Troubleshooting Ddca_get_display_refs() Only Returning One Display In Ddcutil

by StackCamp Team 78 views

Hey guys,

We've got an interesting issue to dive into today concerning the ddca_get_display_refs() function in the ddcutil library. It seems like some users are encountering a situation where this function only returns one display reference, even when multiple monitors are connected. This can be a real head-scratcher, so let's break down the problem, explore the details, and see how we can work around it.

The Issue: ddca_get_display_refs() and Multiple Monitors

So, here's the deal. The ddca_get_display_refs() function is supposed to give us a list of display references for all connected monitors. These references are crucial because they allow us to interact with specific displays using ddcutil. However, some users, like our friend in this scenario, have found that when they call ddca_get_display_refs(), they only get a reference to the first monitor, leaving the others out in the cold. This can be a major roadblock if you're trying to control multiple displays, which is a pretty common use case.

Diving into the Code Snippet

To really understand the problem, let's look at the code snippet provided. The user tried to get display references using ddca_get_display_refs() and store them in an array drefs. They then attempted to open and represent each display handle. The code looks something like this:

DDCA_Display_Ref *drefs[2];
DDCA_Display_Handle dh;

ddca_get_display_refs(false, drefs); // 'true' also doesn't work

ddca_open_display2(*drefs[0], false, &dh);
printf("%s\n", ddca_dh_repr(dh)); // Display_Handle[i2c-6: fd=4]
ddca_close_display(dh);

ddca_open_display2(*drefs[1], false, &dh);
printf("%s\n", ddca_dh_repr(dh)); // segfaults
ddca_close_display(dh);

As you can see, the code attempts to retrieve two display references. The first ddca_open_display2 call works fine, but the second one, using drefs[1], results in a segfault. This strongly suggests that ddca_get_display_refs() isn't populating the drefs array as expected, leaving drefs[1] uninitialized and causing the program to crash when dereferenced.

The Workaround: ddca_get_display_info_list2() to the Rescue!

Now, here's the interesting part. The user discovered a workaround using another function, ddca_get_display_info_list2(). This function provides a list of display information, including the display references. And guess what? It works like a charm! Here's the code snippet that demonstrates the workaround:

DDCA_Display_Info_List *dlist;
DDCA_Display_Handle dh;

ddca_get_display_info_list2(false, &dlist);

ddca_open_display2(dlist->info[0].dref, false, &dh);
printf("%s\n", ddca_dh_repr(dh)); // Display_Handle[i2c-6: fd=4]
ddca_close_display(dh);

ddca_open_display2(dlist->info[1].dref, false, &dh);
printf("%s\n", ddca_dh_repr(dh)); // Display_Handle[i2c-7: fd=4]
ddca_close_display(dh);

ddca_free_display_info_list(dlist);

In this code, ddca_get_display_info_list2() successfully retrieves information for both monitors. The display references are accessed through dlist->info[0].dref and dlist->info[1].dref, and the code can open and represent both displays without any issues. This clearly indicates that the underlying system is capable of detecting both monitors; the problem lies specifically within the ddca_get_display_refs() function.

Why the Discrepancy?

The million-dollar question, of course, is why ddca_get_display_refs() fails while ddca_get_display_info_list2() succeeds. This could be due to a number of reasons, including:

  • A bug in the implementation of ddca_get_display_refs(). It's possible that there's a conditional check or a loop that isn't handling multiple monitors correctly.
  • Different underlying mechanisms. The two functions might be using different system calls or APIs to retrieve display information. If one of those mechanisms has limitations in certain environments, it could explain the discrepancy.
  • Initialization issues. Perhaps ddca_get_display_refs() relies on some initialization steps that aren't being completed correctly when multiple monitors are present.

Without digging deeper into the ddcutil source code, it's hard to say for sure what's going on. However, the fact that ddca_get_display_info_list2() works provides a valuable clue and a viable workaround.

Implications and Recommendations

So, what does this all mean for you, the ddcutil user? Here are a few key takeaways:

  • Be aware of the issue: If you're working with multiple monitors, there's a chance that ddca_get_display_refs() might not return all the display references.
  • Use the workaround: ddca_get_display_info_list2() seems to be a reliable alternative for getting display references when multiple monitors are involved. Use this function instead of ddca_get_display_refs() to ensure you can access all your displays.
  • Report the issue: If you encounter this problem, it's a good idea to report it to the ddcutil developers. This will help them identify and fix the underlying bug, making the library more robust for everyone.
  • Consider contributing: If you're comfortable with C and have some time to spare, you could even try to investigate the issue yourself and submit a patch. Open-source projects thrive on community contributions!

Deep Dive into ddca_get_display_info_list2()

Let's explore ddca_get_display_info_list2() a bit more. As we've seen, it's a reliable way to get display references, but what exactly does it do under the hood? Understanding this can help us appreciate its robustness and potentially shed light on why it works when ddca_get_display_refs() doesn't.

What does it return?

The function ddca_get_display_info_list2() returns a pointer to a DDCA_Display_Info_List structure. This structure is essentially a container that holds an array of DDCA_Display_Info structures, one for each detected display. The DDCA_Display_Info structure, in turn, contains various pieces of information about the display, including the all-important DDCA_Display_Ref.

So, when you call ddca_get_display_info_list2(), you're getting a list of detailed information about each display, which includes the reference needed to interact with it. This comprehensive approach might be one reason why it's more reliable than ddca_get_display_refs(), which seems to focus solely on the references themselves.

How to use the returned list?

The code snippet we saw earlier demonstrates the typical usage pattern. First, you call ddca_get_display_info_list2() to get the list. Then, you iterate through the info array within the DDCA_Display_Info_List structure, accessing the dref member of each DDCA_Display_Info structure. This dref is the display reference you need to pass to functions like ddca_open_display2().

Important: Don't forget to free the memory allocated for the display info list when you're done with it! This is done using the ddca_free_display_info_list() function, as shown in the example. Failing to do so can lead to memory leaks, which can cause performance issues or even crashes over time.

Potential advantages of using ddca_get_display_info_list2()

Besides being a workaround for the ddca_get_display_refs() issue, ddca_get_display_info_list2() offers some potential advantages:

  • More information: As the name suggests, it provides more than just display references. You also get information like the display's serial number, model name, and EDID data. This can be useful for identifying specific displays or for more advanced use cases.
  • Robustness: As we've seen, it seems to be more reliable in multi-monitor setups.
  • Future-proofing: It's possible that ddca_get_display_info_list2() uses a more modern or robust API internally, which might make it less susceptible to compatibility issues in the future.

Impact of Shared Library Versions

The user who reported the issue mentioned that they tested with shared library versions 2.2.0 and 2.2.1 and encountered the same problem in both. This suggests that the bug, if it is indeed a bug, has been present for at least two releases. It also reinforces the idea that the issue is likely not due to a recent change or regression.

This information is valuable for the ddcutil developers because it helps narrow down the possible causes. It also highlights the importance of thorough testing across different environments and configurations.

Thank You and Conclusion

Finally, the user expressed their gratitude for ddcutil, calling it "incredibly useful software." This is a sentiment that many ddcutil users share. The library provides a powerful way to control and monitor displays, and it's a valuable tool for system administrators, power users, and anyone who wants more control over their display settings.

In conclusion, the issue with ddca_get_display_refs() only returning one display reference in multi-monitor setups is a real problem, but thankfully, there's a solid workaround using ddca_get_display_info_list2(). By being aware of this issue and using the workaround, you can continue to leverage the power of ddcutil to manage your displays effectively. And remember, reporting issues and contributing to open-source projects helps make them even better for everyone!