My code prints a hex that is not valid for an image, it's not a value from the array that I am comparing. I don't understand where is coming from

huangapple
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英文:

My code prints a hex that is not valid for an image, it's not a value from the array that I am comparing. I don't understand where is coming from

问题

以下是您要翻译的代码部分：

  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <string.h>
  5. int main()
  6. {
  7.     // 打开图像文件
  8.     FILE *image = fopen("me.jpg", "r");
  9.     // 读取图像
  10.     // 检查文件是否为NULL
  11.     if (image == NULL)
  12.     {
  13.         return 1;
  14.     }
  15.     // 存储字符签名在数组中
  16.     unsigned char signature[4];
  17.     // 存储额外数据
  18.     unsigned char extra[16] =
  19.     {
  20.         0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
  21.         0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef
  22.     };
  23.     // 读取签名
  24.     fread(signature, 4, 1, image);
  25.     // 读取额外数据
  26.     char extrahex = fread(extra, 16, 1, image);
  27.     // 在图像中查找额外数据
  28.     for (int i = 0; i < 16; i++)
  29.     {
  30.         if (extra[i] == extrahex)
  31.         {
  32.             printf("%#04x\n", extrahex);
  33.             return 0;
  34.             char hexnum = extrahex;
  35.             // JPEG的条件
  36.             if (signature[0] == 0xff && signature[1] == 0xd8 && signature[2] == 0xff && signature[3] == hexnum)
  37.             {
  38.                 printf("有效的JPEG格式\n");
  39.             }
  40.             else
  41.             {
  42.                 printf("不是有效的JPEG格式\n");
  43.             }
  44.             return 0;
  45.         }
  46.     }
  47.     fclose(image);
  48. }

请注意，您提供的代码存在一些逻辑错误，可能会导致不正确的结果。如果您需要关于代码逻辑方面的帮助，请提出具体的问题。

英文: 
  1. #include &lt;stdio.h&gt;
  2. #include &lt;stdlib.h&gt;
  3. #include &lt;string.h&gt;
  5. int main() 
  6. {
  7.     // open image
  8.     FILE *image = fopen(&quot;me.jpg&quot;, &quot;r&quot;);
  9.     // read image
  10.     // check if file is NULL
  11.     if(image == NULL) 
  12.     {
  13.         return 1;
  14.     }
  15.     // store char signature in array
  16.     unsigned char signature[4];
  17.     // store extra
  18.     unsigned char extra[16] = 
  19.     {
  20.       0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
  21.       0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef
  22.     };
  23.     // read signature
  24.     fread(signature, 4, 1, image);
  25.     // read extra
  26.     char extrahex = fread(extra, 16, 1, image);
  27.     // find extra in the image
  28.     for (int i = 0; i &lt; 16; i++) 
  29.     {
  30.         if(extra[i] == extrahex)
  31.         {
  32.             printf(&quot;%#04x\n&quot;, extrahex);
  33.             return 0;
  34.             char hexnum = extrahex;
  35.             // condition for jpeg
  36.             if(signature[0] == 0xff &amp;&amp; signature[1] == 0xd8 &amp;&amp; signature[2] == 0xff &amp;&amp; signature[3] == hexnum) 
  37.             {
  38.                 printf(&quot;valid jpeg format\n&quot;);
  39.             } 
  40.             else 
  41.             {
  42.                 printf(&quot;not a valid jpeg format\n&quot;);
  43.             }
  44.             return 0;
  45.         }
  46.     }
  47.     fclose(image);
  48. }

the return value that is printed is: 0x01, this is not a value from the extra array that I am checking against the image.
And of course, the result is "not a valid jpg format". I tried a different image, it prints the same 0x01 value.

My code prints a hex that is not valid for an image, it's not a value from the array that I am comparing. I don't understand where is coming from

答案1

得分: 0

根据这里提到的内容：https://en.cppreference.com/w/c/io/fread，fread的返回值是函数读取的完整项目的数量。在你的情况下，你尝试读取一个大小为16字节的元素。所以extrahex将始终为0x01。

只是一个猜测：你可能想要使用**char extrahex = signature[3]**而不是。

英文:

As mentioned here: https://en.cppreference.com/w/c/io/fread the return value of fread is the number of full items the function read. In your case you try to read 1 element of 16 bytes in size. So extrahex will always be 0x01.

Just a guess: you might want char extrahex = signature[3] instead

答案2

得分: 0

不需要再调用fread。相反，您必须检查signature[3]是否是存储在extra中的已知值之一。

  1. #include <stdio.h>
  3. int main(void)
  4. {
  5.     FILE *image = fopen("me.jpg", "r");
  7.     if (!image)
  8.     {
  9.         return 1;
  10.     }
  12.     unsigned char signature[4];
  13.     unsigned char extra[16] = {
  14.         0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
  15.         0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef
  16.     };
  18.     fread(signature, 4, 1, image);
  19.     fclose(image);
  21.     if (signature[0] == 0xff && signature[1] == 0xd8 && signature[2] == 0xff)
  22.     {
  23.         for (size_t i = 0; i < 16; i++)
  24.         {
  25.             if (signature[3] == extra[i])
  26.             {
  27.                 puts("有效的 JPEG 格式");
  28.                 return 0;
  29.             }
  30.         }
  31.     }
  33.     puts("不是有效的 JPEG 格式");
  34. }

或者，检查signature[3]的前四位是否为1110（0xe）。

  1. #include <stdio.h>
  3. int main(void)
  4. {
  5.     FILE *image = fopen("me.jpg", "r");
  7.     if (!image)
  8.     {
  9.         return 1;
  10.     }
  12.     unsigned char signature[4];
  14.     fread(signature, 4, 1, image);
  15.     fclose(image);
  17.     if (signature[0] == 0xff &&
  18.         signature[1] == 0xd8 &&
  19.         signature[2] == 0xff &&
  20.         (signature[3] >> 4) == 0xe)
  21.     {
  22.         puts("有效的 JPEG 格式");
  23.     }
  24.     else
  25.     {
  26.         puts("不是有效的 JPEG 格式");
  27.     }
  28. }
英文:

You do not need to make another call to fread. Instead, you must check if signature[3] is one of the known values stored in extra.

  1. #include &lt;stdio.h&gt;
  3. int main(void)
  4. {
  5.     FILE *image = fopen(&quot;me.jpg&quot;, &quot;r&quot;);
  7.     if (!image)
  8.     {
  9.         return 1;
  10.     }
  12.     unsigned char signature[4];
  13.     unsigned char extra[16] = {
  14.         0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
  15.         0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef
  16.     };
  18.     fread(signature, 4, 1, image);
  19.     fclose(image);
  21.     if (signature[0] == 0xff &amp;&amp; signature[1] == 0xd8 &amp;&amp; signature[2] == 0xff)
  22.     {
  23.         for (size_t i = 0; i &lt; 16; i++)
  24.         {
  25.             if (signature[3] == extra[i])
  26.             {
  27.                 puts(&quot;valid jpeg format&quot;);
  28.                 return 0;
  29.             }
  30.         }
  31.     }
  33.     puts(&quot;not a valid jpeg format&quot;);
  34. }

Alternatively, check that the first four bits of signature[3] are 1110 (0xe).

  1. #include &lt;stdio.h&gt;
  3. int main(void)
  4. {
  5.     FILE *image = fopen(&quot;me.jpg&quot;, &quot;r&quot;);
  7.     if (!image)
  8.     {
  9.         return 1;
  10.     }
  12.     unsigned char signature[4];
  14.     fread(signature, 4, 1, image);
  15.     fclose(image);
  17.     if (signature[0] == 0xff &amp;&amp;
  18.         signature[1] == 0xd8 &amp;&amp;
  19.         signature[2] == 0xff &amp;&amp;
  20.         (signature[3] &gt;&gt; 4) == 0xe)
  21.     {
  22.         puts(&quot;valid jpeg format&quot;);
  23.     }
  24.     else
  25.     {
  26.         puts(&quot;not a valid jpeg format&quot;);
  27.     }
  28. }

答案3

得分: 0

用Oka的示例解决了。有趣的是，我使用地址和指针进行了操作，但在printf中仍然得到了1作为返回值。My code prints a hex that is not valid for an image, it's not a value from the array that I am comparing. I don't understand where is coming from

英文:

Solved with the example of Oka. Interesting though, I played with addresses and pointers and still got 1 as return in printf.My code prints a hex that is not valid for an image, it's not a value from the array that I am comparing. I don't understand where is coming from

huangapple
  • 本文由 发表于 2023年7月12日 20:41:21
  • 转载请务必保留本文链接：https://go.coder-hub.com/76670667.html
  • arrays
  • c
  • cs50
  • image
  • printf
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