Ifstream从文本文件中读取错误的字符

Question

I have the following simple code, that reads contents of a text file into array of chars:

const char* name = "test.txt";
std::cout << "Loading file " << name << std::endl;
std::ifstream file;
file.open(name);
file.seekg (0, std::ios::end);
int length = file.tellg();
std::cout << "Size: " << length << " bytes" << std::endl;
file.seekg (0, std::ios::beg);
char* buffer = new char[length];
file.read(buffer,length);
file.close();
std::cout.write(buffer,length);

However, it seems ifstream reads wrong number of chars from the file: 1 additional char for each line. I searched through the web and it looks like in win7 text files have carriage return symbol (\\r) in addition to newline (\\n) in the end of each line. However, the stream somehow does not see these \\r, but still uses the original number of symbols in the file, reading additional bytes from beyond the end of the file. Is it possible to somehow solve this problem?

If it helps: I use MinGW compiler and Windows 7 64bit.

Answer 1

You might want to open the file in binary mode:

file.open(name, ios_base::in | ios_base::binary);

Otherwise what happens is that the standard library translates every Windows newline (CR+LF) into a single \\n for you.

This means that the number of characters that you can read from the file is not the same as the size of the file. When you call read() , it reads as many characters as it can. If it can't read the number of characters you requested, it sets the stream's failbit .

Answer 2

了解有关打开文件进行binary读取的信息（google或在此处查看 ）。

Answer 3

You're starting from some very erroneous (but widespread) opinions. file.tellg() doesn't return an int ; it returns an implementation defined object of type streampos , which must be a class type, and may or may not be convertible into an integral type. And if it is convertable into an integral type (and I don't know of an implementation where it isn't, even if it is not required), there is no guarantee that the resulting integer represents anything more than a magic cookie which would allow reseeking to the same position.

In practice, this is probably not a big issue on modern machines: both Unix and Windows return the offset in bytes from the start of the file. In the case of Unix, this works fine, because the mapping of the internal representation to the external one is one to one. In the case of Windows, there is a remapping of line endings: in a text file, a line ending is a two byte sequence of 0x0D, 0x0A, which becomes, when read, the single char '\\n' . And streampos (converted to an integral type) gives the offset in bytes to where you have to seek in the file, and not the number of char you have to read to get to that position. For things like what you seem to be doing, this is not a problem; the allocated buffer may be a little larger than necessary, but it will never be too small.

Be aware that this may not be true on mainframes. Historically, at least, mainframes used block oriented files, and the integral value of a streampos could easily be something broken up into fields, with a certain number of bits for the block number, and other bits for the byte offset in the block. Depending on how these are laid out in the word, a buffer allocated as you do could easily be several orders of magnitude too big, or if the offset is placed on the high order bits, too small.

The only reliable way of getting the exact size of buffer you need is system dependent, and on some systems (including Windows), there may be no other way except by reading all of the characters and counting them.

(The reason streampos is required to be a class type is because, historically, many older multibyte encodings had an encoding state; you couldn't correctly decode a character without knowing what characters preceded it. So streampos is required to contain two different information: the position to seek in the file, and information about this state. I don't think that there are any state dependent multibyte encodings in wide use today, however.)