vecLibFort is lightweight but flexible "shim" designed to rectify the incompatibilities between the Accelerate/vecLib BLAS and LAPACK libraries shipped with Mac OS X and FORTRAN code compiled with modern compilers such as GNU Fortran.
You will want this code if you are...
- compiling your code directly from FORTRAN source; and
- using Apple's BLAS and/or LAPACK for your linear algebra; and
- using single-precision or complex arithmetic.
You will not need this code if you are...
- using some other linear algebra package; or
- calling BLAS and LAPACK only from C; or
- using an alternative BLAS/LAPACK package (OpenBlas,MKL); or
- using only double-precision real arithmetic.
You may want this code if you are...
- running a pre-compiled program, or linking to a pre-compiled library, that seems to exhibit bugs described in the next section. See the section Preloaded (interposing) library for more details on how you may be able to fix these programs without recompilation.
Apple's vecLib framework provides both C and FORTRAN bindings for BLAS and LAPACK, the de-facto standard libraries for dense numerical linear algebra. Because there remains quite a bit of useful FORTRAN code out there that in turn depend on BLAS and LAPACK, this is certainly a welcome provision from Apple.
Unfortunately, those FORTRAN bindings follow an F2C-style return value convention, while GNU Fortran uses a different convention. Most subroutines and functions work without modification; in particular, if you rely solely on double-precision real arithmetic, you are fine. For single precision or complex arithmetic, there are two fatal incompatibilities:
- Functions whose FORTRAN specifications call for returning single-precision
real values, such as
sdot_andsnrm2_, actually return double-precision results in the Apple/F2C calling convention. GNU Fortran, on the other hand, expects to receive the single-precision result. - Functions designed to return complex values, whether single-precision or double-precision, are converted to subroutines in the Apple/F2C convention, with a pointer to the return value serving as the first argument. (Note that this differs from the CBLAS convention of passing a pointer to the return value as the final argument.) GNU Fortran, on the other hand, expects these values to be returned as a C-style return value.
For programs that use single-precision or complex arithmetic, then, these incompatibilities must be addressed or incorrect results and crashes can occur. In some projects, these errors go uncorrected, because the use cases that exercise them are uncommon.
One solution is to force GNU Fortran to adopt the older, F2C-style return
value convention, using the -ff2c flag. If that solution is sufficient
for you, then I encourage you to adopt it. Unfortunately, this may not be
possible if there is other code or other libraries that you rely upon that
assume the default GNU Fortran convention. And don't forget to rewrite your
C code according to the F2C return value conventions.
The approach taken by vecLibFort is to provide a thin translation layer between the F2C and GFortran worlds, for the few functions where there is a difference. For BLAS, this is simply a matter of wrapping Apple's CBLAS calls in a FORTRAN-friendly wrapper. For LAPACK, a bit of dlopen/dlsym trickery is required to avoid name conflicts.
Still another option is to use a different BLAS and LAPACK library, such as MKL or OpenBlas. I am sure there are good arguments to be made for all three options.
This code can be used in one of three ways, and the included Makefile
builds all three for you. The only variable you may want to modify is the
PREFIX variable, which determines the install location $(PREFIX)/lib.
The most straightforward way to use vecLibFort is by linking with the
standard dynamic library using -lvecLibFort. Of course, if you installed
the library in a non-standard location, you will need an -L<path> linker
flag as well.
If you use this apporach, you do not need to add -framework vecLib or
-framework Accelerate as well. That is because vecLibFort is built to
re-export all of Accelerate's symbols, even those it does not "fix". Thus it
serves as a full replacement for vecLib/Accelerate.
For new projects, feel free to add vecLibFort.c, static.h, and
cloak.h to your project, or link with the static library. You will also
need to link -framework vecLib or -framework Accelerate.
Suppose you have a program that is already compiled, but which apparently exhibits the errors discussed herein. Or perhaps you are using a precompiled third-party library that has not implemented measures like these itself; but because it has already been linked to vecLib, the bugs are baked in. (If you can alter the linking information of a dynamic library, I bow to your skill.)
In these cases, there is a preload feature of Mac OSX's dyld system that
can come in quite handy. The OS makes it possible to specify a library to be
preloaded before the application, with a list of instructions to replace
functions with alternate versions, a process known as interposing. The
source file vecLibFort.c includes this interposing code, but it is
wrapped with #ifdef VECLIBFORT_INTERPOSE to avoid clashing with the
non-interposing code.
To use this library, you must add the full path to libvecLibFortI.dylib
to the DYLD_INSERT_LIBRARIES environment variable. For instance,
if it has been installed in the default location, the command
DYLD_INSERT_LIBRARIES=/usr/local/lib/libvecLibFortI.dylib program
will run the program program but with the BLAS and LAPACK calls corrected.
Of course, this may not work---it may be that the bugs you are seeing are not in fact caused by the specific issues addressed by vecLibFort. Or I might not have implemented something correctly. (Bug reports are welcome.) And you should not use this if the program or library already uses the F2C calling conventions correctly; you will break it.
This code in vecLibFort.c is new, but the concepts that undergird it are
most certainly not. The inspirations include:
- The dotwrp project project provides a simple FORTRAN-based wrapper for the 5 most common problematic BLAS functions. Thanks to vecLib's CBLAS interface, the substitutions can be made statically. We have extended this approach to cover all of the relevant BLAS calls, and implemented it in C.
- The dynamic substitution approach is heavily inspired by the method used by GNU Octave, as contributed by Jarno Rajahaime. You can see the here. vecLibFort differs from Octave in that it resolves the replacements lazily, eliminating the need for lookup tables and (hopefully) improving performance. It also implements the full set of BLAS/LAPACK replacements, whereas Octave replaces only a subset.
- The interposing implementation is explained in a variety of places on the
Internet, including section 2.6.3.4 of Amit Singh's book "Mac OSX
Internals." (http://osxbook.com). Point your favorite search engine to the
term
DYLD_INSERT_LIBRARIESto find a wealth of material. - In order to make the primary source file as compact as possible, this code employs a simple preprocessor library by Paul Fultz II called Cloak. The Boost Preprocessor Library is perhaps a more well known example of this kind of work, but it is far more complex than needed in this case.
I've released this under the Boost Software License. So do whatever you wish with it. You do not have to redistribute the source code; but if you do, you must include the license with it.
If you do use this in your projects, I would appreciate it if you would give me credit, as I have attempted to do in the previous section. But I'm not going to get bent out of shape about it. Large piles of cash are welcome, as are simple emails of gratitude, or pull requests!
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