rand-bytes.h: use getentropy() instead of getrandom() to support macos

1 files changed, 24 insertions, 8 deletions

diff --git a/rand-bytes.h b/rand-bytes.h
index b9d38d4..14b2861 100644
--- a/rand-bytes.h
+++ b/rand-bytes.h
@@ -2,18 +2,34 @@
 #define RAND_BYTES_H
 
 #include <stddef.h> // size_t
-#include <sys/random.h> // getrandom()
+#include <sys/random.h> // getentropy()
 #include <err.h> // errx()
 
-// Fill `buf` with `len` random bytes using `getrandom()`.
+// Fill `buf` with `len` random bytes using `getentropy()`.
 //
-// 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) {
+// Prints an error message to standard error and exits with a non-zero
+// error code if `len` random bytes could not be read.
+static void rand_bytes(void * const buf, size_t len) {
+  uint8_t *u8 = (uint8_t*) buf;
+
+  // the maximum length for getentropy() is 256 bytes, so split requests
+  // for more than 256 bytes into multiple calls to `getentropy()`.
+  while (len > 256) {
+    // read 256 bytes, check for error
+    if (getentropy(u8, 256) == -1) {
+      // print error message, exit with error
+      errx(-1, "getentropy() failed");
+    }
+
+    // increment pointer, decrement length
+    u8 += 256;
+    len -= 256;
+  }
+
+  // read remaining bytes, check for error
+  if (getentropy(u8, len) == -1) {
     // print error message, exit with error
-    errx(-1, "getrandom() failed");
+    errx(-1, "getentropy() failed");
   }
 }