libc: Provide ucontext_t/mcontext_t/<sys/ucontext.h>

This patch updates the C library headers to provide ucontext_t
definitions for three architectures.

+ Fix <signal.h> to always define 'struct sigcontext'.

The new declarations are announced with new macros defined in
<sys/cdefs.h> in order to make it easier to adapt client code
that already defines its own, incompatible, versions of the
structures seen here.

http://code.google.com/p/android/issues/detail?id=34784

Change-Id: Ie78c48690a4ce61c50593f6c39639be7fead3596

libc/include/signal.h

@@ -29,29 +29,61 @@
 #define _SIGNAL_H_
 
 #include <sys/cdefs.h>
-#include <limits.h>		/* For LONG_BIT */
-#include <string.h>		/* For memset() */
 #include <sys/types.h>
-#include <asm/signal.h>
+#include <machine/signal.h>
+#include <machine/ucontext.h>
 
-#define __ARCH_SI_UID_T __kernel_uid32_t
-#include <asm/siginfo.h>
-#undef __ARCH_SI_UID_T
+#include <limits.h>             /* For LONG_BIT */
+#include <string.h>             /* For memset() */
 
 __BEGIN_DECLS
 
-typedef int sig_atomic_t;
-
-/* _NSIG is used by the SIGRTMAX definition under <asm/signal.h>, however
- * its definition is part of a #if __KERNEL__ .. #endif block in the original
- * kernel headers and is thus not part of our cleaned-up versions.
+/* Previous versions of <signal.h> didn't define 'struct sigcontext'. This
+ * has led client code to redefine the structure, sometimes in a custom and
+ * incompatible way.
  *
- * Looking at the current kernel sources, it is defined as 64 for all
- * architectures except for the 'mips' one which set it to 128.
+ * The __BIONIC_HAVE_STRUCT_SIGCONTEXT macro is defined in <sys/cdefs.h>
+ * to indicate that <signal.h> does indeed define the structure. For
+ * maximum source compatibility, client code should follow these guidelines:
+ *
+ *  - If client code is guaranteed to build against a recent NDK/platform
+ *    version, just include <signal.h> as usual, and use the
+ *    'struct sigcontext'.
+ *
+ *  - If client code wants to build against older NDK/platform versions,
+ *    and only uses 'struct sigcontext' without accessing _any_
+ *    of its fields, just always include <asm/sigcontext.h> on Android
+ *    to ensure maximum portability (i.e. to old platform/NDK releases).
+ *    As in:
+ *
+ *        #include <signal.h>
+ *        #ifdef __BIONIC__
+ *        #include <asm/sigcontext.h>
+ *        #endif
+ *
+ *  - Otherwise, if the client has a custom sigcontext definition and wants to
+ *    keep it to minimize source changes, use a macro trick as below to avoid
+ *    naming conflicts:
+ *
+ *        #include <signal.h>
+ *        #ifdef __BIONIC__
+ *        #  include <asm/sigcontext.h>
+ *        #  ifdef __BIONIC_HAVE_STRUCT_SIGCONTEXT
+ *        #    undef sigcontext
+ *        #    define sigcontext  my_custom_sigcontext
+ *        #  endif
+ *        #endif
  */
-#ifndef _NSIG
-#  define _NSIG  64
-#endif
+
+/* Similarly, previous versions of the C library didn't define ucontext_t
+ * and mcontext_t in <signal.h>, or <sys/ucontext.h>. Client code can check
+ * against the macro __BIONIC_HAVE_UCONTEXT_T to determine if it does.
+ *
+ * Beware, that does _not_ mean that <ucontext.h> and related functions are
+ * available.
+ */
+
+typedef int sig_atomic_t;
 
 extern const char * const sys_siglist[];
 extern const char * const sys_signame[];