当seekg查找超过文件末尾时，未设置failbit（C ++，Linux） [英] failbit not being set when seekg seeks past end of file (C++,Linux)

问题描述

我看到我认为istream :: seekg的奇怪行为。具体来说，看起来
当我寻找一个明显偏离文件结尾的点时，不设置failbit。

I'm seeing what I think is odd behaviour of istream::seekg. Specifically, it appears to NOT set the failbit when I seek to a point that is clearly way off the end of the file.

在文档中，failbit应该设置，但它不是。

From what I can tell in the documentation, the failbit should be set, yet it is not.

任何人都可以解释这种行为吗？相关代码段：

Can anyone explain this behaviour? snippet of relevant code:

class Tester 
{

  ... 

  void testTriggered()
  {
    fs.open("/pathtofile/testFile.TEST", std::ios_base::in|std::ios_base::binary);
    prv_testbits("testTriggered(): OpeningFile");

    fs.seekg(2000,std::ios_base::beg);
    prv_testbits("testTriggered(): seekTwoThousand");
    int g = fs.tellg();
    std::cout << "get pointer is:" << g << std::endl; 
  }

  void prv_testbits(std::string msg){
    if (fs.fail()) {
      std::cout << msg << ": failbit set." << std::endl;
    } else {
      std::cout << msg << ": failbit NOT set." << std::endl;
    }
    if (fs.bad()) {
      std::cout << msg << ": badbit set." << std::endl;
    }else {
      std::cout << msg << ": badbit NOT set." << std::endl;
    }
    if (fs.eof()) {
      std::cout << msg << ": eofbit set." << std::endl;
    } else {
      std::cout << msg << ": eofbit NOT set." << std::endl;
    }
  }

   ....

 private:
  std::ifstream fs;
}; 

输入文件包含20个字节：
0123456789abcdefghij

input file consists of twenty bytes: 0123456789abcdefghij

示例运行的输出：

testTriggered(): OpeningFile: failbit NOT set.
testTriggered(): OpeningFile: badbit NOT set.
testTriggered(): OpeningFile: eofbit NOT set.
testTriggered(): seekTwoThousand: failbit NOT set.
testTriggered(): seekTwoThousand: badbit NOT set.
testTriggered(): seekTwoThousand: eofbit NOT set.
get pointer is:2000

g ++版本信息：
$ g ++ -v
使用内置specs。
目标：x86_64-linux-gnu
配置为：../src/configure -v --with-pkgversion ='Ubuntu 4.4.3-4ubuntu5'-with-gxx-include-dir = / usr / include / c ++ / 4.4 [snip]
gcc版本4.4.3（Ubuntu 4.4.3-4ubuntu5）

g++ version info: $ g++ -v Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5' -with-gxx-include-dir=/usr/include/c++/4.4 [snip] gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)

$ uname -a
Linux hostx 2.6.32-36-server＃79-Ubuntu SMP Tue Nov 8 22:44:38 UTC 2011 x86_64 GNU / Linux

$ uname -a Linux hostx 2.6.32-36-server #79-Ubuntu SMP Tue Nov 8 22:44:38 UTC 2011 x86_64 GNU/Linux

推荐答案

需要 fseek（）（可能用于实现 fstream :: seekg ）的Open Group规范允许超出当前文件结束位置的文件位置：

The Open Group specification for fseek() (likely used to implement fstream::seekg) is required to allow file positions beyond the current end-of-file:

fseek（）函数应允许文件位置指示符设置超出文件中现有数据的结尾。如果此后写入数据，则间隙中的数据的后续读取将返回值为0的字节，直到数据实际写入间隙。

The fseek() function shall allow the file-position indicator to be set beyond the end of existing data in the file. If data is later written at this point, subsequent reads of data in the gap shall return bytes with the value 0 until data is actually written into the gap.

http://pubs.opengroup.org/onlinepubs/009696699/ functions / fseek.html

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