diff --git a/libc/include/fcntl.h b/libc/include/fcntl.h
index 779a089..cd68154 100644
--- a/libc/include/fcntl.h
+++ b/libc/include/fcntl.h
@@ -37,6 +37,20 @@
 
 __BEGIN_DECLS
 
+#ifdef __LP64__
+/* LP64 kernels don't have flock64 because their flock is 64-bit. */
+struct flock64 {
+  short l_type;
+  short l_whence;
+  off64_t l_start;
+  off64_t l_len;
+  pid_t l_pid;
+};
+#define F_GETLK64  F_GETLK
+#define F_SETLK64  F_SETLK
+#define F_SETLKW64 F_SETLKW
+#endif
+
 #ifndef O_ASYNC
 #define O_ASYNC  FASYNC
 #endif
diff --git a/tests/fcntl_test.cpp b/tests/fcntl_test.cpp
index 4aac468..725ac4a 100644
--- a/tests/fcntl_test.cpp
+++ b/tests/fcntl_test.cpp
@@ -116,3 +116,19 @@
   ASSERT_EQ(0, fstat(tf.fd, &sb));
   ASSERT_EQ(4, sb.st_size);
 }
+
+TEST(fcntl, f_getlk64) {
+  int fd = open64("/proc/version", O_RDONLY);
+  ASSERT_TRUE(fd != -1);
+
+  struct flock64 check_lock;
+  check_lock.l_type = F_WRLCK;
+  check_lock.l_start = 0;
+  check_lock.l_whence = SEEK_SET;
+  check_lock.l_len = 0;
+
+  int rc = fcntl(fd, F_GETLK64, &check_lock);
+  ASSERT_EQ(0, rc);
+
+  close(fd);
+}