# Frequently Asked Questions¶

## Why do I get Illegal instruction error?¶

The short answer is that the binary of Owl or the program using Owl have been compiled on one platform and are run on another.

If you use opam or dune to install or build your code you can use OWL_CFLAGS to override the -march-option that is used to build the binary, by default it’s set to native, but if you want to maximize compatibility you should set it to x86-64:

env OWL_CFLAGS="-g -O3 -Ofast -march=x86-64 -mfpmath=sse -funroll-loops -ffast-math -DSFMT_MEXP=19937 -msse2 -fno-strict-aliasing -Wno-tautological-constant-out-of-range-compare" opam install owl


If you use esy you can inject environment variables into the sandboxed build environment using the resolutions field in you package.json:

{
"resolutions": {
"@opam/owl": {
"source": "owlbarn/owl:owl.opam#7f6ae99",
"override": {
"buildEnv": {
"OWL_CFLAGS": "-g -O3 -Ofast -march=x86-64 -mfpmath=sse -funroll-loops -ffast-math -DSFMT_MEXP=19937 -msse2 -fno-strict-aliasing -Wno-tautological-constant-out-of-range-compare"
}
}
}
}
}


The somewhat longer answer is that some parts of Owl are written in C. Since Owl is trying to be as performant as possible it is compiled with the CFLAGS option -march=native. This tells GCC (the c compiler used) to use all available instruction-sets the machine’s current CPU supports aka make the fastes binary possible. The downside of this is that the compiled binary may not run on older CPUs.