Marshalling

How JavaScript values cross the WASM boundary to the Gmsh C API and back. This is handled entirely by src/runtime.mjs, driven by the generated descriptor — you do not write any of this, but understanding it explains the API shapes.

Inputs

Gmsh argument	You pass	Sent to C as
bool	boolean	int (0/1)
int, size_t	number	int / size_t
double	number	double
string	string	const char* (UTF-8, heap-allocated)
vector<int>	number[]	const int*, size_t n
vector<double>	number[]	const double*, size_t n
vector<string>	string[]	const char* const*, size_t n
vector<pair>	number[] flat	const int*, size_t n (n = flattened length)
vector<vector<int>>	number[][]	const int* const*, const size_t*, size_t nn

All temporary heap allocations for inputs are freed after the call.

Outputs

Output parameters are allocated by the wrapper, filled by Gmsh (which mallocs the arrays), copied into JS values, and then released with gmshFree.

Gmsh output	You receive
int* / size_t* / double*	number
char**	string
int**+size_t* (vector)	number[]
char***+size_t* (vector of strings)	string[]
int***+size_t**+size_t* (vector of vectors)	number[][]

Return shape rules

For a function f:

1. No outputs, no scalar return → returns undefined.
2. Scalar return only (e.g. addPoint → a tag) → returns that number.
3. One or more output parameters → returns an object keyed by each output's name. If the function also has a scalar return, it is included as value.

// rule 2
const tag = gmsh.model.geo.addPoint(0, 0, 0);          // number

// rule 3
const { nodeTags, coord } = gmsh.model.mesh.getNodes(); // object

Memory & numeric notes

• The boundary is wasm32: pointers and size_t are 4 bytes; double is 8.
• size_t outputs are read unsigned (HEAPU32), so large tags/counts up to 2³² are represented correctly.
• Heap views are re-fetched on every access, so memory growth during a call cannot leave a stale typed-array view.
• Errors are surfaced by reading gmshLoggerGetLastError and throwing.