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https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=185337
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2008-02-24  Jan Kratochvil  <jan.kratochvil@redhat.com>
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	Port to GDB-6.8pre.
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currently for trivial nonthreaded helloworld with no debug info up to -ggdb2 you
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will get:
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        (gdb) p errno
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        [some error]
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* with -ggdb2 and less "errno" in fact does not exist anywhere as it was
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  compiled to "(*__errno_location ())" and the macro definition is not present.
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  Unfortunately gdb will find the TLS symbol and it will try to access it but
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  as the program has been compiled without -lpthread the TLS base register
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  (%gs on i386) is not setup and it will result in:
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        Cannot access memory at address 0x8
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Attached suggestion patch how to deal with the most common "errno" symbol
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for the most common under-ggdb3 compiled programs.
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Original patch hooked into target_translate_tls_address.  But its inferior
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call invalidates `struct frame *' in the callers - RH BZ 690908.
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https://bugzilla.redhat.com/show_bug.cgi?id=1166549
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2007-11-03  Jan Kratochvil  <jan.kratochvil@redhat.com>
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	* ./gdb/dwarf2read.c (read_partial_die, dwarf2_linkage_name): Prefer
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	DW_AT_MIPS_linkage_name over DW_AT_name now only for non-C.
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glibc-debuginfo-2.7-2.x86_64: /usr/lib/debug/lib64/libc.so.6.debug:
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  <81a2>     DW_AT_name        : (indirect string, offset: 0x280e): __errno_location
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  <81a8>     DW_AT_MIPS_linkage_name: (indirect string, offset: 0x2808): *__GI___errno_location
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Index: gdb-7.10.90.20160211/gdb/printcmd.c
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===================================================================
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--- gdb-7.10.90.20160211.orig/gdb/printcmd.c	2016-02-15 23:29:53.062420893 +0100
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+++ gdb-7.10.90.20160211/gdb/printcmd.c	2016-02-15 23:33:31.068968762 +0100
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@@ -1003,6 +1003,10 @@
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   if (exp && *exp)
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     {
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+      /* '*((int *(*) (void)) __errno_location) ()' is incompatible with
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+	 function descriptors.  */
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+      if (target_has_execution && strcmp (exp, "errno") == 0)
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+	exp = "*(*(int *(*)(void)) __errno_location) ()";
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       expr = parse_expression (exp);
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       make_cleanup (free_current_contents, &expr;;
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       val = evaluate_expression (expr);
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Index: gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno.c
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===================================================================
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--- /dev/null	1970-01-01 00:00:00.000000000 +0000
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+++ gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno.c	2016-02-15 23:30:20.703617148 +0100
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@@ -0,0 +1,28 @@
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+/* This testcase is part of GDB, the GNU debugger.
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+
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+   Copyright 2005, 2007 Free Software Foundation, Inc.
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+
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+   This program is free software; you can redistribute it and/or modify
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+   it under the terms of the GNU General Public License as published by
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+   the Free Software Foundation; either version 3 of the License, or
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+   (at your option) any later version.
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+
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+   This program is distributed in the hope that it will be useful,
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+   but WITHOUT ANY WARRANTY; without even the implied warranty of
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+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
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+   GNU General Public License for more details.
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+
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+   You should have received a copy of the GNU General Public License
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+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
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+
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+   Please email any bugs, comments, and/or additions to this file to:
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+   bug-gdb@prep.ai.mit.edu  */
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+
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+#include <errno.h>
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+
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+int main()
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+{
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+  errno = 42;
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+
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+  return 0;	/* breakpoint */
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+}
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Index: gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno.exp
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===================================================================
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--- /dev/null	1970-01-01 00:00:00.000000000 +0000
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+++ gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno.exp	2016-02-15 23:35:25.582781821 +0100
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@@ -0,0 +1,60 @@
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+# Copyright 2007 Free Software Foundation, Inc.
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+
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+# This program is free software; you can redistribute it and/or modify
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+# it under the terms of the GNU General Public License as published by
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+# the Free Software Foundation; either version 3 of the License, or
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+# (at your option) any later version.
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+#
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+# This program is distributed in the hope that it will be useful,
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+# but WITHOUT ANY WARRANTY; without even the implied warranty of
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+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
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+# GNU General Public License for more details.
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+#
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+# You should have received a copy of the GNU General Public License
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+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
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+
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+set testfile dw2-errno
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+set srcfile ${testfile}.c
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+set binfile [standard_output_file ${testfile}]
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+
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+proc prep {} {
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+    global srcdir subdir binfile
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+    gdb_exit
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+    gdb_start
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+    gdb_reinitialize_dir $srcdir/$subdir
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+    gdb_load ${binfile}
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+
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+    runto_main
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+
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+    gdb_breakpoint [gdb_get_line_number "breakpoint"]
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+    gdb_continue_to_breakpoint "breakpoint"
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+}
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+
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+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable "additional_flags=-g2"] != "" } {
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+    untested "Couldn't compile test program"
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+    return -1
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+}
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+prep
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+gdb_test "print errno" ".* = 42" "errno with macros=N threads=N"
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+
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+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable "additional_flags=-g3"] != "" } {
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+    untested "Couldn't compile test program"
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+    return -1
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+}
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+prep
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+gdb_test "print errno" ".* = 42" "errno with macros=Y threads=N"
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+
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+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable "additional_flags=-g2"] != "" } {
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+    return -1
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+}
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+prep
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+gdb_test "print errno" ".* = 42" "errno with macros=N threads=Y"
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+
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+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable "additional_flags=-g3"] != "" } {
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+    return -1
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+}
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+prep
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+gdb_test "print errno" ".* = 42" "errno with macros=Y threads=Y"
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+
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+# TODO: Test the error on resolving ERRNO with only libc loaded.
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+# Just how to find the current libc filename?
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Index: gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno2.c
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===================================================================
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--- /dev/null	1970-01-01 00:00:00.000000000 +0000
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+++ gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno2.c	2016-02-15 23:30:20.703617148 +0100
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@@ -0,0 +1,28 @@
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+/* This testcase is part of GDB, the GNU debugger.
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+
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+   Copyright 2005, 2007 Free Software Foundation, Inc.
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+
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+   This program is free software; you can redistribute it and/or modify
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+   it under the terms of the GNU General Public License as published by
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+   the Free Software Foundation; either version 3 of the License, or
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+   (at your option) any later version.
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+
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+   This program is distributed in the hope that it will be useful,
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+   but WITHOUT ANY WARRANTY; without even the implied warranty of
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+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
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+   GNU General Public License for more details.
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+
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+   You should have received a copy of the GNU General Public License
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+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
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+
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+   Please email any bugs, comments, and/or additions to this file to:
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+   bug-gdb@prep.ai.mit.edu  */
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+
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+#include <errno.h>
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+
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+int main()
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+{
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+  errno = 42;
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+
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+  return 0;	/* breakpoint */
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+}
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Index: gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno2.exp
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===================================================================
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--- /dev/null	1970-01-01 00:00:00.000000000 +0000
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+++ gdb-7.10.90.20160211/gdb/testsuite/gdb.dwarf2/dw2-errno2.exp	2016-02-15 23:35:25.582781821 +0100
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@@ -0,0 +1,71 @@
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+# Copyright 2007 Free Software Foundation, Inc.
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+
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+# This program is free software; you can redistribute it and/or modify
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+# it under the terms of the GNU General Public License as published by
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+# the Free Software Foundation; either version 3 of the License, or
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+# (at your option) any later version.
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+#
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+# This program is distributed in the hope that it will be useful,
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+# but WITHOUT ANY WARRANTY; without even the implied warranty of
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+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
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+# GNU General Public License for more details.
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+#
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+# You should have received a copy of the GNU General Public License
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+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
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+
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+set testfile dw2-errno2
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+set srcfile ${testfile}.c
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+set binfile [standard_output_file ${testfile}]
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+
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+proc prep { message {do_xfail 0} } { with_test_prefix $message {
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+    global srcdir subdir binfile variant
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+    gdb_exit
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+    gdb_start
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+    gdb_reinitialize_dir $srcdir/$subdir
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+    gdb_load ${binfile}${variant}
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+
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+    runto_main
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+
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+    gdb_breakpoint [gdb_get_line_number "breakpoint"]
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+    gdb_continue_to_breakpoint "breakpoint"
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+
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+    gdb_test "gcore ${binfile}${variant}.core" "\r\nSaved corefile .*" "gcore $variant"
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+
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+    gdb_test "print errno" ".* = 42"
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+
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+    gdb_test "kill" ".*" "kill" {Kill the program being debugged\? \(y or n\) } "y"
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+    gdb_test "core-file ${binfile}${variant}.core" "\r\nCore was generated by .*" "core-file"
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+    if $do_xfail {
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+	setup_xfail "*-*-*"
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+    }
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+    gdb_test "print errno" ".* = 42" "print errno for core"
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+}}
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+
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+set variant g2thrN
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+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}${variant}" executable "additional_flags=-g2"] != "" } {
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+    untested "Couldn't compile test program"
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+    return -1
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+}
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+prep "macros=N threads=N" 1
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+
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+set variant g3thrN
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+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}${variant}" executable "additional_flags=-g3"] != "" } {
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+    untested "Couldn't compile test program"
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+    return -1
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+}
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+prep "macros=Y threads=N" 1
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+
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+set variant g2thrY
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+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}${variant}" executable "additional_flags=-g2"] != "" } {
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+    return -1
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+}
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+prep "macros=N threads=Y"
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+
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+set variant g3thrY
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+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}${variant}" executable "additional_flags=-g3"] != "" } {
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+    return -1
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+}
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+prep "macros=Y threads=Y" 1
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+
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+# TODO: Test the error on resolving ERRNO with only libc loaded.
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+# Just how to find the current libc filename?