blocks|key|1013325|text|我在想read系统调用。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1013326|请记住，fread是“read”的包装器。|1013327|另一方面，fread有一个内部缓冲区，所以'read‘可能更快，但我认为'fread’会更有效率。|1013328|entityMap|0|LINK|mutability|MUTABLE|url|http://linux.die.net/man/2/read^0|3|4|0|0|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|U|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]]]

I'm thinking of the <a href="http://linux.die.net/man/2/read" rel="nofollow noreferrer">read</a> system call.

Keep in mind that fread is a wrapper for 'read'.

On the other hand fread has an internal buffer, so 'read' may be faster but i think 'fread' will be more efficient.

blocks|key|1013426|text|如果您愿意超越C规范而编写特定于操作系统的代码，那么内存映射通常被认为是最有效的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1013427|对于Posix，请查看mmap；对于Windows，请查看OpenFileMapping|offset|length|style|CODE|1013428|entityMap^0|0|B|4|T|F|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|I|$]]

If you are willing to go beyond the C spec into OS specific code, memory mapping is generally considered the most efficient way.

For Posix, check out <code>mmap</code> and for Windows check out <code>OpenFileMapping</code>

blocks|key|1227980|text|如果fread很慢，那是因为它向底层操作系统机制添加了额外的层，以便从文件中读取数据，而这些层干扰了特定程序使用fread的方式。换句话说，它慢是因为你没有以优化过的方式使用它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1227981|话虽如此，通过理解操作系统I/O函数是如何工作的，并提供您自己的抽象来更好地处理程序的特定I/O访问模式，可以更快地读取文件。大多数情况下，您可以使用内存映射文件来做到这一点。|1227982|但是，如果您正在运行的机器达到了极限，那么内存映射可能是不够的。在这一点上，真正由您决定如何优化您的I/O代码。|1227983|entityMap^0|2|5|1K|5|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|R|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|K|$]]

If <code>fread</code> is slow it is because of the additional layers it adds to the underlying operating system mechanism to read from a file that interfere with how your particular program is using <code>fread</code>. In other words, it's slow because you aren't using it the way it has been optimized for.

Having said that, faster file reading would be done by understanding how the operating system I/O functions work and providing your own abstraction that handles your program's particular I/O access patterns better. Most of the time you can do this with memory mapping the file.

However, if you are hitting the limits of the machine you are running on, memory mapping probably won't be sufficient. At that point it's really up to you to figure out how to optimize your I/O code.

blocks|key|1228087|text|一些人已经注意到的问题是，根据你的源，你的目标缓冲区大小等，你可以为特定的情况创建一个自定义的处理程序，但也有其他的情况，比如块/字符设备，也就是/dev/*，这样的标准规则是否适用，你的支持源可能是没有任何缓冲的连续弹出字符的东西，比如I2C总线，标准RS-232，等等。还有一些其他来源，字符设备是内存可映射的内存大段，就像nvidia与他们的视频驱动字符设备(/dev/nvidiactl)一样。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1228088|许多人在高性能应用程序中选择的另一个设计实现是异步而不是同步I/O，用于处理数据读取的方式。查看libaio，以及为异步I/O提供预打包解决方案的libaio的移植版本，以及在工作线程和使用者线程之间使用具有共享内存的读取(但请记住，如果您采用这种方式，这将增加编程复杂性)。异步I/O也是在标准OS系统调用中无法通过stdio获得的现成功能。只是要小心，因为根据规范，有一些读取是“可移植的”，但并不是所有的操作系统(例如FreeBSD+)都支持POSIX流(根据选择)。|1228089|您可以做的另一件事(取决于您的数据的可移植性)是查看压缩和/或转换为二进制格式，如数据库格式，即BDB、SQL等。一些数据库格式可以使用字节顺序转换函数跨机器移植。|1228090|一般来说，最好采用一组算法和方法，使用不同的方法运行性能测试，并评估服务于应用程序将服务的平均任务的最佳算法。这将帮助您确定最佳性能算法是什么。|1228091|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

The problem that some people have noted here, is that depending on your source, your target buffer size, etc, you can create a custom handler for that specific case, but there are other cases, like block/character devices, i.e. /dev/* where standard rules like that do or don't apply and your backing source might be something that pops character off serially without any buffering, like an I2C bus, standard RS-232, etc. And there are some other sources where character devices are memory mappable large sections of memory like nvidia does with their video driver character device (/dev/nvidiactl).

One other design implementation that many people have chosen in high-performance applications is asynchronous instead of synchronous I/O for handling how data is read. Look into libaio, and the ported versions of libaio which provide prepackaged solutions for asynchronous I/O, as well as look into using read with shared memory between a worker and consumer thread (but keep in mind that this will increase programming complexity if you go this route). Asynchronous I/O is also something that you can't get out of the box with stdio that you can get with standard OS system calls. Just be careful as there are bits of read which are `portable' according to the spec, but not all operating systems (like FreeBSD for instance) support POSIX STREAMs (by choice).

Another thing that you can do (depending on how portable your data is) is look into compression and/or conversion into a binary format like database formats, i.e. BDB, SQL, etc. Some database formats are portable across machines using endianness conversion functions.

In general it would be best to take a set of algorithms and methods, run performance tests using the different methods, and evaluate the best algorithm that serves the mean task that your application would serve. That would help you determine what the best performing algorithm is.

blocks|key|1013626|text|也许可以看看perl是如何做到这一点的。perl的I/O例程经过了优化，我想这就是为什么使用perl过滤器处理文本比使用sed进行同样的转换快两倍的原因。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1013627|显然，perl相当复杂，I/O只是它所做工作的一小部分。我从来没有看过它的来源，所以我不能给你更好的方向，除了给你指出here。|1013628|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cpan.org/src/README.html^0|1O|3|0|1N|4|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@]|D|@$9|T|A|U|1|V]]|E|$]]|$1|H|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

Maybe check out how perl does it. Perl's I/O routines are optimized, and are, I gather, the reason why processing text with a perl filter can be twice as fast as doing the same transformation with <code>sed</code>.

Obviously perl is pretty complex, and I/O is only one small part of what it does. I've never looked at its source so I couldn't give you any better directions than to point you <a href="http://www.cpan.org/src/README.html" rel="nofollow noreferrer">here</a>.

blocks|key|1228148|text|它不是最快的，但它很好，而且很短。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1228149|#include+<fcntl.h>
#include+<unistd.h>

int+main()+{
++++int+f+=+open("file1",+O_RDWR);

++++char+buffer[4096];

++++while+(+read(f,+buffer,+4096)+>+0+)+{
++++++++printf("%25s",+buffer);
++++}

}|code-block|syntax|javascript|1228150|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

It's not the fastest but it's pretty good and short.
<pre class="lang-c prettyprint-override"><code>#include &lt;fcntl.h&gt;
#include &lt;unistd.h&gt;

int main() {
 int f = open(&quot;file1&quot;, O_RDWR);

 char buffer[4096];

 while ( read(f, buffer, 4096) &gt; 0 ) {
 printf(&quot;%s&quot;, buffer);
 }

}
</code></pre>

Right now I am using fread() to read a file, but in other language fread() is inefficient i'v been told. Is this the same in C? If so, how would faster file reading be done?

Fastest file reading in C

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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现在我正在使用fread()来读取一个文件，但是在其他语言中，fread()是低效的。这在C中也是一样的吗？如果是这样，如何实现更快的文件读取速度？

问在C中读取文件的速度最快
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