Fixing LAPACKE Header Names With SYMBOLPREFIX In OpenBLAS

Hey everyone, let's dive into a rather peculiar issue that pops up when using SYMBOLPREFIX with OpenBLAS, specifically concerning the lapacke.h header file. The problem? The function names in lapacke.h don't always align correctly with the prefix you've set. This leads to potential headaches when linking your code, as the compiler can't find the functions it expects. Let's break down the issue, why it happens, and what we can do to fix it. This is important for anyone building and using OpenBLAS with custom prefixes, especially when integrating it into larger projects like SciPy or other scientific computing libraries.

The Core Problem: Mismatched Function Names

So, what exactly goes wrong? When you use SYMBOLPREFIX (e.g., setting it to scipy_), OpenBLAS is supposed to prepend this prefix to all the function names in the library. This is super useful for avoiding naming conflicts when you're linking multiple libraries together. However, in the case of lapacke.h, things don't always work as expected. As the original poster points out, the lapacke.h header file might not have the correct prefix applied to the function declarations, even though the compiled library (.so file) does have the prefix. This creates a mismatch: your code looks for scipy_dlange, but lapacke.h might declare LAPACKE_dlange. This is a classical example of a header/library mismatch, which is never fun to debug!

Let's go through a basic example. The poster shows this using the command grep to check inside the include and library files after building OpenBLAS with SYMBOLPREFIX=scipy_. When checking the lapacke.h header, the declared function is double LAPACKE_dlange. But when checking the shared library (.so), using nm -D to list symbols, the symbol is scipy_LAPACKE_dlange, or even scipy_dlange_. Clearly there's a discrepancy.

This issue doesn't affect cblas.h. When checking the cblas header, the function is declared as scipy_cblas_dnrm2, and when checking the library, the symbol is also scipy_cblas_dnrm2. This is a key point because it means that OpenBLAS can handle prefixes correctly; the problem seems to be specific to how lapacke.h is handled during the build process. The cblas.h and lapacke.h files are created differently, so the build steps are different.

Diving Deeper into the Discrepancy

The discrepancy arises because of how the lapacke.h header is integrated during the build process. Instead of generating the lapacke.h with the prefix applied, the build process appears to simply copy the original header files. This means the preprocessor directives and variable substitutions that should be applying the prefix are not working correctly. This leads to an incorrect header file, breaking the expected link between header and library.

This is also likely related to the process in which the header files are created or modified during the build. It's possible that the necessary transformations to apply the SYMBOLPREFIX are not being applied correctly to lapacke.h, leaving the function declarations untouched.

Why is this Happening? Potential Causes and Solutions

The root cause is most likely in the build system's handling of the lapacke.h header. Let's talk about the potential causes and how we could fix them. This involves understanding the Makefile and how it processes the headers.

One potential cause is that the build process might not be correctly applying the SYMBOLPREFIX when generating or copying the lapacke.h header file. The build system might be missing a step or a directive that's supposed to perform this substitution. Another cause could be that the build system uses a simple copy command instead of using a more sophisticated approach that processes the header to substitute the prefix.

Addressing the Root Cause

The most direct solution would be to modify the Makefile to correctly handle the lapacke.h header. Here's what we need to consider:

1. Modify the Makefile: The Makefile needs to be updated to process the lapacke.h header. The existing approach of simply copying the header is insufficient. Instead, the build system must either generate the header file with the prefix applied or modify the copied header using tools like sed to apply the prefix.
2. Prefix Application: Make sure the SYMBOLPREFIX is correctly applied during the header generation or modification stage. This likely involves using preprocessor directives or a substitution command to replace the function names with the prefixed versions.
3. Testing: After making these changes, rigorous testing is essential. Build OpenBLAS with different SYMBOLPREFIX values and verify that the header files and the library symbols are consistent and correct.

Manual sed fix

The poster mentioned that manually using sed on the header file fixes the issue. This confirms the problem and shows that a simple find-and-replace operation on the header file can correct the function names. Specifically, sed is a stream editor that can be used to perform text transformations. It's a powerful tool for making automated edits to text files, including header files. For instance, if the prefix is scipy_, and the original function declaration is LAPACKE_dlange, a sed command might look like: sed 's/LAPACKE_dlange/scipy_LAPACKE_dlange/g' lapacke.h. This would replace all instances of LAPACKE_dlange with scipy_LAPACKE_dlange in the lapacke.h file. The key is to integrate this sed command (or a similar mechanism) into the OpenBLAS build process to ensure the header files are correctly modified.

How to Contribute to the Solution

If you're up for the challenge, here's how you can contribute:

1. Understand the Build Process: Start by familiarizing yourself with the OpenBLAS build system, specifically the Makefile. Locate the section that handles the installation of header files. Understand how lapacke.h is handled. This is where you'll need to make changes.
2. Implement the Fix: Implement the fix. This might involve adding a new rule in the Makefile to process the lapacke.h header, or modifying the existing one. This could involve generating the header with the prefix, using sed or another tool, or using preprocessor directives.
3. Test Thoroughly: Test your changes thoroughly. Build OpenBLAS with different SYMBOLPREFIX values. Make sure the function names in the header files and the library symbols match. Write a small test program that uses the OpenBLAS functions and ensures it compiles and links correctly.
4. Create a Pull Request: Once you're confident that your solution works, create a pull request on the OpenBLAS repository. Be sure to explain your changes clearly and provide any relevant testing results.

Conclusion: Bridging the Gap

Fixing the lapacke.h header issue is important for ensuring consistency and preventing linking errors when using SYMBOLPREFIX. By modifying the build process to correctly handle the header file, we can ensure that the function names in the header and the library symbols match. This will lead to a more robust and user-friendly experience when working with OpenBLAS. This is an ongoing effort to improve the library's usability and compatibility across a wide range of projects.

For further information, you can check the following links:

• OpenBLAS GitHub Repository: For the latest updates and to contribute: https://github.com/OpenMathLib/OpenBLAS. This is where you can find the source code, report issues, and submit your pull requests. This is the heart of the OpenBLAS project. Good luck!