CODEWITHSUNDEEP
c++
Duc Le
Duc Le

Reputation: 13

fstream::read() read empty if input size too big

I have tried to read a file by using istream& read (char* s, streamsize n). I have read the description at: http://www.cplusplus.com/reference/istream/istream/read/ saying

If the input sequence runs out of characters to extract (i.e., the end-of-file is reached) before n characters have been successfully read, the array pointed to by s contains all the characters read until that point, and both the eofbit and failbit flags are set for the stream.

Because of that I have put the n with a very large number because I trust the caller that able to allocate enough buffer to read. But I always receive 0 read, I have tried following code to read txt file with 90 bytes:

std::wstring name(L"C:\\Users\\dle\\Documents\\01_Project\\01_VirtualMachine\\99_SharedFolder\\lala.txt");
std::ifstream ifs;
ifs.open(name, ifstream::binary | ifstream::in);
if (ifs)
{
    // get length of file:
    ifs.seekg(0, ifs.end);
    int length = ifs.tellg();
    ifs.seekg(0, ifs.beg);
    char *buffer = new char[length];


    ifs.read(buffer, UINT32_MAX);
    int success = ifs.gcount();
    cout << "success: " << success << endl;
    cout << "size: " << size;
    ifs.close();
}

I even tried with smaller number, eg: 500,000 and it still failed. I have realized that the "n" and the size of file related somehow, the "n" could not be larger than file size too much or else it will read empty....

I know we could fix that easily by putting correct size to read() but I wonder why it happened like that? I should read till EOF then stop right? Could anyone explain to me why please?

EDIT: I just simply want to read to EOF by utilizing istream& read without caring about file size. According to the definition of istream& read(char*s, streamsize n)it should work.

Upvotes: 0

Views: 883

Answers (1)

DevSolar
DevSolar

Reputation: 70243

ifs.read(buffer, UINT32_MAX);

The second parameter to fstream::read is std::streamsize, which is defined as (emphasis mine)...

...a signed integral type...

I therefore guess (as I don't have a Windows environment to test on at this point) that you're working on a machine where std::streamsize is 32bit, and you're looking at your UINT32_MAX ending up as a -1 (and @john testing on a machine where sizeof( std::streamsize ) > 4 so that his UINT32_MAX doesn't wrap into the negative.)

Try again with std::numeric_limits< std::streamsize >::max()... or even better yet, use length because, well, you have the file size right there and don't have to rely on the EOF behavior of fstream::read to save you.

I am not sure whether C++ changed the definition of streams from what the C standard says, but note that C's definition on binary streams states that they...

...may, however, have an implementation-defined number of null characters appended to the end of the stream.

So your, or the user's, assumption that a buffer big enough to hold the data written earlier is big enough to hold the data read till EOF might actually fail.

Upvotes: 1

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