FreeFem++ mesh (.msh)

The FreeFem++ .msh mesh format (as handled by FEconv). ASCII, with a volume-element block and a boundary-element block, each entity carrying an integer region/boundary label.

Format name	freefem
Extensions	.msh
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

# `.msh` is shared with ansys and gmsh — be explicit on read:
mesh = meshioplusplus.read("mesh.msh", file_format="freefem")
meshioplusplus.freefem.write("out.msh", mesh)

write takes no keyword arguments.

File structure

nver n_el1 n_el2
x y [z] ref        # nver rows
v0 v1 ... ref      # n_el1 rows (volume element)
v0 v1 ... ref      # n_el2 rows (boundary element)

The header is 3 integers: vertex count, then the two element-block counts. The spatial dimension is inferred from the first vertex row's token count minus one (must resolve to 2 or 3) — there is no explicit dimension field.

• 2D: n_el1 rows are triangle (3 nodes + ref), n_el2 rows are line (2 nodes + ref, the boundary edges).
• 3D: n_el1 rows are tetra (4 nodes + ref), n_el2 rows are triangle (3 nodes + ref, the boundary faces).

All connectivity is 1-based. Blank lines are ignored; the reader consumes tokens from non-blank lines only (_nonblank_lines).

Cell types

Linear only, dimension-dependent as above: {triangle, line} for 2D meshes, {tetra, triangle} for 3D meshes.

Data mapping

• point_data["freefem:ref"] — the per-vertex label.
• cell_data["freefem:ref"] — the per-element label, one array per cell block; defaults to zero if not present when writing.

Quirks & limitations

• The .msh extension is shared with ansys and gmsh. On extension-based auto-detection, the three formats are tried in registration order and freefem is attempted last — pass file_format="freefem" explicitly to select it reliably (see test_explicit_file_format).
• The writer only ever emits the two cell types appropriate for the mesh's dimension; any other cell type triggers a Python warn (and, in the C++ writer, a hard WriteError that forces the Python fallback, which then performs the warn-and-skip).
• Points are written with full Python repr() precision in the Python writer vs. %.16e in the C++ writer — same effective precision, different string form (does not affect round-trip correctness).

Notes

• Fully handled by the C++ core.
• No reference fixture exists under tests/meshes/freefem/; tests round-trip tri_mesh_2d, tri_mesh, and tet_mesh.