diff --git a/prewarm-launcher.h b/prewarm-launcher.h
index 72120d2a7..9cdf10e0d 100644
--- a/prewarm-launcher.h
+++ b/prewarm-launcher.h
@@ -218,9 +218,9 @@ setup_signal_handler(void) {
 static void
 setup_stdio_handles(void) {
     int pos = 0;
-    if (isatty(STDIN_FILENO)) stdin_pos = pos++;
-    if (isatty(STDOUT_FILENO)) stdout_pos = pos++;
-    if (isatty(STDERR_FILENO)) stderr_pos = pos++;
+    if (!isatty(STDIN_FILENO)) stdin_pos = pos++;
+    if (!isatty(STDOUT_FILENO)) stdout_pos = pos++;
+    if (!isatty(STDERR_FILENO)) stderr_pos = pos++;
 }
 
 static bool
@@ -257,15 +257,17 @@ create_launch_msg(int argc, char *argv[]) {
     for (int i = 0; i < argc; i++) w("argv", argv[i]);
     char **s = environ;
     for (; *s; s++) w("env", *s);
+    int num_fds = 0, fds[8];
+#define sio(which, x) if (which##_pos > -1) { snprintf(buf, sizeof(buf), "%d", which##_pos); w(#which, buf); fds[num_fds++] = x;  }
+    sio(stdin, STDIN_FILENO); sio(stdout, STDOUT_FILENO); sio(stderr, STDERR_FILENO);
+#undef sio
+    w("finish", "");
     struct cmsghdr *cmsg = CMSG_FIRSTHDR(&launch_msg_container);
     cmsg->cmsg_level = SOL_SOCKET;
     cmsg->cmsg_type = SCM_RIGHTS;
-    cmsg->cmsg_len = 0;
-#define sio(which, x) if (which##_pos > -1) { snprintf(buf, sizeof(buf), "%d", which##_pos); w(#which, buf); int fd = x; memcpy(CMSG_DATA(cmsg) + cmsg->cmsg_len, &fd, sizeof(fd)); cmsg->cmsg_len += sizeof(fd);  }
-    sio(stdin, STDIN_FILENO); sio(stdout, STDOUT_FILENO); sio(stderr, STDERR_FILENO);
-#undef sio
-    launch_msg_container.msg_controllen = CMSG_SPACE(cmsg->cmsg_len);
-    w("finish", "");
+    cmsg->cmsg_len = CMSG_LEN(sizeof(fds[0]) * num_fds);
+    memcpy(CMSG_DATA(cmsg), fds, num_fds * sizeof(fds[0]));
+    launch_msg_container.msg_controllen = cmsg->cmsg_len;
     return true;
 #undef w
 }
@@ -275,7 +277,7 @@ static char from_child_buf[64] = {0};
 static size_t from_child_buf_pos = 0;
 
 static bool
-read_child_data(int socket_fd) {
+read_child_data(void) {
     ssize_t n;
     if (from_child_buf_pos >= sizeof(from_child_buf) - 2) { print_error("Too much data from prewarm socket", 0); return false; }
     while ((n = read(socket_fd, from_child_buf, sizeof(from_child_buf) - 2 - from_child_buf_pos)) < 0 && errno == EINTR);
@@ -315,7 +317,7 @@ send_launch_msg(void) {
 
 
 static void
-loop(int socket_fd) {
+loop(void) {
 #define fail(s) { print_error(s, errno); return; }
 #define check_fd(which, name) { if (poll_data[which].revents & POLLERR) { pe("File descriptor %s failed", #name); return; } if (poll_data[which].revents & POLLHUP); { pe("File descriptor %s hungup", #name); return; } }
     struct pollfd poll_data[4];
@@ -330,14 +332,16 @@ loop(int socket_fd) {
         for (size_t i = 0; i < arraysz(poll_data); i++) poll_data[i].revents = 0;
         while (poll(poll_data, arraysz(poll_data), -1) == -1) { if (errno != EINTR) fail("poll() failed"); }
 
-        check_fd(0, self_ttyfd); check_fd(1, child_master_fd); check_fd(3, signal_read_fd);
+        check_fd(0, self_ttyfd);
+        check_fd(1, child_master_fd);
+        check_fd(3, signal_read_fd);
 
         // socket_fd
         if (poll_data[2].revents & POLLERR) {
             print_error("File descriptor socket_fd failed", 0); return;
         }
         if (poll_data[2].revents & POLLIN) {
-            if (!read_child_data(socket_fd)) fail("reading information about child failed");
+            if (!read_child_data()) fail("reading information about child failed");
         }
         if (poll_data[2].revents & POLLHUP) {
             if (from_child_buf[0]) { char *p = memchr(from_child_buf, ':', sizeof(from_child_buf)); if (p) parse_int(p+1, &exit_status); }
@@ -373,7 +377,7 @@ use_prewarmed_process(int argc, char *argv[]) {
     if (socket_fd < 0) fail("Failed to connect to prewarm socket");
 #undef fail
 
-    loop(socket_fd);
+    loop();
     cleanup();
     exit(exit_status);
 }