diff --git a/src/hru.rs b/src/hru.rs
index 1ed5bdd..c6009a7 100644
--- a/src/hru.rs
+++ b/src/hru.rs
@@ -29,6 +29,10 @@ extern crate sysexits;
 use strerror::StrError;
 use sysexits::{ EX_DATAERR, EX_IOERR, EX_SOFTWARE, EX_USAGE };
 
+#[cfg(target_os="openbsd")] use sysexits::EX_OSERR;
+#[cfg(target_os="openbsd")] extern crate openbsd;
+#[cfg(target_os="openbsd")] use openbsd::{ Promises, pledge };
+
 /* list of SI prefixes */
 const LIST: [(u32, &str); 10] = [
 	(3, "k"), /* kilo */
@@ -76,10 +80,20 @@ fn convert(input: u128) -> Result<(f64, (u32, &'static str)), String> {
 
 fn main() -> ExitCode {
 	let argv = args().collect::<Vec<String>>();
+
 	if let Some(_) = argv.get(1) {
 		eprintln!("Usage: {}", argv[0]);
 		return ExitCode::from(EX_USAGE as u8);
 	}
+
+	if cfg!(target_os="openbsd") {
+		let promises = Promises::new("stdio");
+		if let Err(e) = pledge(Some(promises), None) {
+			eprintln!("{}: {}", argv[0], e.strerror());
+			return ExitCode::from(EX_OSERR as u8);
+		}
+	}
+
 	let mut buf = String::new();
 
 	while let Ok(_) = stdin().read_line(&mut buf) {