Seeking Past EOF

When you use SDL_RWseek() to move the read/write offset past the end of a file, the behavior depends on whether you subsequently try to read or write data.

If you try to read data after seeking past the end of the file, SDL_RWread() will return 0, indicating that it could not read any data. It will not generate an error in this case.

If you try to write data after seeking past the end of the file, the file will be extended to accommodate the new data. The contents of the file between the previous end of the file and the new data are undefined, meaning they could contain any values. It's therefore important to be careful about data integrity if you're relying on this behaviour.

Appending to a File

You can use this behavior to append data to a file. By seeking to the end of the file using RW_SEEK_END with an offset of 0, you can then write data, and it will be added to the end of the file.

This effectively extends the file, adding your new content to the existing data. Here's a simple example to demonstrate this:

1#include <SDL.h>
2#include <iostream>
3
4int main() {
5  // Create a file with some initial content
6  SDL_RWops* File{SDL_RWFromFile(
7    "example.txt", "w+b")};
8  if (!File) {
9    std::cerr << "Error opening file: "
10      << SDL_GetError() << "\n";
11    return 1;
12  }
13
14  const char* InitialContent{"Hello "};
15  SDL_RWwrite(File, InitialContent, 1, 6);
16
17  // Seek to the end and append more data
18  SDL_RWseek(File, 0, RW_SEEK_END);
19  const char* AppendedContent{"World!"};
20  SDL_RWwrite(File, AppendedContent, 1, 6);
21
22  // Verify the content
23  SDL_RWseek(File, 0, RW_SEEK_SET);
24  char Buffer[13];
25  Buffer[12] = '\0';
26  SDL_RWread(File, Buffer, 1, 12);
27
28  std::cout << "File content: " << Buffer;
29
30  SDL_RWclose(File);
31  return 0;
32}

1File content: Hello World!

In this example, we first write "Hello " to the file. Then, we use SDL_RWseek(File, 0, RW_SEEK_END) to move the offset to the end of the file.

Finally, we write "World!", which is appended to the file. The output shows that the file now contains "Hello World!".