C++: handle glibc's ptrace(enum __ptrace_request, ...)

Building in C++ mode issues ~40 warnings like this:

 ../../src/gdb/linux-nat.c: In function ‘int linux_handle_extended_wait(lwp_info*, int, int)’:
 ../../src/gdb/linux-nat.c:2016:51: warning: invalid conversion from ‘int’ to ‘__ptrace_request’ [-fpermissive]
	ptrace (PTRACE_GETEVENTMSG, pid, 0, &new_pid);

The issue is that in glibc, ptrace's first parameter is an enum.
That's not a problem if we pick the PTRACE_XXX requests from
sys/ptrace.h, as those will be values of the corresponding enum.
However, we have fallback definitions for PTRACE_XXX symbols when the
system headers miss them (such as PTRACE_GETEVENTMSG above), and those
are plain integer constants.  E.g., nat/linux-ptrace.h:

 #define PTRACE_GETEVENTMSG	0x4201

One idea would be to fix this by defining those fallbacks like:

 -#define PTRACE_GETEVENTMSG	0x4201
 +#define PTRACE_GETEVENTMSG	((enum __ptrace_request) 0x4201)

However, while glibc's ptrace uses enum __ptrace_request for first
parameter:

  extern long int ptrace (enum __ptrace_request __request, ...) __THROW;

other libc's, like e.g., Android's bionic do not -- in that case, the
first parameter is int:

  long ptrace(int request, pid_t pid, void * addr, void * data);

So the fix I came up is to make configure/ptrace.m4 also detect the
type of the ptrace's first parameter and defin PTRACE_TYPE_ARG1, as
already does the for parameters 3-4, and then simply wrap ptrace with
a macro that casts the first argument to the detected type.  (I'm
leaving adding a nicer wrapper for when we drop building in C).

While this adds the wrapper, GNU/Linux files won't use it until the
next patch, which makes all native GNU/Linux files include
gdb_ptrace.h.

gdb/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	* ptrace.m4 (ptrace tests): Test in C++ mode.  Try with 'enum
	__ptrace_request as first parameter type instead of int.
	(PTRACE_TYPE_ARG1): Define.
	* nat/gdb_ptrace.h [!PTRACE_TYPE_ARG5] (ptrace): Define as wrapper
	that casts first argument to PTRACE_TYPE_ARG1.
	* config.in: Regenerate.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	* config.in: Regenerate.
	* configure: Regenerate.

1 files changed, 13 insertions, 1 deletions

diff --git a/gdb/ptrace.m4 b/gdb/ptrace.m4
index e5fe7fa..f01c692 100644
--- a/gdb/ptrace.m4
+++ b/gdb/ptrace.m4
@@ -22,6 +22,12 @@ AC_DEFUN([GDB_AC_PTRACE],
 
 AC_CHECK_HEADERS([sys/ptrace.h ptrace.h])
 
+# Needs to be tested in C++ mode, to detect whether we need to cast
+# the first argument to enum __ptrace_request.
+if test "$enable_build_with_cxx" = "yes"; then
+    AC_LANG_PUSH([C++])
+fi
+
 gdb_ptrace_headers='
 #include <sys/types.h>
 #if HAVE_SYS_PTRACE_H
@@ -52,7 +58,7 @@ AC_DEFINE_UNQUOTED(PTRACE_TYPE_RET, $gdb_cv_func_ptrace_ret,
 AC_CACHE_CHECK([types of arguments for ptrace], gdb_cv_func_ptrace_args, [
   AC_TRY_COMPILE($gdb_ptrace_headers,
     [extern long ptrace (enum __ptrace_request, ...);],
-    [gdb_cv_func_ptrace_args='int,int,long,long'],[
+    [gdb_cv_func_ptrace_args='enum __ptrace_request,int,long,long'],[
 for gdb_arg1 in 'int' 'long'; do
  for gdb_arg2 in 'pid_t' 'int' 'long'; do
   for gdb_arg3 in 'int *' 'caddr_t' 'int' 'long' 'void *'; do
@@ -81,6 +87,8 @@ ac_save_IFS=$IFS; IFS=','
 set dummy `echo "$gdb_cv_func_ptrace_args" | sed 's/\*/\*/g'`
 IFS=$ac_save_IFS
 shift
+AC_DEFINE_UNQUOTED(PTRACE_TYPE_ARG1, $[1],
+  [Define to the type of arg 1 for ptrace.])
 AC_DEFINE_UNQUOTED(PTRACE_TYPE_ARG3, $[3],
   [Define to the type of arg 3 for ptrace.])
 AC_DEFINE_UNQUOTED(PTRACE_TYPE_ARG4, $[4],
@@ -89,4 +97,8 @@ if test -n "$[5]"; then
   AC_DEFINE_UNQUOTED(PTRACE_TYPE_ARG5, $[5],
     [Define to the type of arg 5 for ptrace.])
 fi
+
+if test "$enable_build_with_cxx" = "yes"; then
+    AC_LANG_POP([C++])
+fi
 ])