fread用二进制文件过早发出EOF信号 [英] fread is signalling EOF prematurely with a binary file

问题描述

我是C的新手。我正在尝试制作我自己的 base64 版本;程序从stdin获取输入并输出相当于stdout的base64。在针对二进制文件测试我的程序时，我注意到来自stdin的 fread -ing似乎在实际到达EOF之前提前返回了一个短计数。

I'm a newcomer to C. I'm attempting to make my own version of base64; the program takes input from stdin and outputs its base64 equivalent to stdout. While testing my program against a binary file, I noticed that fread-ing from stdin seemed to be returning a short count early before actually reaching EOF.

以下是我的主要方法的相关部分：

Here is the relevant portion of my main method:

int main(void)
{
    unsigned char buffer[BUFFER_SIZE];
    unsigned char base64_buffer[BASE64_BUFFER];

    while (1)
    {
        TRACE_PUTS("Reading in data from stdin...");
        size_t read = fread(buffer, 1, sizeof(buffer), stdin); /* Read the data in using fread(3) */

        /* Process the buffer */

        TRACE_PRINTF("Amount read: %zu\n", read);
        TRACE_PUTS("Beginning base64 encode of buffer");
        size_t encoded = base64_encode(buffer, read, base64_buffer, sizeof(base64_buffer));

        /* Write the data to stdout */
        TRACE_PUTS("Writing data to standard output");
        ...

        if (read < sizeof(buffer))
        {
            break; /* We reached EOF or had an error during the read */
        }
    }

    if (ferror(stdin))
    {
        /* Handle errors */
        fprintf(stderr, "%s\n", "There was a problem reading from the file.");
        exit(1);
    }

    puts(""); /* Output a newline before finishing */

    return 0;
}

如您所见，主循环调用 fread stdin上的每次迭代都进入一个缓冲区，然后在最后检查读取的数量是否小于缓冲区的大小。如果是，我们假设有一个错误（在这种情况下返回0）或达到EOF，并退出循环。

As you can see, the main loop calls fread every iteration on stdin into a buffer, then at the end checks if the amount read is less than the size of the buffer. If it is, we assume there was either an error (in which case 0 was returned) or EOF was reached, and exit from the loop.

我假设它可以检查读取是< sizeof（缓冲区），而不仅仅是！= 0 ，基于来自fread的联机帮助页的引用：

I am assuming that it is OK to check read to be < sizeof(buffer), rather than just != 0, based on this quote from fread's manpage:

成功时，fread（）和fwrite（）返回读取或写入的项目数。此数字等于仅在大小为1时传输的字节数。如果发生错误或达到文件末尾，则返回值为短项目计数（或零）。

这意味着如果没有读入完整缓冲区，则达到EOF。

This implies that if the full buffer is not read into, then EOF is reached.

有了这个，这是我在针对 cat / bin / echo 运行我的应用程序时获得的跟踪：

With that established, this is the trace I get when I run my app against cat /bin/echo:

$ cat /bin/echo | bin/base64 >/dev/null # only view the trace output
TRACE: C:/Users/James/Code/c/base64/main.c:23: Reading in data from stdin...
TRACE: C:/Users/James/Code/c/base64/main.c:28: Amount read: 600
TRACE: C:/Users/James/Code/c/base64/main.c:29: Beginning base64 encode of buffer
TRACE: C:/Users/James/Code/c/base64/main.c:43: Writing data to standard output
TRACE: C:/Users/James/Code/c/base64/main.c:23: Reading in data from stdin...
TRACE: C:/Users/James/Code/c/base64/main.c:28: Amount read: 600
TRACE: C:/Users/James/Code/c/base64/main.c:29: Beginning base64 encode of buffer
TRACE: C:/Users/James/Code/c/base64/main.c:43: Writing data to standard output
TRACE: C:/Users/James/Code/c/base64/main.c:23: Reading in data from stdin...
TRACE: C:/Users/James/Code/c/base64/main.c:28: Amount read: 600
TRACE: C:/Users/James/Code/c/base64/main.c:29: Beginning base64 encode of buffer
TRACE: C:/Users/James/Code/c/base64/main.c:43: Writing data to standard output
TRACE: C:/Users/James/Code/c/base64/main.c:23: Reading in data from stdin...
TRACE: C:/Users/James/Code/c/base64/main.c:28: Amount read: 569
TRACE: C:/Users/James/Code/c/base64/main.c:29: Beginning base64 encode of buffer
TRACE: C:/Users/James/Code/c/base64/main.c:43: Writing data to standard output

$

这里有多大 / bin / echo 实际上是：

$ cat /bin/echo | wc -c
28352

如您所见，整个文件长度为28352字节，但我的应用程序只能在它停止之前阅读约2400个。知道为什么吗？ fread 特别处理空终止符吗？

So as you can see, the whole file is 28352 bytes long, but my app is only reading in about ~2400 of them before it stops. Any idea why? Does fread handle null terminators specially?

如果有帮助，我将MinGW-w64与GCC一起使用;谢谢。

I am using MinGW-w64 with GCC if that helps; thanks.

推荐答案

你在Windows上吗？是的，路径名开始 C：所以你是。你可能有一个Control-Z（'\ x1A'或'\ 32'）字符文件。它（Windows C运行时，因此你的程序）不会将标准输入视为二进制文件，除非你以某种方式调整它，所以Control-Z标记输入的结束。

Are you on Windows? Yes, the pathname starts C: so you are. You've probably got a Control-Z ('\x1A' or '\32') character in the file. It (the Windows C run-time, and hence your program) won't treat standard input as a binary file unless you tweak it somehow, so the Control-Z marks the end of the input.

调整模式的一种可能方式是 _set_fmode（） 。但是，您更有可能需要 _setmode（） ：

One possible 'somehow' to tweak the mode is _set_fmode(). However, it is more likely that you need _setmode():

_setmode(fileno(stdin), O_BINARY);

我保留判断这是否是最佳或唯一的方法。你也可以研究这些手册。我没办法测试 fileno（） - 或者微软世界中的 _fileno（） - 是否可用。

I reserve judgement on whether that's the best or only method for doing so. You can research the manuals as well as I can. I have no way to test that fileno() — or perhaps _fileno() in the Microsoft world — is available.

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