ifstream.read only reads half the file - c++

Question

I'm trying to make a simple image converter (ppm format to a custom one) and i'm having a problem with the ifstream.read method. Despite having this:
int rows,cols, maxV;
char header [100], *ptr;
std::ifstream im;
//open image in binary format
im.open(name.c_str(), std::ios::in | std::ios::binary);
if (!im)
{
std::cout << "Can't read image!" << std::endl;
exit(1);
}
//read the header of the image
im.getline(header, 3,'\n');
//test if header is P6
if ((header[0] != 80) || (header[1] != 54))
{
std::cout << "Image" << name << "is not .ppm format" << std::endl;
}
//get next line for height and width
im.getline(header,100,'\n');
//dont read the comments
while (header[0] == '#')
im.getline(header,100,'\n');
//number of columns, rows
cols = strtol(header, &ptr, 0);
rows = strtol(header, &ptr, 0);
maxV = strtol(header, &ptr, 0);
const int rows1=rows;
const int cols1=cols;
Component * tbuffer;
const_cast<Component*> (tbuffer);
tbuffer = new Component[rows1*cols1 * 3];
im.read((char *)tbuffer, cols*rows * 3);
std::cout << tbuffer[3000000] << std::endl;
im.close();
It only reads 2.700.007 elements out of 4.320.000 of the image i'm trying to read. so tbuffer[3.000.000] will "cout" NULL. Am i missing anything?
Edit: About component:
typedef unsigned char Component;
Edit2: The image is 1200*1200 (cols*rows).
2.700.007 is the last index of the tbuffer with a value in it. the rest of the tbuffer remains empty

Answer 1

The PPM format that you read does not guarantee that the magic number P6 is followed ended by a newline, nor that the rest of the header is followed by a newline, nor that lentgh, heigth and maxV are on the same line.
But the main problem that you have is
cols = strtol(header, &ptr, 0); // you start at the begin of the header
rows = strtol(header, &ptr, 0); // you start again at the begin of the header
maxV = strtol(header, &ptr, 0); // and another time !!
So your rows and maxV might not be the values in the file. You should --regardless of the other changes mentionned above-- rather use:
cols = strtol(header, &ptr, 0); // you start at the begin of the header
rows = strtol(ptr, &ptr, 0); // continue after the first number
maxV = strtol(ptr, &ptr, 0); // ...
But keep also in mind that you should not assume that the three are on the same line. And that there might be additional comments.
I propose you the following utility function to skip whitespace and comments according to the PPM format:
ifstream& skipwcmt(ifstream& im) {
char c;
do {
while ((c = im.get()) != EOF && isspace(c)) ;
if (isdigit(c))
im.unget();
else if (c == '#')
while ((c = im.get()) != EOF && c != '\n' && c != '\r');
} while (isspace(im.peek()));
return im;
}
You can use this function for reading the header as here:
// ...
// check magic number
im.read(header, 2);
if ((header[0] != 'P') || (header[1] != '6'))
{
std::cout << "Image" << name << "is not .ppm format" << std::endl;
exit(1);
}
skipwcmt(im) >> cols;
skipwcmt(im) >> rows;
skipwcmt(im) >> maxV;
if (!isspace(im.get())) { // folowed by exactly one whitespace !
std::cout << "Image" << name << "has a corrupted header" << std::endl;
exit(1);
}
// display the header to check the data
cout << "cols=" << cols << ", rows=" << rows << ", maxcol=" << maxV << endl;
Remark: I don't know if the files you have to read are guaranteed to have maxV<=255. In theory you could have values up to 65535 in which case you'd need to read 2 bytes for a color cmponent instead of one.