Make errno a thread-local variable.

errno is supposed to be thread local and we do that here in a rather
makeshift way by implementing a function that returns a pointer to
the __errno variable in the thread block.

If more serious TLS is required, perhaps it would be worthwhile
implementing it using the linker and the "__thread" storage attribute.
That's a bit overkill just for this. I'm also not liking what I saw
the compiler producing.

Change-Id: I03bc0bd6a89f6e3d6bae7653284ee01054614f9a
Reviewed-on: http://gerrit.rockbox.org/803
Reviewed-by: Michael Sevakis <jethead71@rockbox.org>
Tested: Michael Sevakis <jethead71@rockbox.org>

apps/plugin.c

@@ -684,7 +684,7 @@ static const struct plugin_api rockbox_api = {
 
     /* misc */
 #if (CONFIG_PLATFORM & PLATFORM_NATIVE)
-    &errno,
+    __errno,
 #endif
     srand,
     rand,

apps/plugin.h

@@ -160,12 +160,12 @@ void* plugin_get_buffer(size_t *buffer_size);
 #define PLUGIN_MAGIC 0x526F634B /* RocK */
 
 /* increase this every time the api struct changes */
-#define PLUGIN_API_VERSION 229
+#define PLUGIN_API_VERSION 230
 
 /* update this to latest version if a change to the api struct breaks
    backwards compatibility (and please take the opportunity to sort in any
    new function which are "waiting" at the end of the function table) */
-#define PLUGIN_MIN_API_VERSION 228
+#define PLUGIN_MIN_API_VERSION 230
 
 /* plugin return codes */
 /* internal returns start at 0x100 to make exit(1..255) work */
@@ -829,7 +829,7 @@ struct plugin_api {
 
     /* misc */
 #if (CONFIG_PLATFORM & PLATFORM_NATIVE)
-    int* __errno;
+    int * (*__errno)(void);
 #endif
     void (*srand)(unsigned int seed);
     int  (*rand)(void);

firmware/kernel/include/thread.h

@@ -210,6 +210,7 @@ struct thread_entry
     volatile intptr_t retval;  /* Return value from a blocked operation/
                                   misc. use */
 #endif
+    int __errno;               /* Thread error number (errno tls) */
 #ifdef HAVE_PRIORITY_SCHEDULING
     /* Priority summary of owned objects that support inheritance */
     struct blocker *blocker;   /* Pointer to blocker when this thread is blocked

firmware/libc/errno.c

@@ -1 +1,5 @@
-int errno;
+#include "thread.h"
+int * __errno(void)
+{
+    return &thread_self_entry()->__errno;
+}

firmware/libc/include/errno.h

@@ -10,10 +10,12 @@
 
 #ifndef _SYS_ERRNO_H_
 
+extern int * __errno(void);
+
 #ifdef PLUGIN
-#define errno   (*rb->__errno)
+#define errno  (*rb->__errno())
 #else
-extern int errno;
+#define errno  (*__errno())
 #endif
 
 #define EPERM 1         /* Not super-user */