Detecting and Managing Errors

As with any code, interacting with SDL can sometimes result in errors. For example, let’s imagine we’re trying to create a window that uses Metal, which is Apple’s API for creating high-performance graphics.

To do this, we pass the SDL_WINDOW_METAL flag to SDL_CreateWindow():

1#include <SDL.h>
2
3int main(int argc, char** argv) {
4  SDL_Init(SDL_INIT_VIDEO);
5  SDL_Window* Window{SDL_CreateWindow(
6    "Hello World",
7    SDL_WINDOWPOS_UNDEFINED,
8    SDL_WINDOWPOS_UNDEFINED,
9    1024, 768,
10    SDL_WINDOW_METAL
11  )};
12  
13  SDL_DestroyWindow(Window);
14  SDL_Quit();
15  return 0;
16}

If this program is run on a platform that doesn’t support Metal (such as a Windows machine), window creation will fail, and it might not be obvious why that is. In this lesson, we’ll introduce the ways we can detect and cover from errors coming from SDL.

Retrieving Error Messages

The SDL_GetError() function is the main way we retrieve information about errors coming from SDL. It returns a string, representing the most recent error that occurred.

If we call this function after attempting to create our window, we’ll get more information about what’s going wrong:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_Window* Window{SDL_CreateWindow(
7    "Hello World",
8    SDL_WINDOWPOS_UNDEFINED,
9    SDL_WINDOWPOS_UNDEFINED,
10    1024, 768,
11    SDL_WINDOW_METAL
12  )};
13
14  std::cout << SDL_GetError();
15  
16  SDL_DestroyWindow(Window);
17  SDL_Quit();
18  return 0;
19}

1Metal support is either not configured in SDL or not available in current SDL video driver (windows) or platform

SDL_GetError() can only return the most recent error message. In the following example, we have two errors - one when we create the window, as before, and a second error when we then try to get the surface associated with that window:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_Window* Window{SDL_CreateWindow(
7    "Hello World",
8    SDL_WINDOWPOS_UNDEFINED,
9    SDL_WINDOWPOS_UNDEFINED,
10    1024, 768,
11    SDL_WINDOW_METAL 
12  )};
13
14  SDL_GetWindowSurface(Window);
15
16  std::cout << SDL_GetError();
17  
18  SDL_DestroyWindow(Window);
19  SDL_Quit();
20  return 0;
21}

When we called SDL_GetError(), the most recent error was from the SDL_GetWindowSurface() call. SDL_CreateWindow() failed in the previous example but information about that error was overwritten by the error from SDL_GetWindowSurface():

1Invalid window

Additionally, SDL_GetError() does not clear the error state. Subsequent calls to SDL_GetError() will return the same message until a new error occurs:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_GetWindowSurface(nullptr);
7
8  std::cout << SDL_GetError() << '\n';
9  std::cout << SDL_GetError() << '\n';
10  std::cout << SDL_GetError() << '\n';
11
12  SDL_Quit();
13  return 0;
14}

1Invalid window
2Invalid window
3Invalid window

The char* Data Type

In previous lessons, we’ve been using std::string when working with strings of text. SDL uses a more primitive type, sometimes called a C-style string.

Its specific type is a char* - that is, a pointer to an individual character (char):

1const char* Error = SDL_GetError();

Like any pointer, a char* represents a location in memory. In this case, it represents the first character in the string.

Subsequent characters are then stored in subsequent memory addresses, until we encounter a special character that is designed to represent the end of the string. That special character is called the null terminator and is typically represented as a backslash followed by 0, \0:

Through this simple convention, a single memory address can represent strings of any length. If the first character in a string is the null terminator \0, that is equivalent to the string being empty:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv){
5  SDL_Init(SDL_INIT_VIDEO);
6
7  const char* Error{SDL_GetError()};
8  if (*Error == '\0') {
9    std::cout << "There is no error";
10  } else {
11    std::cout << "Error: " << Error;
12  }
13
14  SDL_Quit();
15  return 0;
16}

1There is no error

C-Style Strings and std::string

C-Style strings are primitive, and working with raw memory addresses is error-prone. Often, we’ll want to use a more powerful and modern string representation, such as std::string, in our projects. This means we need to deal with conversions between std::string and char* when interacting with the SDL API.

std::string has a constructor that accepts a char*, so converting a string returned from SDL to a std::string is easy:

1const char* Error{SDL_GetError()};
2std::string Error{SDL_GetError()};

The std::string type also has much friendlier API:

1const char* Error{SDL_GetError()};
2bool isEmpty{*Error == '\0'};
3
4std::string Error{SDL_GetError()};
5bool isEmpty{Error.empty()};

Updating our earlier code to use a std::string might look like this:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6
7  std::string Error{SDL_GetError()};
8  if (Error.empty()) {
9    std::cout << "There is no error";
10  } else {
11    std::cout << "Error: " << Error;
12  }
13  
14  SDL_Quit();
15  return 0;
16}

1There is no error

When we have a std::string and need an equivalent char* to pass to some SDL function, the c_str() method can help us:

1std::string SomeString{"Hello"};
2const char* SomeCString{SomeString.c_str()};

SDL_CreateWindow() is an example of where this might be needed, as the window title needs to be provided as a char*:

1#include <SDL.h>
2#include <string>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6
7  std::string GameName{"My Game"}; 
8  SDL_Window* Window{SDL_CreateWindow(
9    GameName.c_str(), 
10    SDL_WINDOWPOS_UNDEFINED,
11    SDL_WINDOWPOS_UNDEFINED,
12    1024, 768, 0
13  )};
14
15  SDL_DestroyWindow(Window);
16  SDL_Quit();
17  return 0;
18}

Our advanced course has a dedicated chapter that goes much deeper on C-style strings and std::string. That chapter starts here:

Characters, Unicode and Encoding

An introduction to C++ character types, the Unicode standard, character encoding, and C-style strings

Clearing Errors

Once we’ve processed and acknowledged an error, we can call SDL_ClearError().

This will cause SDL_GetError() to return an empty string, unless another error has occured between the SDL_ClearError() call and the next SDL_GetError() call:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_GetWindowSurface(nullptr);
7  std::cout << "Error: " << SDL_GetError();
8  SDL_ClearError();
9  std::cout << "\nError: " << SDL_GetError();
10
11  SDL_Quit();
12  return 0;
13}

1Error: Invalid window
2Error:

Creating an Error Handler

If we’re going to be doing a lot of error checking, it can be helpful to create a simple function that makes this easier. The following function:

• Accepts a string, letting the caller explain what action is associated with the error check
• If SDL_Error() returns a non-empty string, it logs that error, as well as the action that caused it for context
• Clears SDL’s error string using SDL_ClearError()

1// ErrorHandling.h
2#pragma once
3#include <SDL.h>
4#include <iostream>
5
6void CheckSDLError(const std::string& Action) {
7  const char* Error{SDL_GetError()};
8  if (*Error != '\0') {
9    std::cout << Action << " Error: "
10      << Error << '\n';
11    SDL_ClearError();
12  }
13}

We can use it like this:

1#include <SDL.h>
2#include <iostream>
3#include "ErrorHandling.h"
4
5int main(int argc, char** argv) {
6  SDL_Init(SDL_INIT_VIDEO);
7  SDL_Window* Window{SDL_CreateWindow(
8    "Hello World",
9    SDL_WINDOWPOS_UNDEFINED,
10    SDL_WINDOWPOS_UNDEFINED,
11    1024, 768,
12    SDL_WINDOW_METAL 
13  )};
14  CheckSDLError("Creating Window");
15
16  SDL_GetWindowSurface(Window);
17  CheckSDLError("Getting Surface");
18
19  std::cout << "Hello World\n";
20  // No error has occurred since the last call
21  // to CheckSDLError, so this won't log anything
22  CheckSDLError("Saying Hello");
23
24  SDL_DestroyWindow(Window);
25  SDL_Quit();
26  return 0;
27}

1Creating Window Error: Metal support is either not configured in SDL or not available in current SDL video driver (windows) or platform
2Getting Surface Error: Invalid window
3Hello World

Toggling Error Logs at Build Time

We may want to disable calls to this function in our final released games, so we can have the preprocessor include them only if some macro, such as ERROR_LOGGING, is defined.

This definition would typically be set through our build management tools but, for this example, we’ll just define it in our code file:

1// ErrorHandling.h
2#pragma once
3#include <SDL.h>
4#include <iostream>
5
6// Define me to enable error logging:
7#define ERROR_LOGGING
8
9void CheckSDLError(const std::string& Action) {
10#ifdef ERROR_LOGGING
11  const char* Error{SDL_GetError()};
12  if (*Error != '\0') {
13    std::cout << Action << " Error: "
14      << Error << '\n';
15    SDL_ClearError();
16  }
17#endif
18}

1#include <SDL.h>
2#include "ErrorHandling.h"
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_Window* Window{SDL_CreateWindow(
7    "Hello World",
8    SDL_WINDOWPOS_UNDEFINED,
9    SDL_WINDOWPOS_UNDEFINED,
10    1024, 768,
11    SDL_WINDOW_METAL 
12  )};
13  
14#ifdef ERROR_LOGGING 
15  CheckSDLError("Creating Window");
16#endif 
17
18  SDL_DestroyWindow(Window);
19  SDL_Quit();
20  return 0;
21}

Detecting Specific Errors

In most advanced programs, our error handling will often want to go beyond simply logging out what went wrong. We’ll often want to implement logic that reacts and potentially recovers from certain types of errors.

In addition to updating the string returned by SDL_GetError(), SDL functions will often use their return values to indicate something went wrong.

We need to refer to the official documentation for information on any specific function. However, as a general pattern, functions that return a pointer, such as SDL_CreateWindow(), will return a nullptr if something went wrong:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_Window* Window{SDL_CreateWindow(
7    "Hello World",
8    SDL_WINDOWPOS_UNDEFINED,
9    SDL_WINDOWPOS_UNDEFINED,
10    1024, 768,
11    SDL_WINDOW_METAL 
12  )};
13
14  if (!Window) {
15    std::cout << "Couldn't create window -"
16                 "trying without Metal\n";
17
18    Window = SDL_CreateWindow(
19      "Hello World",
20      SDL_WINDOWPOS_UNDEFINED,
21      SDL_WINDOWPOS_UNDEFINED,
22      1024, 768,
23      0 
24    );
25  }
26
27  if (Window) {
28    std::cout << "Window created successfully";
29    SDL_ClearError();
30  }
31
32  SDL_DestroyWindow(Window);
33  SDL_Quit();
34  return 0;
35}

1Couldn't create window - trying without Metal
2Window created successfully

Other functions that report errors will typically do so through an integer they return, with negative values indicating an error. For example, SDL_Init() returns 0 if it was successful, or a negative value if it failed:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
6    std::cout << "Init failed: "
7              << SDL_GetError();
8  }
9
10  SDL_Quit();
11  return 0;
12}

Setting Custom Errors

While SDL_SetError() is primarily used internally by SDL functions, we can also use it in our own code if we want to use SDL's error reporting system.

This can be useful when we want to use a consistent error-handling mechanism throughout our project, even for non-SDL related errors:

1#include <SDL.h>
2#include <iostream>
3
4int Divide(int a, int b) {
5  if (b == 0) {
6    SDL_SetError("Cannot divide by 0");
7    return 0;
8  }
9  return a / b;
10}
11
12int main(int argc, char** argv) {
13  SDL_Init(SDL_INIT_VIDEO);
14  Divide(2, 0);
15  std::cout << SDL_GetError();
16  // Alternatively, using the function
17  // we created earlier:
18  CheckSDLError("Dividing");
19
20  SDL_Quit();
21  return 0;
22}

1Cannot divide by 0

Summary

In this lesson, we've explored essential techniques for handling errors when working with SDL. We learned:

• How to use SDL_GetError() to retrieve error messages
• An introduction to working with C-style strings, and how they interact with std::string
• The use of SDL_ClearError() to manage the error state
• How to implement custom error handling functions
• How to check for specific errors by examining the return values from SDL functions
• Calling SDL_SetError() to use SDL to handle our own custom error states

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