Existing toolchain builds rely on the similarity between picolibc and
newlib when building libstdc++ and use --with-newlib.

Switch to the picolibc 16-bit _ctype_wide array which provides
separate values for ctype_base::blank and ctype_base::space.

This fixes a bug where libstdc++ was including '\f', '\n', '\r' and
'\v' in the set of 'blank' chars. Afterwards, only ' ' and '\t' are in
this set, as specified by C++ 11.

Signed-off-by: Keith Packard <[email protected]>
---
 libstdc++-v3/acinclude.m4                     |   4 +
 libstdc++-v3/config/os/picolibc/ctype_base.h  |  61 ++++++++++
 .../os/picolibc/ctype_configure_char.cc       | 105 ++++++++++++++++++
 .../config/os/picolibc/ctype_inline.h         |  74 ++++++++++++
 libstdc++-v3/config/os/picolibc/os_defines.h  |  61 ++++++++++
 libstdc++-v3/configure.ac                     |  62 +++++++++++
 6 files changed, 367 insertions(+)
 create mode 100644 libstdc++-v3/config/os/picolibc/ctype_base.h
 create mode 100644 libstdc++-v3/config/os/picolibc/ctype_configure_char.cc
 create mode 100644 libstdc++-v3/config/os/picolibc/ctype_inline.h
 create mode 100644 libstdc++-v3/config/os/picolibc/os_defines.h

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 103515c000f..1eb575d3202 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -81,6 +81,10 @@ AC_DEFUN([GLIBCXX_CONFIGURE], [
     AC_HELP_STRING([--with-newlib],
                   [assume newlib as a system C library]))
 
+  AC_ARG_WITH([picolibc],
+    AC_HELP_STRING([--with-picolibc],
+                  [assume picolibc as a system C library]))
+
   # Will set LN_S to either 'ln -s', 'ln', or 'cp -p' (if linking isn't
   # available).  Uncomment the next line to force a particular method.
   AC_PROG_LN_S
diff --git a/libstdc++-v3/config/os/picolibc/ctype_base.h 
b/libstdc++-v3/config/os/picolibc/ctype_base.h
new file mode 100644
index 00000000000..0d73c74387f
--- /dev/null
+++ b/libstdc++-v3/config/os/picolibc/ctype_base.h
@@ -0,0 +1,61 @@
+// Locale support -*- C++ -*-
+
+// Copyright (C) 2000-2026 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+//
+// ISO C++ 14882: 22.1  Locales
+//
+
+// Information as gleaned from /usr/include/ctype.h
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  /// @brief  Base class for ctype.
+  struct ctype_base
+  {
+    // Non-standard typedefs.
+    typedef const int*                 __to_type;
+
+    // NB: Offsets into ctype<char>::_M_table force a particular size
+    // on the mask type. Because of this, we don't use an enum.
+    typedef short              mask;
+    static const mask upper            = mask (__CTYPE_UPPER);
+    static const mask lower    = mask (__CTYPE_LOWER);
+    static const mask alpha    = mask (__CTYPE_UPPER | __CTYPE_LOWER);
+    static const mask digit    = mask (__CTYPE_DIGIT);
+    static const mask xdigit   = mask (__CTYPE_HEX | __CTYPE_DIGIT);
+    static const mask space    = mask (__CTYPE_SPACE);
+    static const mask print    = mask (__CTYPE_PUNCT | __CTYPE_UPPER | 
__CTYPE_LOWER | __CTYPE_DIGIT | __CTYPE_BLANK);
+    static const mask graph    = mask (__CTYPE_PUNCT | __CTYPE_UPPER | 
__CTYPE_LOWER | __CTYPE_DIGIT);
+    static const mask cntrl    = mask (__CTYPE_CNTRL);
+    static const mask punct    = mask (__CTYPE_PUNCT);
+    static const mask alnum    = mask (__CTYPE_UPPER | __CTYPE_LOWER | 
__CTYPE_DIGIT);
+#if __cplusplus >= 201103L
+    static const mask blank    = mask (__CTYPE_BLANK | __CTYPE_TAB);
+#endif
+  };
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
diff --git a/libstdc++-v3/config/os/picolibc/ctype_configure_char.cc 
b/libstdc++-v3/config/os/picolibc/ctype_configure_char.cc
new file mode 100644
index 00000000000..74370d4be49
--- /dev/null
+++ b/libstdc++-v3/config/os/picolibc/ctype_configure_char.cc
@@ -0,0 +1,105 @@
+// Locale support -*- C++ -*-
+
+// Copyright (C) 2011-2026 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file ctype_configure_char.cc */
+
+//
+// ISO C++ 14882: 22.1  Locales
+//
+
+#include <locale>
+#include <cstdlib>
+#include <cstring>
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+// Information as gleaned from /usr/include/ctype.h
+
+  const ctype_base::mask*
+  ctype<char>::classic_table() throw()
+  { return _ctype_wide + 1; }
+
+  ctype<char>::ctype(__c_locale, const mask* __table, bool __del,
+                    size_t __refs)
+  : facet(__refs), _M_del(__table != 0 && __del),
+  _M_toupper(NULL), _M_tolower(NULL),
+  _M_table(__table ? __table : classic_table())
+  {
+    memset(_M_widen, 0, sizeof(_M_widen));
+    _M_widen_ok = 0;
+    memset(_M_narrow, 0, sizeof(_M_narrow));
+    _M_narrow_ok = 0;
+  }
+
+  ctype<char>::ctype(const mask* __table, bool __del, size_t __refs)
+  : facet(__refs), _M_del(__table != 0 && __del),
+  _M_toupper(NULL), _M_tolower(NULL),
+  _M_table(__table ? __table : classic_table())
+  {
+    memset(_M_widen, 0, sizeof(_M_widen));
+    _M_widen_ok = 0;
+    memset(_M_narrow, 0, sizeof(_M_narrow));
+    _M_narrow_ok = 0;
+  }
+
+  char
+  ctype<char>::do_toupper(char __c) const
+  {
+    int __x = __c;
+    return (this->is(ctype_base::lower, __c) ? (__x - 'a' + 'A') : __x);
+  }
+
+  const char*
+  ctype<char>::do_toupper(char* __low, const char* __high) const
+  {
+    while (__low < __high)
+      {
+       *__low = this->do_toupper(*__low);
+       ++__low;
+      }
+    return __high;
+  }
+
+  char
+  ctype<char>::do_tolower(char __c) const
+  {
+    int __x = __c;
+    return (this->is(ctype_base::upper, __c) ? (__x - 'A' + 'a') : __x);
+  }
+
+  const char*
+  ctype<char>::do_tolower(char* __low, const char* __high) const
+  {
+    while (__low < __high)
+      {
+       *__low = this->do_tolower(*__low);
+       ++__low;
+      }
+    return __high;
+  }
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
diff --git a/libstdc++-v3/config/os/picolibc/ctype_inline.h 
b/libstdc++-v3/config/os/picolibc/ctype_inline.h
new file mode 100644
index 00000000000..950983800e7
--- /dev/null
+++ b/libstdc++-v3/config/os/picolibc/ctype_inline.h
@@ -0,0 +1,74 @@
+// Locale support -*- C++ -*-
+
+// Copyright (C) 2000-2026 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/ctype_inline.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{locale}
+ */
+
+//
+// ISO C++ 14882: 22.1  Locales
+//
+
+// ctype bits to be inlined go here. Non-inlinable (ie virtual do_*)
+// functions go in ctype.cc
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  bool
+  ctype<char>::
+  is(mask __m, char __c) const
+  { return _M_table[static_cast<unsigned char>(__c)] & __m; }
+
+  const char*
+  ctype<char>::
+  is(const char* __low, const char* __high, mask* __vec) const
+  {
+    while (__low < __high)
+      *__vec++ = _M_table[static_cast<unsigned char>(*__low++)];
+    return __high;
+  }
+
+  const char*
+  ctype<char>::
+  scan_is(mask __m, const char* __low, const char* __high) const
+  {
+    while (__low < __high && !this->is(__m, *__low))
+      ++__low;
+    return __low;
+  }
+
+  const char*
+  ctype<char>::
+  scan_not(mask __m, const char* __low, const char* __high) const
+  {
+    while (__low < __high && this->is(__m, *__low) != 0)
+      ++__low;
+    return __low;
+  }
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
diff --git a/libstdc++-v3/config/os/picolibc/os_defines.h 
b/libstdc++-v3/config/os/picolibc/os_defines.h
new file mode 100644
index 00000000000..fa180715d9e
--- /dev/null
+++ b/libstdc++-v3/config/os/picolibc/os_defines.h
@@ -0,0 +1,61 @@
+// Specific definitions for newlib  -*- C++ -*-
+
+// Copyright (C) 2000-2026 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/os_defines.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{iosfwd}
+ */
+
+#ifndef _GLIBCXX_OS_DEFINES
+#define _GLIBCXX_OS_DEFINES 1
+
+// System-specific #define, typedefs, corrections, etc, go here.  This
+// file will come before all others.
+
+#ifdef __CYGWIN__
+#define _GLIBCXX_GTHREAD_USE_WEAK 0
+
+#if defined (_GLIBCXX_DLL)
+#define _GLIBCXX_PSEUDO_VISIBILITY_default __attribute__ ((__dllimport__))
+#else
+#define _GLIBCXX_PSEUDO_VISIBILITY_default
+#endif
+#define _GLIBCXX_PSEUDO_VISIBILITY_hidden
+
+#define _GLIBCXX_PSEUDO_VISIBILITY(V) _GLIBCXX_PSEUDO_VISIBILITY_ ## V
+
+// See libstdc++/20806.
+#define _GLIBCXX_HAVE_DOS_BASED_FILESYSTEM 1
+
+// Enable use of GetModuleHandleEx (requires Windows XP/2003) in
+// __cxa_thread_atexit to prevent modules from being unloaded before
+// their dtors are called
+#define _GLIBCXX_THREAD_ATEXIT_WIN32 1
+
+// See libstdc++/69506
+#define _GLIBCXX_USE_WEAK_REF 0
+
+#endif
+
+#endif
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index 339e7bb17a4..cd038ee85ac 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -100,6 +100,7 @@ GLIBCXX_CONFIGURE
 
 # Libtool setup.
 if test "x${with_newlib}" != "xyes" &&
+    test "x${with_picolibc}" != "xyes" &&
     test "x${with_avrlibc}" != "xyes" &&
     test "x$with_headers" != "xno"; then
   AC_LIBTOOL_DLOPEN
@@ -428,6 +429,67 @@ dnl # rather than hardcoding that information.
         glibcxx_cv_mkdir=yes
         ;;
     esac
+  elif test "x${with_picolibc}" = "xyes"; then
+    os_include_dir="os/picolibc"
+    AC_DEFINE(HAVE_HYPOT)
+
+    # GLIBCXX_CHECK_STDLIB_SUPPORT
+    AC_DEFINE(HAVE_STRTOF)
+
+    AC_DEFINE(HAVE_ACOSF)
+    AC_DEFINE(HAVE_ASINF)
+    AC_DEFINE(HAVE_ATAN2F)
+    AC_DEFINE(HAVE_ATANF)
+    AC_DEFINE(HAVE_CEILF)
+    AC_DEFINE(HAVE_COSF)
+    AC_DEFINE(HAVE_COSHF)
+    AC_DEFINE(HAVE_EXPF)
+    AC_DEFINE(HAVE_FABSF)
+    AC_DEFINE(HAVE_FLOORF)
+    AC_DEFINE(HAVE_FMODF)
+    AC_DEFINE(HAVE_FREXPF)
+    AC_DEFINE(HAVE_HYPOTF)
+    AC_DEFINE(HAVE_LDEXPF)
+    AC_DEFINE(HAVE_LOG10F)
+    AC_DEFINE(HAVE_LOGF)
+    AC_DEFINE(HAVE_MODFF)
+    AC_DEFINE(HAVE_POWF)
+    AC_DEFINE(HAVE_SINF)
+    AC_DEFINE(HAVE_SINHF)
+    AC_DEFINE(HAVE_SQRTF)
+    AC_DEFINE(HAVE_TANF)
+    AC_DEFINE(HAVE_TANHF)
+
+    # Support for iconv in picolibc is configurable.
+    AC_TRY_COMPILE([#include <picolibc.h>], [
+      #ifndef _ICONV_ENABLED
+      #error
+      #endif], [ac_picolibc_iconv_enabled=yes], [ac_picolibc_iconv_enabled=no])
+    if test "$ac_picolibc_iconv_enabled" = yes; then
+      AC_DEFINE(HAVE_ICONV)
+    fi
+
+    AC_DEFINE(HAVE_MEMALIGN)
+
+    # Check for TLS
+    AC_TRY_COMPILE([#include <picolibc.h>], [
+      #ifndef __THREAD_LOCAL_STORAGE
+      #error
+      #endif], [ac_picolibc_tls_enabled=yes], [ac_picolibc_tls_enabled=no])
+    if test "$ac_picolibc_tls_enabled" = yes; then
+      AC_DEFINE(HAVE_TLS)
+    fi
+    
+    AC_DEFINE(HAVE_ALIGNED_ALLOC)
+    AC_DEFINE(HAVE_AT_QUICK_EXIT)
+    AC_DEFINE(HAVE_LINK)
+    AC_DEFINE(HAVE_SYS_STAT_H)
+    AC_DEFINE(HAVE_SYS_TYPES_H)
+    AC_DEFINE(HAVE_SETENV)
+    AC_DEFINE(HAVE_STRERROR_L)
+    AC_DEFINE(HAVE_S_ISREG)
+    AC_DEFINE(HAVE_UNISTD_H)
+
   elif test "x$with_headers" != "xno"; then
     GLIBCXX_CROSSCONFIG
   fi
-- 
2.51.0

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