add rand-bytes.h

1 files changed, 20 insertions, 0 deletions

diff --git a/rand-bytes.h b/rand-bytes.h
new file mode 100644
index 0000000..b9d38d4
--- /dev/null
+++ b/rand-bytes.h
@@ -0,0 +1,20 @@
+#ifndef RAND_BYTES_H
+#define RAND_BYTES_H
+
+#include <stddef.h> // size_t
+#include <sys/random.h> // getrandom()
+#include <err.h> // errx()
+
+// Fill `buf` with `len` random bytes using `getrandom()`.
+//
+// Prints an error and exits with an error code if `len` random bytes
+// could not be read.
+static void rand_bytes(void * const buf, const size_t len) {
+  const ssize_t got = getrandom(buf, len, 0);
+  if (got < (ssize_t) len) {
+    // print error message, exit with error
+    errx(-1, "getrandom() failed");
+  }
+}
+
+#endif /* RAND_BYTES_H */