浮点长度在32和40位之间变化 [英] Length of float changes between 32 and 40 bit

问题描述

在将 float 值导出到文件时，遇到了一个奇怪的问题。我会期望每个浮点是相同的长度（显然），但我的程序有时导出一个32位数，有时作为一个40位数。
仍然显示此行为的程序的最小工作示例是：

I encountered an odd problem when exporting float values to a file. I would expect every float to be of the same length (obviously), but my programme sometimes exports it a 32 bit number and sometimes as a 40 bit number. A minimal working example of a programme that still shows this behaviour is:

#include <stdio.h>

const char* fileName = "C:/Users/Path/To/TestFile.txt";
float array [5];

int main(int argc, char* argv [])
{
    float temp1 = 1.63006e-33f;
    float temp2 = 1.55949e-32f;

    array[0] = temp1;
    array[1] = temp2;
    array[2] = temp1;
    array[3] = temp2;
    array[4] = temp2;

    FILE* outputFile;
    if (!fopen_s(&outputFile, fileName, "w")) 
    {
        fwrite(array, 5 * sizeof(float), 1, outputFile);
        fclose(outputFile);
    }

     return true;
}

我希望输出文件包含正好20（5乘4）字节，每四个表示一个浮点。但是，我得到这个：

I would expect the output file to contain exactly 20 (5 times 4) bytes, each four of which represent a float. However, I get this:

 8b 6b 07 09      // this is indeed 1.63006e-33f 
 5b f2 a1 0d 0a   // I don't know what this is but it's a byte too long
 8b 6b 07 09      
 5b f2 a1 0d 0a   
 5b f2 a1 0d 0a 

因此，float temp2 需要5个字节而不是4个字节，文件的长度是23.这是怎么可能的？数字不是那么小，它们是次正常数字，我不能想到任何其他原因为什么会有大小的差异。

So the float temp2 takes 5 bytes instead of four, and the total length of he file is 23. How is this possible?! The number aren't so small that they are subnormal numbers, and I can't think of any other reason why there would be a difference in size.

我使用在64位Windows 7系统上的MSVC 2010编译器。

I am using the MSVC 2010 compiler on a 64-bit Windows 7 system.

注意：我已经问过一个非常类似的问题，当我意识到问题更笼统，我决定以更简洁的方式转发它。
QDataStream有时使用32位，有时40位浮点数。

Note: I already asked a very similar question here, but when I realised the problem was more general, I decided to repost it in a more concise way. QDataStream uses sometimes 32 bit and sometimes 40 bit floats

推荐答案

问题是在Windows上，你必须区分文本和二进制文件。您已将文件打开为文本，这意味着 0d （回车）在每个 0a 饲料）。打开文件如下：

The problem is that on Windows, you have to differentiate between text and binary files. You have the file opened as text, which means 0d (carriage-return) is inserted before every 0a (line-feed) written. Open the file like this:

if (!fopen_s(&outputFile, fileName, "wb"))

剩下的就像以前一样工作。

The rest as before, and it should work.

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