tellg() function give wrong size of file?

by

tellg does not report the size of the file, nor the offset
from the beginning in bytes. It reports a token value which can
later be used to seek to the same place, and nothing more.
(It’s not even guaranteed that you can convert the type to an
integral type.)

At least according to the language specification: in practice,
on Unix systems, the value returned will be the offset in bytes
from the beginning of the file, and under Windows, it will be
the offset from the beginning of the file for files opened in
binary mode. For Windows (and most non-Unix systems), in text
mode, there is no direct and immediate mapping between what
tellg returns and the number of bytes you must read to get to
that position. Under Windows, all you can really count on is
that the value will be no less than the number of bytes you have
to read (and in most real cases, won’t be too much greater,
although it can be up to two times more).

If it is important to know exactly how many bytes you can read,
the only way of reliably doing so is by reading. You should be
able to do this with something like:

#include <limits>

file.ignore( std::numeric_limits<std::streamsize>::max() );
std::streamsize length = file.gcount();
file.clear();   //  Since ignore will have set eof.
file.seekg( 0, std::ios_base::beg );

Finally, two other remarks concerning your code:

First, the line:

*buffer = new char[length];

shouldn’t compile: you have declared buffer to be a char*,
so *buffer has type char, and is not a pointer. Given what
you seem to be doing, you probably want to declare buffer as
a char**. But a much better solution would be to declare it
as a std::vector<char>& or a std::string&. (That way, you
don’t have to return the size as well, and you won’t leak memory
if there is an exception.)

Second, the loop condition at the end is wrong. If you really
want to read one character at a time,

while ( file.get( buffer[i] ) ) {
    ++ i;
}

should do the trick. A better solution would probably be to
read blocks of data:

while ( file.read( buffer + i, N ) || file.gcount() != 0 ) {
    i += file.gcount();
}

or even:

file.read( buffer, size );
size = file.gcount();

EDIT: I just noticed a third error: if you fail to open the
file, you don’t tell the caller. At the very least, you should
set the size to 0 (but some sort of more precise error
handling is probably better).

More Related Contents:

  1. How to read huge file in c++
  2. How to extract a multiline text segment between two delimiters under a certain heading from a text file using C++ [closed]
  3. How do I print the content of a file?
  4. Reading and writing binary file
  5. Fast textfile reading in c++
  6. Getting std :: ifstream to handle LF, CR, and CRLF?
  7. How does ifstream’s eof() work?
  8. Do I need to manually close an ifstream?
  9. Read Unicode UTF-8 file into wstring
  10. Create a directory if it doesn’t exist
  11. How to embed a file into an executable?
  12. C++ string to double conversion
  13. reading a line from ifstream into a string variable
  14. Where can I get a copy of the file libstdc++.so.6.0.15 [closed]
  15. How do I get a list of files in a directory in C++? [duplicate]
  16. C++ Filehandling: Difference between ios::app and ios::ate?
  17. c++ template and header files [duplicate]
  18. C++ ifstream failbit and badbit
  19. How to Read from a Text File, Character by Character in C++
  20. Write a file in a specific path in C++
  21. How to write to middle of a file in C++?
  22. std::fstream buffering vs manual buffering (why 10x gain with manual buffering)?
  23. Difference between opening a file in binary vs text [duplicate]
  24. ifstream not reading EOF character
  25. counting the number of lines in a text file
  26. How can a C++ binary replace itself?
  27. Is it possible to use threads to speed up file reading?
  28. How to write an object to file in C++
  29. Reading a Matrix txt file and storing as an array
  30. Why does wide file-stream in C++ narrow written data by default?

Leave a Comment