From 7b0d75ee0af44d91b46d22853a55e2f491d92385 Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <michael@stapelberg.de>
Date: Fri, 11 Jan 2013 19:09:41 +0100
Subject: [PATCH] ipc_send_message: use stack frame with fixed size

---
 include/i3/ipc.h         |  9 +++++++-
 include/libi3.h          |  4 ++--
 libi3/ipc_send_message.c | 45 +++++++++++++++++++++++++---------------
 3 files changed, 38 insertions(+), 20 deletions(-)

diff --git a/include/i3/ipc.h b/include/i3/ipc.h
index 93b2ae87..7ce9ac65 100644
--- a/include/i3/ipc.h
+++ b/include/i3/ipc.h
@@ -2,7 +2,7 @@
  * vim:ts=4:sw=4:expandtab
  *
  * i3 - an improved dynamic tiling window manager
- * © 2009-2012 Michael Stapelberg and contributors (see also: LICENSE)
+ * © 2009-2013 Michael Stapelberg and contributors (see also: LICENSE)
  *
  * This public header defines the different constants and message types to use
  * for the IPC interface to i3 (see docs/ipc for more information).
@@ -11,6 +11,13 @@
 #ifndef I3_I3_IPC_H
 #define I3_I3_IPC_H
 
+typedef struct i3_ipc_header {
+    /* 6 = strlen(I3_IPC_MAGIC) */
+    char magic[6];
+    uint32_t size;
+    uint32_t type;
+} __attribute__ ((packed)) i3_ipc_header_t;
+
 /*
  * Messages from clients to i3
  *
diff --git a/include/libi3.h b/include/libi3.h
index 54fa0cc7..2037da2e 100644
--- a/include/libi3.h
+++ b/include/libi3.h
@@ -202,8 +202,8 @@ int ipc_connect(const char *socket_path);
  * Returns 0 on success.
  *
  */
-int ipc_send_message(int sockfd, uint32_t message_size,
-                     uint32_t message_type, const uint8_t *payload);
+int ipc_send_message(int sockfd, const uint32_t message_size,
+                     const uint32_t message_type, const uint8_t *payload);
 
 /**
  * Reads a message from the given socket file descriptor and stores its length
diff --git a/libi3/ipc_send_message.c b/libi3/ipc_send_message.c
index 88d87a6a..c5560c0d 100644
--- a/libi3/ipc_send_message.c
+++ b/libi3/ipc_send_message.c
@@ -2,7 +2,7 @@
  * vim:ts=4:sw=4:expandtab
  *
  * i3 - an improved dynamic tiling window manager
- * © 2009-2011 Michael Stapelberg and contributors (see also: LICENSE)
+ * © 2009-2013 Michael Stapelberg and contributors (see also: LICENSE)
  *
  */
 #include <string.h>
@@ -24,24 +24,35 @@
  * Returns 0 on success.
  *
  */
-int ipc_send_message(int sockfd, uint32_t message_size,
-                     uint32_t message_type, const uint8_t *payload) {
-    int buffer_size = strlen(I3_IPC_MAGIC) + sizeof(uint32_t) + sizeof(uint32_t) + message_size;
-    char msg[buffer_size];
-    char *walk = msg;
-
-    strncpy(walk, I3_IPC_MAGIC, buffer_size - 1);
-    walk += strlen(I3_IPC_MAGIC);
-    memcpy(walk, &message_size, sizeof(uint32_t));
-    walk += sizeof(uint32_t);
-    memcpy(walk, &message_type, sizeof(uint32_t));
-    walk += sizeof(uint32_t);
-    memcpy(walk, payload, message_size);
+int ipc_send_message(int sockfd, const uint32_t message_size,
+                     const uint32_t message_type, const uint8_t *payload) {
+    const i3_ipc_header_t header = {
+        /* We don’t use I3_IPC_MAGIC because it’s a 0-terminated C string. */
+        .magic = { 'i', '3', '-', 'i', 'p', 'c' },
+        .size = message_size,
+        .type = message_type
+    };
 
     int sent_bytes = 0;
-    while (sent_bytes < buffer_size) {
-        int n = write(sockfd, msg + sent_bytes, buffer_size - sent_bytes);
-        if (n == -1) {
+    int n = 0;
+
+    /* This first loop is basically unnecessary. No operating system has
+     * buffers which cannot fit 14 bytes into them, so the write() will only be
+     * called once. */
+    while (sent_bytes < sizeof(i3_ipc_header_t)) {
+        if ((n = write(sockfd, ((void*)&header) + sent_bytes, sizeof(i3_ipc_header_t) - sent_bytes)) == -1) {
+            if (errno == EAGAIN)
+                continue;
+            return -1;
+        }
+
+        sent_bytes += n;
+    }
+
+    sent_bytes = 0;
+
+    while (sent_bytes < message_size) {
+        if ((n = write(sockfd, payload + sent_bytes, message_size - sent_bytes)) == -1) {
             if (errno == EAGAIN)
                 continue;
             return -1;
-- 
2.39.5