diff --git a/src/libguac/guacamole/socket.h b/src/libguac/guacamole/socket.h
index 74eeb881..a9d63d74 100644
--- a/src/libguac/guacamole/socket.h
+++ b/src/libguac/guacamole/socket.h
@@ -205,6 +205,11 @@ struct guac_socket {
      */
     pthread_mutex_t __instruction_write_lock;
 
+    /**
+     * Lock which is acquired when the buffer is being modified or flushed.
+     */
+    pthread_mutex_t __buffer_lock;
+
 };
 
 /**
@@ -251,6 +256,23 @@ void guac_socket_instruction_begin(guac_socket* socket);
  */
 void guac_socket_instruction_end(guac_socket* socket);
 
+/**
+ * Marks the beginning of a socket's buffer modification. If threadsafety is
+ * enabled on the socket, other functions which modify the buffer will be
+ * blocked until this modification is complete.
+ *
+ * @param socket The guac_socket whose buffer is being updated.
+ */
+void guac_socket_update_buffer_begin(guac_socket* socket);
+
+/**
+ * Marks the end of a socket's buffer modification. If threadsafety is enabled
+ * on the socket, other functions which modify the buffer will now be allowed
+ * to continue.
+ *
+ * @param socket The guac_socket whose buffer is done being updated.
+ */
+void guac_socket_update_buffer_end(guac_socket* socket);
 
 /**
  * Allocates and initializes a new guac_socket object with the given open
diff --git a/src/libguac/socket.c b/src/libguac/socket.c
index af7bba14..e25c5f27 100644
--- a/src/libguac/socket.c
+++ b/src/libguac/socket.c
@@ -125,6 +125,7 @@ int guac_socket_select(guac_socket* socket, int usec_timeout) {
 
 guac_socket* guac_socket_alloc() {
 
+    pthread_mutexattr_t lock_attributes;
     guac_socket* socket = malloc(sizeof(guac_socket));
 
     /* If no memory available, return with error */
@@ -155,10 +156,14 @@ guac_socket* guac_socket_alloc() {
     socket->__instructionbuf_parse_start = 0;
     socket->__instructionbuf_elementc = 0;
 
-
     /* Default to unsafe threading */
     socket->__threadsafe_instructions = 0;
-    pthread_mutex_init(&(socket->__instruction_write_lock), NULL);
+
+    pthread_mutexattr_init(&lock_attributes);
+    pthread_mutexattr_setpshared(&lock_attributes, PTHREAD_PROCESS_SHARED);
+
+    pthread_mutex_init(&(socket->__instruction_write_lock), &lock_attributes);
+    pthread_mutex_init(&(socket->__buffer_lock),            &lock_attributes);
     
     /* No handlers yet */
     socket->read_handler   = NULL;
@@ -184,12 +189,28 @@ void guac_socket_instruction_begin(guac_socket* socket) {
 
 void guac_socket_instruction_end(guac_socket* socket) {
 
-    /* Lock writes if threadsafety enabled */
+    /* Unlock writes if threadsafety enabled */
     if (socket->__threadsafe_instructions)
         pthread_mutex_unlock(&(socket->__instruction_write_lock));
 
 }
 
+void guac_socket_update_buffer_begin(guac_socket* socket) {
+
+    /* Lock if threadsafety enabled */
+    if (socket->__threadsafe_instructions)
+        pthread_mutex_lock(&(socket->__buffer_lock));
+
+}
+
+void guac_socket_update_buffer_end(guac_socket* socket) {
+
+    /* Unlock if threadsafety enabled */
+    if (socket->__threadsafe_instructions)
+        pthread_mutex_unlock(&(socket->__buffer_lock));
+
+}
+
 void guac_socket_free(guac_socket* socket) {
 
     /* Call free handler if defined */
@@ -214,6 +235,8 @@ ssize_t guac_socket_write_string(guac_socket* socket, const char* str) {
 
     char* __out_buf = socket->__out_buf;
 
+    guac_socket_update_buffer_begin(socket);
+
     for (; *str != '\0'; str++) {
 
         __out_buf[socket->__written++] = *str; 
@@ -221,10 +244,12 @@ ssize_t guac_socket_write_string(guac_socket* socket, const char* str) {
         /* Flush when necessary, return on error. Note that we must flush within 4 bytes of boundary because
          * __guac_socket_write_base64_triplet ALWAYS writes four bytes, and would otherwise potentially overflow
          * the buffer. */
-        if (socket->__written >= GUAC_SOCKET_OUTPUT_BUFFER_SIZE - 4) {
+        if (socket->__written > GUAC_SOCKET_OUTPUT_BUFFER_SIZE - 4) {
 
-            if (guac_socket_write(socket, __out_buf, socket->__written))
+            if (guac_socket_write(socket, __out_buf, socket->__written)) {
+                guac_socket_update_buffer_end(socket);
                 return 1;
+            }
 
             socket->__written = 0;
 
@@ -232,6 +257,7 @@ ssize_t guac_socket_write_string(guac_socket* socket, const char* str) {
 
     }
 
+    guac_socket_update_buffer_end(socket);
     return 0;
 
 }
@@ -264,7 +290,7 @@ ssize_t __guac_socket_write_base64_triplet(guac_socket* socket, int a, int b, in
     /* At this point, 4 bytes have been socket->__written */
 
     /* Flush when necessary, return on error */
-    if (socket->__written >= GUAC_SOCKET_OUTPUT_BUFFER_SIZE - 4) {
+    if (socket->__written > GUAC_SOCKET_OUTPUT_BUFFER_SIZE - 4) {
 
         if (guac_socket_write(socket, __out_buf, socket->__written))
             return -1;
@@ -309,14 +335,18 @@ ssize_t guac_socket_write_base64(guac_socket* socket, const void* buf, size_t co
     const unsigned char* char_buf = (const unsigned char*) buf;
     const unsigned char* end = char_buf + count;
 
+    guac_socket_update_buffer_begin(socket);
     while (char_buf < end) {
 
         retval = __guac_socket_write_base64_byte(socket, *(char_buf++));
-        if (retval < 0)
+        if (retval < 0) {
+            guac_socket_update_buffer_end(socket);
             return retval;
+        }
 
     }
 
+    guac_socket_update_buffer_end(socket);
     return 0;
 
 }
@@ -324,14 +354,18 @@ ssize_t guac_socket_write_base64(guac_socket* socket, const void* buf, size_t co
 ssize_t guac_socket_flush(guac_socket* socket) {
 
     /* Flush remaining bytes in buffer */
+    guac_socket_update_buffer_begin(socket);
     if (socket->__written > 0) {
 
-        if (guac_socket_write(socket, socket->__out_buf, socket->__written))
+        if (guac_socket_write(socket, socket->__out_buf, socket->__written)) {
+            guac_socket_update_buffer_end(socket);
             return 1;
+        }
 
         socket->__written = 0;
     }
 
+    guac_socket_update_buffer_end(socket);
     return 0;
 
 }
@@ -341,12 +375,18 @@ ssize_t guac_socket_flush_base64(guac_socket* socket) {
     int retval;
 
     /* Flush triplet to output buffer */
+    guac_socket_update_buffer_begin(socket);
     while (socket->__ready > 0) {
+
         retval = __guac_socket_write_base64_byte(socket, -1);
-        if (retval < 0)
+        if (retval < 0) {
+            guac_socket_update_buffer_end(socket);
             return retval;
+        }
+
     }
 
+    guac_socket_update_buffer_end(socket);
     return 0;
 
 }