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docs: improvements to the User Manual

tags/v0.3.28^2
Ralf Gommers 1 year ago
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## Compile the library

This user manual covers compiling OpenBLAS itself, linking your code to OpenBLAS,
example code to use the C (CBLAS) and Fortran (BLAS) APIs, and some troubleshooting
tips. Compiling OpenBLAS is optional, since you may be able to install with a
package manager.

!!! Note BLAS API reference documentation

The OpenBLAS documentation does not contain API reference documentation for
BLAS or LAPACK, since these are standardized APIs, the documentation for
which can be found in other places. If you want to understand every BLAS
function and definition, we recommend reading the
[Intel MKL reference manual](https://software.intel.com/en-us/intel-mkl/documentation)
or the [Netlib BLAS documentation](http://netlib.org/blas/).

OpenBLAS does contain a limited number of functions that are non-standard,
these are documented at [OpenBLAS extension functions](extensions.md).


## Compiling OpenBLAS

### Normal compile
* type `make` to detect the CPU automatically.
or
* type `make TARGET=xxx` to set target CPU, e.g. `make TARGET=NEHALEM`. The full target list is in file TargetList.txt.

### Cross compile
Please set `CC` and `FC` with the cross toolchains. Then, set `HOSTCC` with your host C compiler. At last, set `TARGET` explicitly.
The default way to build and install OpenBLAS from source is with Make:
```
make # add `-j4` to compile in parallel with 4 processes
make install
```

Examples:
By default, the CPU architecture is detected automatically when invoking
`make`, and the build is optimized for the detected CPU. To override the
autodetection, use the `TARGET` flag:

* On x86 box, compile the library for ARM Cortex-A9 linux.
```
# `make TARGET=xxx` sets target CPU: e.g. for an Intel Nehalem CPU:
make TARGET=NEHALEM
```
The full list of known target CPU architectures can be found in
`TargetList.txt` in the root of the repository.

Install only gnueabihf versions. Please check https://github.com/xianyi/OpenBLAS/issues/936#issuecomment-237596847
### Cross compile

make CC=arm-linux-gnueabihf-gcc FC=arm-linux-gnueabihf-gfortran HOSTCC=gcc TARGET=CORTEXA9
For a basic cross-compilation with Make, three steps need to be taken:

* On X86 box, compile this library for loongson3a CPU.
- Set the `CC` and `FC` environment variables to select the cross toolchains
for C and Fortran.
- Set the `HOSTCC` environment variable to select the host C compiler (i.e. the
regular C compiler for the machine on which you are invoking the build).
- Set `TARGET` explicitly to the CPU architecture on which the produced
OpenBLAS binaries will be used.

#### Cross-compilation examples

Compile the library for ARM Cortex-A9 linux on an x86-64 machine
_(note: install only `gnueabihf` versions of the cross toolchain - see
[this issue comment](https://github.com/OpenMathLib/OpenBLAS/issues/936#issuecomment-237596847)
for why_):
```
make BINARY=64 CC=mips64el-unknown-linux-gnu-gcc FC=mips64el-unknown-linux-gnu-gfortran HOSTCC=gcc TARGET=LOONGSON3A
make CC=arm-linux-gnueabihf-gcc FC=arm-linux-gnueabihf-gfortran HOSTCC=gcc TARGET=CORTEXA9
```

* On X86 box, compile this library for loongson3a CPU with loongcc (based on Open64) compiler.
Compile OpenBLAS for a loongson3a CPU on an x86-64 machine:
```
make BINARY=64 CC=mips64el-unknown-linux-gnu-gcc FC=mips64el-unknown-linux-gnu-gfortran HOSTCC=gcc TARGET=LOONGSON3A
```

Compile OpenBLAS for loongson3a CPU with the `loongcc` (based on Open64) compiler on an x86-64 machine:
```
make CC=loongcc FC=loongf95 HOSTCC=gcc TARGET=LOONGSON3A CROSS=1 CROSS_SUFFIX=mips64el-st-linux-gnu- NO_LAPACKE=1 NO_SHARED=1 BINARY=32
```

### Debug version
### Building a debug version

make DEBUG=1
Add `DEBUG=1` to your build command, e.g.:
```
make DEBUG=1
```

### Install to the directory (optional)
### Install to a specific directory

Example:
!!! note

make install PREFIX=your_installation_directory
Installing to a directory is optional; it is also possible to use the shared or static
libraries directly from the build directory.

The default directory is /opt/OpenBLAS. Note that any flags passed to `make` during build should also be passed to `make install` to circumvent any install errors, i.e. some headers not being copied over correctly.
Use `make install` with the `PREFIX` flag to install to a specific directory:

For more information, please read [Installation Guide](install.md).
```
make install PREFIX=/path/to/installation/directory
```

The default directory is `/opt/OpenBLAS`.

!!! important

Note that any flags passed to `make` during build should also be passed to
`make install` to circumvent any install errors, i.e. some headers not
being copied over correctly.

## Link the library
For more detailed information on building/installing from source, please read
the [Installation Guide](install.md).

* Link shared library

## Linking to OpenBLAS

OpenBLAS can be used as a shared or a static library.

### Link a shared library

The shared library is normally called `libopenblas.so`, but not that the name
may be different as a result of build flags used or naming choices by a distro
packager (see [distributing.md] for details). To link a shared library named
`libopenblas.so`, the flag `-lopenblas` is needed. To find the OpenBLAS headers,
a `-I/path/to/includedir` is needed. And unless the library is installed in a
directory that the linker searches by default, also `-L` and `-Wl,-rpath` flags
are needed. For a source file `test.c` (e.g., the example code under _Call
CBLAS interface_ further down), the shared library can then be linked with:
```
gcc -o test test.c -I/your_path/OpenBLAS/include/ -L/your_path/OpenBLAS/lib -Wl,-rpath,/your_path/OpenBLAS/lib -lopenblas
```

The `-Wl,-rpath,/your_path/OpenBLAS/lib` option to linker can be omitted if you ran `ldconfig` to update linker cache, put `/your_path/OpenBLAS/lib` in `/etc/ld.so.conf` or a file in `/etc/ld.so.conf.d`, or installed OpenBLAS in a location that is part of the `ld.so` default search path (usually /lib,/usr/lib and /usr/local/lib). Alternatively, you can set the environment variable LD_LIBRARY_PATH to point to the folder that contains libopenblas.so. Otherwise, linking at runtime will fail with a message like `cannot open shared object file: no such file or directory`
The `-Wl,-rpath,/your_path/OpenBLAS/lib` linker flag can be omitted if you
ran `ldconfig` to update linker cache, put `/your_path/OpenBLAS/lib` in
`/etc/ld.so.conf` or a file in `/etc/ld.so.conf.d`, or installed OpenBLAS in a
location that is part of the `ld.so` default search path (usually `/lib`,
`/usr/lib` and `/usr/local/lib`). Alternatively, you can set the environment
variable `LD_LIBRARY_PATH` to point to the folder that contains `libopenblas.so`.
Otherwise, the build may succeed but at runtime loading the library will fail
with a message like:
```
cannot open shared object file: no such file or directory
```

If the library is multithreaded, please add `-lpthread`. If the library contains LAPACK functions, please add `-lgfortran` or other Fortran libs, although if you only make calls to LAPACKE routines, i.e. your code has `#include "lapacke.h"` and makes calls to methods like `LAPACKE_dgeqrf`, `-lgfortran` is not needed.
More flags may be needed, depending on how OpenBLAS was built:

* Link static library
- If `libopenblas` is multi-threaded, please add `-lpthread`.
- If the library contains LAPACK functions (usually also true), please add
`-lgfortran` (other Fortran libraries may also be needed, e.g. `-lquadmath`).
Note that if you only make calls to LAPACKE routines, i.e. your code has
`#include "lapacke.h"` and makes calls to methods like `LAPACKE_dgeqrf`,
then `-lgfortran` is not needed.

!!! tip Use pkg-config

Usually a pkg-config file (e.g., `openblas.pc`) is installed together
with a `libopenblas` shared library. pkg-config is a tool that will
tell you the exact flags needed for linking. For example:

```
$ pkg-config --cflags openblas
-I/usr/local/include
$ pkg-config --libs openblas
-L/usr/local/lib -lopenblas
```

### Link a static library

Linking a static library is simpler - add the path to the static OpenBLAS
library to the compile command:
```
gcc -o test test.c /your/path/libopenblas.a
```

You can download `test.c` from https://gist.github.com/xianyi/5780018

## Code examples

### Call CBLAS interface
This example shows calling cblas_dgemm in C. https://gist.github.com/xianyi/6930656

This example shows calling `cblas_dgemm` in C:

<!-- Source: https://gist.github.com/xianyi/6930656 -->
```c
#include <cblas.h>
#include <stdio.h>
@@ -83,14 +187,17 @@ void main()
}
```

To compile this file, save it as `test_cblas_dgemm.c` and then run:
```
gcc -o test_cblas_open test_cblas_dgemm.c -I /your_path/OpenBLAS/include/ -L/your_path/OpenBLAS/lib -lopenblas -lpthread -lgfortran
gcc -o test_cblas_open test_cblas_dgemm.c -I/your_path/OpenBLAS/include/ -L/your_path/OpenBLAS/lib -lopenblas -lpthread -lgfortran
```
will result in a `test_cblas_open` executable.

### Call BLAS Fortran interface

This example shows calling dgemm Fortran interface in C. https://gist.github.com/xianyi/5780018
This example shows calling the `dgemm` Fortran interface in C:

<!-- Source: https://gist.github.com/xianyi/5780018 -->
```c
#include "stdio.h"
#include "stdlib.h"
@@ -158,22 +265,41 @@ int main(int argc, char* argv[])
}
```

To compile this file, save it as `time_dgemm.c` and then run:
```
gcc -o time_dgemm time_dgemm.c /your/path/libopenblas.a -lpthread
./time_dgemm <m> <n> <k>
```
You can then run it as: `./time_dgemm <m> <n> <k>`, with `m`, `n`, and `k` input
parameters to the `time_dgemm` executable.

## Troubleshooting
!!! note

* Please read [Faq](faq.md) at first.
* Please use gcc version 4.6 and above to compile Sandy Bridge AVX kernels on Linux/MingW/BSD.
* Please use Clang version 3.1 and above to compile the library on Sandy Bridge microarchitecture. The Clang 3.0 will generate the wrong AVX binary code.
* The number of CPUs/Cores should less than or equal to 256. On Linux x86_64(amd64), there is experimental support for up to 1024 CPUs/Cores and 128 numa nodes if you build the library with BIGNUMA=1.
* OpenBLAS does not set processor affinity by default. On Linux, you can enable processor affinity by commenting the line NO_AFFINITY=1 in Makefile.rule. But this may cause [the conflict with R parallel](https://stat.ethz.ch/pipermail/r-sig-hpc/2012-April/001348.html).
* On Loongson 3A. make test would be failed because of pthread_create error. The error code is EAGAIN. However, it will be OK when you run the same testcase on shell.
When calling the Fortran interface from C, you have to deal with symbol name
differences caused by compiler conventions. That is why the `dgemm_` function
call in the example above has a trailing underscore. This is what it looks like
when using `gcc`/`gfortran`, however such details may change for different
compilers. Hence it requires extra support code. The CBLAS interface may be
more portable when writing C code.

## BLAS reference manual
When writing code that needs to be portable and work across different
platforms and compilers, the above code example is not recommended for
usage. Instead, we advise looking at how OpenBLAS (or BLAS in general, since
this problem isn't specific to OpenBLAS) functions are called in widely
used projects like Julia, SciPy, or R.

If you want to understand every BLAS function and definition, please read [Intel MKL reference manual](https://software.intel.com/en-us/intel-mkl/documentation) or [netlib.org](http://netlib.org/blas/)

Here are [OpenBLAS extension functions](extensions.md)
## Troubleshooting

* Please read the [FAQ](faq.md) first, your problem may be described there.
* Please ensure you are using a recent enough compiler, that supports the
features your CPU provides (example: GCC versions before 4.6 were known to
not support AVX kernels, and before 6.1 AVX512CD kernels).
* The number of CPU cores supported by default is <=256. On Linux x86-64, there
is experimental support for up to 1024 cores and 128 NUMA nodes if you build
the library with `BIGNUMA=1`.
* OpenBLAS does not set processor affinity by default. On Linux, you can enable
processor affinity by commenting out the line `NO_AFFINITY=1` in
`Makefile.rule`.
* On Loongson 3A, `make test` is known to fail with a `pthread_create` error
and an `EAGAIN` error code. However, it will be OK when you run the same
testcase in a shell.

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