Host modules

WasmVM ships with a built-in sysenv host that exposes a small POSIX-style interface to wasm modules. It is split into two sub-modules — sys_fs for filesystem operations and sys_proc for process / environment access — both registered automatically by wasmvm unless --no-system is supplied.

Each function below is registered with both an i32 (wasm32) and an i64 (wasm64) variant for every pointer / length parameter. The matching variant is selected at link time from the importing module’s declared parameter types, so the same host can serve both wasm32 and wasm64 callers.

Pointer / length convention

For functions that take buffers, parameter pairs named ptr and len refer to a contiguous range in memory 0 of the calling module. Negative i32 return values are POSIX-style errors: the magnitude equals the host’s errno after the failed syscall.

sys_fs

Filesystem operations.

Function	Signature (wasm32 form)	Description
open	(path_ptr, path_len, flags:i32, mode:i32) -> i32	Open a path; returns a wasm fd (>= 3) or a negative error.
close	(fd:i32) -> i32	Close a wasm fd.
read	(fd:i32, buf_ptr, buf_len) -> i32	Read up to buf_len bytes; returns bytes read or negative error.
write	(fd:i32, buf_ptr, buf_len) -> i32	Write up to buf_len bytes; returns bytes written or negative error.
lseek	(fd:i32, offset:i64, whence:i32) -> i64	Seek; returns the new offset or a negative error.
stat	(path_ptr, path_len, stat_buf) -> i32	Stat a path into stat_buf.
fstat	(fd:i32, stat_buf) -> i32	Stat by file descriptor.
unlink	(path_ptr, path_len) -> i32	Remove a file.
rename	(old_ptr, old_len, new_ptr, new_len) -> i32	Rename or move a path.
mkdir	(path_ptr, path_len, mode:i32) -> i32	Create a directory.
rmdir	(path_ptr, path_len) -> i32	Remove an empty directory.
getcwd	(buf_ptr, buf_len) -> i32	Write the current working directory into buf; returns length.
opendir	(path_ptr, path_len) -> i32	Open a directory iterator; returns a directory fd.
readdir	(dir_fd:i32, entry_ptr) -> i32	Read the next entry; returns 0 at end-of-directory.
closedir	(dir_fd:i32) -> i32	Close a directory iterator.

The i64 variants accept i64 for every ptr and len argument (mode, flags, whence and fd remain i32); for example open is also registered as (i64, i64, i32, i32) -> i32 for wasm64 callers.

sys_proc

Process and environment access.

Function	Signature (wasm32 form)	Description
exit	(code:i32) -> ()	Terminate the wasm program with the given exit code.
argc	() -> i32	Number of command-line arguments. Argument 0 is the main module path; subsequent arguments come from wasmvm --args.
argv_len	(idx:i32) -> i32	Length in bytes of argument idx.
argv	(idx:i32, buf_ptr, buf_len) -> i32	Copy argument idx into buf; returns bytes written.
getenv	(name_ptr, name_len, buf_ptr, buf_len) -> i32	Look up an environment variable and copy it into buf; returns bytes written or a negative error.
clock_gettime	(clk_id:i32, ts_ptr) -> i32	Read a monotonic / realtime clock into a timespec-shaped buffer.

Disabling the host

Pass --no-system (-ns) to wasmvm to disable automatic instantiation of sys_fs and sys_proc. This is useful when running modules that should not be granted filesystem or process access.