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Argument Passing

Arguments can be passed to WASM programs in two ways: as WASM function parameters (with --call) and as WASI program arguments (via args_get).

Command-Line Syntax​

# Arguments after the WASM file path
wasmrun exec ./program.wasm arg1 arg2 arg3

# Arguments with --call go to the function parameters
wasmrun exec ./math.wasm --call add 10 20

# Use -- to separate wasmrun flags from program arguments
wasmrun exec ./program.wasm -- --flag value

Two Argument Paths​

1. Function Parameters (with --call)​

When using --call, arguments are converted to WASM values and passed directly to the function:

wasmrun exec ./math.wasm --call add 5 3
# → calls add(i32(5), i32(3))

Conversion rules:

InputParsed AsWASM Value
42IntegerValue::I32(42)
-7Negative integerValue::I32(-7)
9876543210Large integerValue::I64(9876543210)
helloNon-numericValue::I32(0)
3.14Non-integerValue::I32(0)

The executor attempts i32 first, then i64. Non-numeric strings become i32(0).

2. WASI Program Arguments (without --call)​

When running a program normally (no --call), arguments are available via WASI syscalls:

wasmrun exec ./echo.wasm hello world

Inside the WASM program, these are accessible through:

  • args_sizes_get — returns argument count and total buffer size
  • args_get — writes argument strings into the program's memory

This is the standard WASI mechanism, equivalent to argc/argv in C.

Example in Rust (compiled to WASI):

fn main() {
    let args: Vec<String> = std::env::args().collect();
    for arg in &args {
        println!("{}", arg);
    }
}
wasmrun exec ./args_demo.wasm one two three
# ./args_demo.wasm
# one
# two
# three

Examples​

Numeric Arguments​

# Parsed as i32
wasmrun exec ./calc.wasm --call add 100 200

# Negative numbers
wasmrun exec ./calc.wasm --call subtract 50 -30

# Large numbers (parsed as i64)
wasmrun exec ./calc.wasm --call big_add 5000000000 3000000000

String Arguments via WASI​

# Program reads args via args_get
wasmrun exec ./grep.wasm "search term" file.txt

# Flags and options
wasmrun exec ./tool.wasm -- --verbose --output result.json

Many Arguments​

# Up to any number of arguments
wasmrun exec ./program.wasm $(seq 1 50)

Special Characters​

# Spaces (quote the argument)
wasmrun exec ./program.wasm "hello world"

# Paths
wasmrun exec ./program.wasm /path/to/file.txt

# Mixed
wasmrun exec ./program.wasm --name "John Doe" --age 30

Limitations​

  • No float arguments — string arguments like 3.14 are not parsed as f32/f64. They become i32(0) when used as function parameters. Float values can only be passed through WASI args as strings.
  • No string function parameters — WASM functions don't have a string type. Strings are passed via WASI args, not function parameters.
  • Argument encoding — WASI args are null-terminated UTF-8 strings in linear memory.

See Also​