Question

I’m trying to read an image into a char array. Here is my try:

ifstream file ("htdocs/image.png", ios::in | ios::binary | ios::ate);
ifstream::pos_type fileSize;
char* fileContents;
if(file.is_open())
{
    fileSize = file.tellg();
    fileContents = new char[fileSize];
    file.seekg(0, ios::beg);
    if(!file.read(fileContents, fileSize))
    {
        cout << "fail to read" << endl;
    }
    file.close();
                
    cout << "size: " << fileSize << endl;
    cout << "sizeof: " << sizeof(fileContents) << endl;
    cout << "length: " << strlen(fileContents) << endl;
    cout << "random: " << fileContents[55] << endl;
    cout << fileContents << endl;
}

And this is the output:

size: 1944
sizeof: 8
length: 8
random: ?
?PNG

Can anyone explain this to me? Is there an end-of-file char at position 8? This example was taken from cplusplus.com

Running Mac OS X and compiling with XCode.

Answer 1

1. Returns the size of the file. size of your image.png is 1944 bytes.

   cout << "size: " << fileSize << endl;

2. Returns the sizeof(char*), which is 8 on your environment. Note that size of any pointer is always the same on any environment.

   cout << "sizeof: " << sizeof(fileContents) << endl;

3. The file you are reading is a binary file so it might contain 0 as a valid data. When you use strlen, it returns the length until a 0 is encountered, which in the case of your file is 8.

   cout << "length: " << strlen(fileContents) << endl;

4. Returns the character at the 56th location (remember array indexing starts from 0) from start of file.

   cout << "random: " << fileContents[55] << endl;

A suggestion:

Do remember to deallocate the dynamic memory allocation for fileContents using:

delete[] fileContents;

if you don’t, you will end up creating a memory leak.