[bug] - ReturnDanglingLifeTime In `ofAppEGLWindow`

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[bug] - returnDanglingLifeTime in ofAppEGLWindow

In this article, we will discuss a bug found in the ofAppEGLWindow class of the openFrameworks library. The bug is related to the eglErrorString function, which returns a pointer to a local variable, causing a dangling pointer issue. We will analyze the code, understand the issue, and provide a fix for it.

The ofAppEGLWindow class is a part of the openFrameworks library, which is a C++ toolkit for creative coding. The class is responsible for creating and managing an EGL window, which is a platform-agnostic windowing system. The eglErrorString function is used to convert EGL error codes into human-readable error messages.

The bug is located in the eglErrorString function, which is defined as follows:

static const char* eglErrorString(EGLint err) {
	std::string str;
	switch (err) {
		CASE_STR(EGL_SUCCESS, "no error");
		CASE_STR(EGL_NOT_INITIALIZED, "EGL not, or could not be, initialized");
		CASE_STR(EGL_BAD_ACCESS, "access violation");
		CASE_STR(EGL_BAD_ALLOC, "could not allocate resources");
		CASE_STR(EGL_BAD_ATTRIBUTE, "invalid attribute");
		CASE_STR(EGL_BAD_CONTEXT, "invalid context specified");
		CASE_STR(EGL_BAD_CONFIG, "invald frame buffer configuration specified");
		CASE_STR(EGL_BAD_CURRENT_SURFACE, "current window, pbuffer or pixmap surface is no longer valid");
		CASE_STR(EGL_BAD_DISPLAY, "invalid display specified");
		CASE_STR(EGL_BAD_SURFACE, "invalid surface specified");
		CASE_STR(EGL_BAD_MATCH, "bad argument match");
		CASE_STR(EGL_BAD_PARAMETER, "invalid paramater");
		CASE_STR(EGL_BAD_NATIVE_PIXMAP, "invalid NativePixmap");
		CASE_STR(EGL_BAD_NATIVE_WINDOW, "invalid NativeWindow");
		CASE_STR(EGL_CONTEXT_LOST, "APM event caused context loss");
		default: str = "unknown error " + err; break;
	}
	return str.c_str();
}

The function returns str.c_str(), but str is a local variable stored on the stack. When the function exits, str is destroyed, making the returned pointer dangling (invalid).

The CppCheck tool, which is used to detect bugs and errors in C++ code, reports the following error message:

Returning pointer to local variable 'str' that will be invalid when returning. [returnDanglingLifetime].

This error message indicates that the function is returning a pointer to a local variable, which will be invalid when the function returns.

To fix the bug, we need to change the function to return a std::string object instead of a const char* pointer. We can do this by modifying the function as follows:

static std::string eglErrorString(EGLint err) {
	std::string str;
	switch (err) {
		CASE_STR(EGL_SUCCESS, "no error");
		CASE_STR(EGL_NOT_INITIALIZED, "EGL not, or could not be, initialized");
		CASE_STR(EGL_BAD_ACCESS, "access violation");
		CASE_STR(EGL_BAD_ALLOC, "could not allocate resources");
		CASE_STR(EGL_BAD_ATTRIBUTE, "invalid attribute");
		CASE_STR(EGL_BAD_CONTEXT, "invalid context specified");
		CASE_STR(EGL_BAD_CONFIG, "invald frame buffer configuration specified");
		CASE_STR(EGL_BAD_CURRENT_SURFACE, "current window, pbuffer or pixmap surface is no longer valid");
		CASE_STR(EGL_BAD_DISPLAY, "invalid display specified");
		CASE_STR(EGL_BAD_SURFACE, "invalid surface specified");
		CASE_STR(EGL_BAD_MATCH, "bad argument match");
		CASE_STR(EGL_BAD_PARAMETER, "invalid paramater");
		CASE_STR(EGL_BAD_NATIVE_PIXMAP, "invalid NativePixmap");
		CASE_STR(EGL_BAD_NATIVE_WINDOW, "invalid NativeWindow");
		CASE_STR(EGL_CONTEXT_LOST, "APM event caused context loss");
		default: str = "unknown error " + err; break;
	}
	return str;
}

By returning a std::string object, we ensure that the returned value is valid and can be used safely.

In this article, we discussed a bug found in the ofAppEGLWindow class of the openFrameworks library. The bug was related to the eglErrorString function, which returned a pointer to a local variable, causing a dangling pointer issue. We analyzed the code, understood the issue, and provided a fix for it. The fix involved changing the function to return a std::string object instead of a const char* pointer. This change ensures that the returned value is valid and can be used safely.
Q&A: [bug] - returnDanglingLifeTime in ofAppEGLWindow

In our previous article, we discussed a bug found in the ofAppEGLWindow class of the openFrameworks library. The bug was related to the eglErrorString function, which returned a pointer to a local variable, causing a dangling pointer issue. In this article, we will provide a Q&A section to answer some common questions related to this bug.

A: A dangling pointer is a pointer that points to a memory location that has already been freed or deallocated. This can cause a program to crash or behave unexpectedly.

A: The eglErrorString function returns a pointer to a local variable, which is stored on the stack. When the function exits, the local variable is destroyed, making the returned pointer dangling.

A: This is a problem because the returned pointer is no longer valid. If the program tries to access the memory location pointed to by the pointer, it will result in undefined behavior.

A: To fix this bug, you can change the eglErrorString function to return a std::string object instead of a const char* pointer. This will ensure that the returned value is valid and can be used safely.

A: Some common symptoms of a dangling pointer bug include:

  • Program crashes or freezes
  • Unexpected behavior or errors
  • Memory leaks or corruption
  • Data loss or corruption

A: To prevent dangling pointer bugs in your code, you can follow these best practices:

  • Use smart pointers instead of raw pointers
  • Avoid returning pointers to local variables
  • Use const correctness to ensure that pointers are not modified
  • Use memory debugging tools to detect memory leaks and corruption

A: Some common tools used to detect dangling pointer bugs include:

  • Memory debugging tools such as Valgrind or AddressSanitizer
  • Code analysis tools such as CppCheck or clang-tidy
  • Static analysis tools such as Coverity or Klocwork

In this article, we provided a Q&A section to answer some common questions related to the ofAppEGLWindow bug. We discussed the problem with the eglErrorString function, the symptoms of a dangling pointer bug, and how to prevent and detect such bugs in your code. By following best practices and using the right tools, you can write safer and more reliable code.