MSYS2/clang OpenBLAS Build Failed: Fix Missing Axpby.c

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Are you encountering a frustrating build failure when trying to compile OpenBLAS on MSYS2/clang? Specifically, is the error message complaining about a missing axpby.c file, like this?

[7901/11858] Building C object interface/CMakeFiles/interface.dir/CMakeFiles/saxpby.c.o
FAILED: [code=1] interface/CMakeFiles/interface.dir/CMakeFiles/saxpby.c.o
/clang64/bin/cc.exe  -I/c/Users/USERNAME/cq/OpenBLAS -I/c/Users/USERNAME/cq/OpenBLAS/build -DHAVE_C11 -Wall -m64 -mavx2 -mavx -msse -msse2 -msse3 -mssse3 -msse4.1 -DF_INTERFACE_GFORT -DSMALL_MATRIX_OPT -DSMP_SERVER -DNO_WARMUP -DMAX_CPU_NUMBER=22 -DMAX_PARALLEL_NUMBER=1 -DMAX_STACK_ALLOC=2048 -DNO_AFFINITY -DVERSION="\"0.3.30.dev\"" -DBUILD_SINGLE -DBUILD_DOUBLE -BUILD_COMPLEX -BUILD_COMPLEX16 -MD -MT interface/CMakeFiles/interface.dir/CMakeFiles/saxpby.c.o -MF interface/CMakeFiles/interface.dir/CMakeFiles/saxpby.c.o.d -o interface/CMakeFiles/interface.dir/CMakeFiles/saxpby.c.o -c /c/Users/USERNAME/cq/OpenBLAS/build/interface/CMakeFiles/saxpby.c
C:/Users/USERNAME/cq/OpenBLAS/build/interface/CMakeFiles/saxpby.c:7:10: fatal error: '/c/Users/USERNAME/cq/OpenBLAS/interface/axpby.c' file not found
    7 | #include "/c/Users/USERNAME/cq/OpenBLAS/interface/axpby.c"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

If so, you're not alone! This issue often arises due to how absolute paths are handled during the build process in the MSYS2 environment. Let's dive into the root cause and explore some effective solutions to get your OpenBLAS build back on track.

Understanding the Problem: Absolute Paths in MSYS2

The error message fatal error: '/c/Users/USERNAME/cq/OpenBLAS/interface/axpby.c' file not found is a clear indicator that the compiler is looking for the axpby.c file at a specific, absolute path. This path, while valid on your system, might not be correctly interpreted within the MSYS2 environment or by the compiler itself. The crux of the problem lies in how CMake generates the include paths and how the compiler resolves them.

When building software, especially with CMake, include directives like #include tell the compiler where to find necessary header files or, in this case, another C source file. The compiler searches these locations to incorporate the code from those files into the current compilation unit. In this scenario, the absolute path /c/Users/USERNAME/cq/OpenBLAS/interface/axpby.c is hardcoded into the generated build files. This can cause issues because:

  • Path Conversion: MSYS2 uses a Unix-like path structure, and sometimes the conversion between Windows paths (like C:\Users\...) and Unix paths (/c/Users/...) isn't seamless, especially when dealing with compilers like Clang.
  • Build Environment Isolation: The build environment might not have direct access to the /c/... path in the way the compiler expects. This can be due to environment variable configurations, security restrictions, or how the build tools are set up within MSYS2.
  • Inconsistent Path Handling: Different parts of the build process (CMake, the compiler, the linker) might interpret paths differently, leading to discrepancies in how files are located.

The core issue, therefore, is that the absolute path baked into the build configuration is not correctly resolved by the compiler during the compilation phase. This is a common pitfall when working with cross-platform build systems and environments like MSYS2 that bridge the gap between Windows and Unix-like systems.

To effectively tackle this problem, we need to find ways to either use relative paths or ensure that the absolute paths are correctly interpreted within the MSYS2 environment. Let's explore some proven solutions.

Solutions to the Rescue: Fixing the Build Failure

Now that we understand the problem, let's explore some solutions to get your OpenBLAS build working smoothly on MSYS2/clang.

1. The CMake CMAKE_INCLUDE_PATH to the Rescue

One of the most reliable solutions is to guide CMake to use relative paths instead of absolute paths. We can achieve this by setting the CMAKE_INCLUDE_PATH variable. This variable tells CMake where to look for include files, and by setting it appropriately, we can encourage the use of relative paths within the generated build files.

How to implement:

Before running the cmake command, set the CMAKE_INCLUDE_PATH environment variable to point to the OpenBLAS interface directory. This will help CMake resolve the include paths correctly.

export CMAKE_INCLUDE_PATH=/c/Users/USERNAME/cq/OpenBLAS/interface
cmake -B build -S . -G Ninja
cmake --build build

Explanation:

  • export CMAKE_INCLUDE_PATH=/c/Users/USERNAME/cq/OpenBLAS/interface: This command sets the CMAKE_INCLUDE_PATH environment variable. The path /c/Users/USERNAME/cq/OpenBLAS/interface should be replaced with the actual path to the interface directory within your OpenBLAS source code. This tells CMake to include this directory when searching for header files.
  • cmake -B build -S . -G Ninja: This is the standard CMake command to configure the build. -B build specifies the build directory, -S . specifies the source directory (current directory), and -G Ninja specifies the Ninja build system.
  • cmake --build build: This command initiates the build process using the configuration generated by the previous CMake command.

By setting CMAKE_INCLUDE_PATH, you're essentially telling CMake, "Hey, look in this directory for include files." This can help CMake generate the correct relative paths in the build files, resolving the axpby.c not found error.

2. Tweaking the Include Directive: A Direct Approach

Another approach, although less recommended for maintainability, is to directly modify the #include directive in the problematic file (C:/Users/USERNAME/cq/OpenBLAS/build/interface/CMakeFiles/saxpby.c).

How to implement:

Open the C:/Users/USERNAME/cq/OpenBLAS/build/interface/CMakeFiles/saxpby.c file in a text editor and change the #include directive from:

#include "/c/Users/USERNAME/cq/OpenBLAS/interface/axpby.c"

to:

#include "../../interface/axpby.c"

Explanation:

  • #include "../../interface/axpby.c": This modified include directive uses a relative path. ../../ moves two directories up from the current file's location (C:/Users/USERNAME/cq/OpenBLAS/build/interface/CMakeFiles/saxpby.c) to C:/Users/USERNAME/cq/OpenBLAS/, and then enters the interface directory to find axpby.c.

Caveats:

While this approach can quickly fix the issue, it's not recommended as a long-term solution. Here's why:

  • Manual Modification: It requires manually editing a generated file, which is prone to errors and can be overwritten if you re-run CMake.
  • Maintainability: This fix is specific to your local build environment. If you share your build configuration or move the project, the fix will likely break.
  • Best Practices: Modifying generated files directly goes against the principles of using a build system like CMake, which is designed to automate the build process.

3. The -I Flag: Explicit Include Directories

The -I flag is a compiler option that allows you to specify additional include directories. We can leverage this to explicitly tell the compiler where to find the axpby.c file.

How to implement:

You can pass the -I flag to the compiler through CMake by setting the CMAKE_C_FLAGS variable. Before running the cmake command, set this variable:

export CMAKE_C_FLAGS="-I/c/Users/USERNAME/cq/OpenBLAS/interface"
cmake -B build -S . -G Ninja
cmake --build build

Explanation:

  • export CMAKE_C_FLAGS="-I/c/Users/USERNAME/cq/OpenBLAS/interface": This command sets the CMAKE_C_FLAGS environment variable, which is used by CMake to pass flags to the C compiler. The -I/c/Users/USERNAME/cq/OpenBLAS/interface part tells the compiler to include the specified directory when searching for header files.
  • cmake -B build -S . -G Ninja: This is the standard CMake command to configure the build, as explained previously.
  • cmake --build build: This command initiates the build process.

By using the -I flag, you're explicitly telling the compiler to look in the OpenBLAS interface directory, ensuring that it finds the axpby.c file.

4. MSYS2 Path Conversion: A Deeper Dive

MSYS2 uses a path conversion mechanism to translate between Windows paths and Unix-like paths. Sometimes, this conversion doesn't work as expected, especially with Clang. You can try using MSYS2's cygpath utility to ensure paths are correctly converted.

How to implement:

  1. Identify the problematic path: In this case, it's /c/Users/USERNAME/cq/OpenBLAS/interface.

  2. Use cygpath to convert it:

    cygpath -u /c/Users/USERNAME/cq/OpenBLAS/interface

    This will output the Unix-style path that MSYS2 uses.

  3. Use the converted path in CMAKE_INCLUDE_PATH or -I flag:

    export CMAKE_INCLUDE_PATH=$(cygpath -u /c/Users/USERNAME/cq/OpenBLAS/interface)
cmake -B build -S . -G Ninja
cmake --build build

Explanation:

  • cygpath -u /c/Users/USERNAME/cq/OpenBLAS/interface: This command uses the cygpath utility with the -u option to convert the Windows-style path to a Unix-style path that MSYS2 understands.
  • export CMAKE_INCLUDE_PATH=$(cygpath -u /c/Users/USERNAME/cq/OpenBLAS/interface): This command sets the CMAKE_INCLUDE_PATH environment variable to the converted path, ensuring that CMake uses the correct path format.

This approach ensures that the paths are correctly formatted for MSYS2, potentially resolving any path conversion issues that might be causing the build failure.

Choosing the Right Solution: A Quick Guide

So, which solution should you choose? Here's a quick guide:

  • Recommended: Setting CMAKE_INCLUDE_PATH (Solution 1) is generally the most robust and recommended approach. It guides CMake to use relative paths, making your build configuration more portable and less prone to issues.
  • Alternative: Using the -I flag (Solution 3) is another good option, especially if you need to explicitly specify include directories for other reasons.
  • For Path Conversion Issues: If you suspect path conversion problems, using cygpath (Solution 4) can be helpful.
  • Avoid (if possible): Modifying the #include directive directly (Solution 2) should be avoided as it's a manual, non-portable fix.

Preventing Future Headaches: Best Practices for MSYS2 Builds

To avoid similar build issues in the future, here are some best practices for building software on MSYS2:

  • Use Relative Paths: Whenever possible, try to use relative paths in your CMake configuration and project files. This makes your build configuration more portable and less dependent on specific directory structures.
  • Set CMAKE_PREFIX_PATH: If you're using external libraries or dependencies, set the CMAKE_PREFIX_PATH variable to point to the installation directories. This helps CMake find the necessary files and headers.
  • Keep MSYS2 Updated: Regularly update your MSYS2 environment and packages to ensure you have the latest bug fixes and improvements.
  • Consult Documentation: Refer to the documentation for both MSYS2 and the software you're building. They often contain valuable information and troubleshooting tips.

Conclusion: Conquering the OpenBLAS Build on MSYS2/clang

Building OpenBLAS on MSYS2/clang can sometimes present challenges, especially with path handling. However, by understanding the root cause of the axpby.c not found error and applying the solutions discussed in this article, you can overcome this hurdle and get your build working smoothly.

Remember, setting CMAKE_INCLUDE_PATH is generally the most reliable solution, but the other approaches can be helpful in specific scenarios. By following the best practices outlined above, you can minimize build issues and enjoy a more seamless development experience on MSYS2.

So go ahead, try these solutions, and get back to the exciting world of high-performance linear algebra with OpenBLAS! You've got this!