary formats (<a href=../../basic_json/from_cbor/ ><code>from_cbor</code></a>, <a href=../../basic_json/from_msgpack/ ><code>from_msgpack</code></a>, ...) read from a <code>FILE*</code> the same way.</p> </div> <div class="admonition note"> <p class=admonition-title>Locking</p> <p>While a value is parsed from a <code>FILE*</code>, the parsing thread holds the file's lock. The lock is recursive, so the parsing thread itself - for instance, a <a href=../../json_sax/ ><abbr title="Simple API for XML">SAX</abbr></a> handler or a <a href=../../basic_json/parser_callback_t/ >parser callback</a> - can keep using the file with stdio functions such as <code>std::ftell</code>. Other threads that use the same <code>FILE</code> object wait until the parser is done. Before, they could interleave their reads with the parser's, which made the parsed input unpredictable anyway.</p> <p>A program in which a <abbr title="Simple API for XML">SAX</abbr> handler or parser callback waits for <strong>another</strong> thread that uses the <strong>same</strong> <code>FILE</code> object would deadlock. Define the macro to <code>1</code> for such programs.</p> </div> <div class="admonition note"> <p class=admonition-title><abbr title="Application Binary Interface">ABI</abbr> compatibility</p> <p>The value of this macro is encoded in the <a href=../../../features/namespace/ >namespace</a> (tag <code>_dufio</code>), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (<abbr title="One Definition Rule">ODR</abbr>) violations, but they cannot exchange instances of library types.</p> </div> <div class="admonition note"> <p class=admonition-title>Other inputs</p> <p>Reading from a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> is not affected by this macro; see <a href=../json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> for that input.</p> </div> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example: parsing from a <code>FILE*</code> (default)</summary> <p>The file is locked once while it is parsed:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;cstdio&gt;</span>
 <span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>