Using heap memory for reading files-CodePudding

To read data from a file, I create heap memory then pass the variable pointer to a function so fread() will put the file data into the pointer. But when the function returns, there is no data in the new created memory.

int main(...) {
  MyFile File;
  File.Open(...);
  int filesize = File.Tell();
  char* buffer = new buffer[filesize]; // Create some memory for the data 
  File.Read((char**)&buffer);

  // Now do something with the buffer. BUT there is trash in it.

  File.Close();
  delete [] buffer;
}

size_t File::Read(void* buf) {
  ...

  ::fseek(fStream, 0, SEEK_END);
  int fileSize = ::ftell(fStream);  // Get file size.
  ::fseek(fStream, 0, SEEK_SET);

  ::fread(buf, 1, fileSize, fStream);

  return (fileSize);
}

Yes, I can put char * myBuffer = new char[fileSize]; inside of File::Read(...) before ::fread(myBuffer, 1, fileSize, fStream);, but I should not have to do this because I already have heap memory (buffer) in main().

CodePudding user response：

You're reading your file contents into the pointer buffer, not the array it points to.

You're overcomplicating things anyway. You don't need a pointer to a pointer, or a void*. You can simply pass a char* to Read. You should really also pass the size of the buffer pointed to into Read as well. Otherwise you risk overflowing your buffer.

int main() {
    MyFile File;
    File.Open(/*.....*/);
    int filesize = File.Tell()
    char* buffer = new buffer[filesize]; // Create some memory for the data 
    File.Read(buffer, filesize);

    // Now do something with the buffer. BUT there is trash in it.

    File.Close();
    delete [] buffer;
}

size_t File::Read(char* buf, size_t count) {
    // ......

    // No need to find the size of the file a second time

    // Return the actual number of bytes read
    return ::fread(buf, 1, count, fStream);
}

CodePudding user response：

I change my function to,

size_t nvFile::Read( char * pszBuffer, const size_t uiCount ) ...

Thank you Miles Budnek! I did not think enought of my problem. I am opening a binary file and it is a byte (char), so it not have to be void *. (I put on my 'cone-of-shame' for not thinking.)

Thank you for help and makeing me think more. :)