第一次调用fread后FTELL错误 [英] ftell error after the first call to fread

问题描述

所以我有一个非常简单的程序，读取一个文件的第3个字节：

So I have a very simple program that reads the 3 first bytes of a file:

int main(void)

{

    FILE *fd = NULL;
    int i;
    unsigned char test = 0;
    fd = fopen("test.bmp", "r");

    printf("position: %ld\n", ftell(fd));

    for (i=0; i<3; i++) {
        fread(&test, sizeof (unsigned char), 1, fd);
        printf("position: %ld char:%X\n", ftell(fd), test);
    }

    return (0);
}

当我用一个文本文件试试它工作得很好：

When I try it with a text file it works fine:

position: 0
position: 1 char: 61
position: 2 char: 62
position: 3 char: 63

但是当我用一个PNG运行它，例如我得到：

but when I run it with a PNG for example I get:

position: 0
position: 147 char:89
position: 148 char:50
position: 149 char:4E

请注意，该文件的第3个字节确实是89 50 4E，但我不知道那里的147来的。
随着bmp文件获得：

Note that the 3 first bytes of the file are indeed 89 50 4E but I don't know where the 147 comes from. With a bmp file I get:

position: 0
position: -1 char:42
position: 0 char:4D
position: 1 char:76

你知道这些第一位置从何而来？
非常感谢您的帮助。

Do you know where these first positions come from? Thanks a lot for your help

推荐答案

您需要以二进制方式打开文件：

You need to open the file in binary mode:

fd = fopen("test.bmp", "rb");

如果您尝试读取的二进制文件，如在文本模式下的位图，分别对应回车换行和字节混淆的东西。

If you try to read a binary file like a bitmap in text mode, the bytes corresponding to carriage returns and linefeeds confuse things.

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