[PATCH 0/2] Support libc with stdio-only I/O in libstdc++

[PATCH 0/2] Support libc with stdio-only I/O in libstdc++

[Keith Packard]

The current libstdc++ basic_file_stdio.cc code assumes a POSIX API
underneath the stdio implementation provided by the host libc. This
means that the host must provide a fairly broad POSIX file API,
including read, write, open, close, lseek and ioctl.

This patch changes basic_file_stdio.cc to only use basic ANSI-C stdio
functions, allowing it to be used with libc implementations like
picolibc which may not have a POSIX operating system underneath.

[PATCH 1/2] libstdc++: Add --enable-pure-stdio-libstdcxx option

[Keith Packard]

This option directs the library to only use simple stdio APIs instead
of using fileno to get the file descriptor for use with POSIX APIs.

Signed-off-by: Keith Packard <keithp@keithp.com>
---
 libstdc++-v3/ChangeLog                     | 13 ++++++
 libstdc++-v3/acinclude.m4                  | 13 ++++++
 libstdc++-v3/config/io/basic_file_stdio.cc | 46 +++++++++++++++++++---
 libstdc++-v3/configure.ac                  |  1 +
 4 files changed, 68 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index d3ee18ed7d1..7f6b0697534 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1147,6 +1147,19 @@
 	* include/bits/uniform_int_dist.h (uniform_int_distribution::_S_nd):
 	Use qualified-id to refer to static member functions.
 
+2020-11-04  Keith Packard  <keithp@keithp.com>
+
+	* acinclude.m4: Add GLIBCXX_ENABLE_PURE_STDIO for
+	--enable-libstdcxx-pure-stdio
+	* configure.ac: Use GLIBCXX_ENABLE_PURE_STDIO for
+	--enable-libstdcxx-pure-stdio
+	* configure: regenerate
+	* config.h.in: regenerate
+	* config/io/basic_file_stdio.cc: Add support for
+	_GLIBCXX_USE_PURE_STDIO. This makes libstdc++ use only defined
+	stdio entry points for all I/O operations, without direct calls to
+	underlying POSIX functions.
+
 2020-11-03  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/syncstream: Include <bits/std_mutex.h>
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index fcd9ea3d23a..863695e68e7 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -2889,6 +2889,19 @@ AC_DEFUN([GLIBCXX_ENABLE_CSTDIO], [
 ])
 
 
+dnl
+dnl Check whether to use 'pure' stdio (no POSIX calls at all)
+dnl
+dnl Default is 'no'
+AC_DEFUN([GLIBCXX_ENABLE_PURE_STDIO], [
+  GLIBCXX_ENABLE(libstdcxx-pure-stdio,$1,,[use only stdio APIs])
+  if test $enable_libstdcxx_pure_stdio = yes; then
+    AC_DEFINE(_GLIBCXX_USE_PURE_STDIO, 1,
+              [Define to restrict code to stdio APIs.])
+  fi
+])
+
+
 dnl
 dnl Check for "unusual" flags to pass to the compiler while building.
 dnl
diff --git a/libstdc++-v3/config/io/basic_file_stdio.cc b/libstdc++-v3/config/io/basic_file_stdio.cc
index ba830fb9e97..d31da8497f4 100644
--- a/libstdc++-v3/config/io/basic_file_stdio.cc
+++ b/libstdc++-v3/config/io/basic_file_stdio.cc
@@ -111,13 +111,21 @@ namespace
 
   // Wrapper handling partial write.
   static std::streamsize
+#ifdef _GLIBCXX_USE_PURE_STDIO
+  xwrite(FILE *__file, const char* __s, std::streamsize __n)
+#else
   xwrite(int __fd, const char* __s, std::streamsize __n)
+#endif
   {
     std::streamsize __nleft = __n;
 
     for (;;)
       {
+#ifdef _GLIBCXX_USE_PURE_STDIO
+	const std::streamsize __ret = fwrite(__file, 1, __nleft, __file);
+#else
 	const std::streamsize __ret = write(__fd, __s, __nleft);
+#endif
 	if (__ret == -1L && errno == EINTR)
 	  continue;
 	if (__ret == -1L)
@@ -133,7 +141,7 @@ namespace
     return __n - __nleft;
   }
 
-#ifdef _GLIBCXX_HAVE_WRITEV
+#if defined(_GLIBCXX_HAVE_WRITEV) && !defined(_GLIBCXX_USE_PURE_STDIO)
   // Wrapper handling partial writev.
   static std::streamsize
   xwritev(int __fd, const char* __s1, std::streamsize __n1,
@@ -286,9 +294,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   __basic_file<char>::is_open() const throw ()
   { return _M_cfile != 0; }
 
+#ifndef _GLIBCCXX_USE_PURE_STDIO
   int
   __basic_file<char>::fd() throw ()
   { return fileno(_M_cfile); }
+#endif
 
   __c_file*
   __basic_file<char>::file() throw ()
@@ -315,28 +325,46 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   {
     streamsize __ret;
     do
+#ifdef _GLIBCXX_USE_PURE_STDIO
+      __ret = fread(__s, 1, __n, this->file());
+#else
       __ret = read(this->fd(), __s, __n);
+#endif
     while (__ret == -1L && errno == EINTR);
     return __ret;
   }
 
   streamsize
   __basic_file<char>::xsputn(const char* __s, streamsize __n)
-  { return xwrite(this->fd(), __s, __n); }
+  {
+#ifdef _GLIBCXX_USE_PURE_STDIO
+    return xwrite(this->file(), __s, __n);
+#else
+    return xwrite(this->fd(), __s, __n);
+#endif
+  }
 
   streamsize
   __basic_file<char>::xsputn_2(const char* __s1, streamsize __n1,
 			       const char* __s2, streamsize __n2)
   {
     streamsize __ret = 0;
-#ifdef _GLIBCXX_HAVE_WRITEV
+#if defined(_GLIBCXX_HAVE_WRITEV) && !defined(_GLIBCXX_USE_PURE_STDIO)
     __ret = xwritev(this->fd(), __s1, __n1, __s2, __n2);
 #else
     if (__n1)
+#ifdef _GLIBCXX_USE_PURE_STDIO
+      __ret = xwrite(this->file(), __s1, __n1);
+#else
       __ret = xwrite(this->fd(), __s1, __n1);
+#endif
 
     if (__ret == __n1)
+#ifdef _GLIBCXX_USE_PURE_STDIO
+      __ret += xwrite(this->file(), __s2, __n2);
+#else
       __ret += xwrite(this->fd(), __s2, __n2);
+#endif
 #endif
     return __ret;
   }
@@ -350,7 +378,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     if (__off > numeric_limits<off_t>::max()
 	|| __off < numeric_limits<off_t>::min())
       return -1L;
+#ifdef _GLIBCXX_USE_PURE_STDIO
+    return fseek(this->file(), __off, __way);
+#else
     return lseek(this->fd(), __off, __way);
+#endif
 #endif
   }
 
@@ -361,7 +393,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   streamsize
   __basic_file<char>::showmanyc()
   {
-#ifndef _GLIBCXX_NO_IOCTL
+#if !defined(_GLIBCXX_NO_IOCTL) && !defined(_GLIBCXX_USE_PURE_STDIO)
 #ifdef FIONREAD
     // Pipes and sockets.
     int __num = 0;
@@ -371,7 +403,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
 #endif
 
-#ifdef _GLIBCXX_HAVE_POLL
+#if defined(_GLIBCXX_HAVE_POLL) && !defined(_GLIBCXX_USE_PURE_STDIO)
     // Cheap test.
     struct pollfd __pfd[1];
     __pfd[0].fd = this->fd();
@@ -395,8 +427,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct stat __buffer;
     const int __err = fstat(this->fd(), &__buffer);
     if (!__err && _GLIBCXX_ISREG(__buffer.st_mode))
+#ifdef _GLIBCXX_USE_PURE_STDIO
+      return __buffer.st_size - fseek(this->file(), 0, ios_base::cur);
+#else
       return __buffer.st_size - lseek(this->fd(), 0, ios_base::cur);
 #endif
+#endif
 #endif
     return 0;
   }
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index cbfdf4c6bad..733154d070b 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -179,6 +179,7 @@ GLIBCXX_ENABLE_EXTERN_TEMPLATE([yes])
 GLIBCXX_ENABLE_PYTHON
 GLIBCXX_ENABLE_WERROR([no])
 GLIBCXX_ENABLE_VTABLE_VERIFY([no])
+GLIBCXX_ENABLE_PURE_STDIO
 
 # Checks for operating systems support that doesn't require linking.
 GLIBCXX_CHECK_STDIO_PROTO
-- 
2.29.2

[PATCH 2/2] Regenerate libstdc++-v3 autoconf files

[Keith Packard]

These are the changes to autoconf files for the pure-stdio patch

Signed-off-by: Keith Packard <keithp@keithp.com>
---
 libstdc++-v3/config.h.in |  3 +++
 libstdc++-v3/configure   | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/libstdc++-v3/config.h.in b/libstdc++-v3/config.h.in
index 72faabfb2c1..76b1c97d2b5 100644
--- a/libstdc++-v3/config.h.in
+++ b/libstdc++-v3/config.h.in
@@ -1022,6 +1022,9 @@
 /* Define if POSIX read/write locks are available in <gthr.h>. */
 #undef _GLIBCXX_USE_PTHREAD_RWLOCK_T
 
+/* Define to restrict code to stdio APIs. */
+#undef _GLIBCXX_USE_PURE_STDIO
+
 /* Define if /dev/random and /dev/urandom are available for the random_device
    of TR1 (Chapter 5.1). */
 #undef _GLIBCXX_USE_RANDOM_TR1
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index d128de2f186..5647c986831 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -929,6 +929,7 @@ enable_extern_template
 with_python_dir
 enable_werror
 enable_vtable_verify
+enable_libstdcxx_pure_stdio
 enable_libstdcxx_time
 enable_tls
 enable_rpath
@@ -1632,6 +1633,8 @@ Optional Features:
                           enable extern template [default=yes]
   --enable-werror         turns on -Werror [default=no]
   --enable-vtable-verify  enable vtable verify [default=no]
+  --enable-libstdcxx-pure-stdio
+                          use only stdio APIs [default=]
   --enable-libstdcxx-time[=KIND]
                           use KIND for check type [default=auto]
   --enable-tls            Use thread-local storage [default=yes]
@@ -18807,6 +18810,26 @@ fi
 
 
 
+   # Check whether --enable-libstdcxx-pure-stdio was given.
+if test "${enable_libstdcxx_pure_stdio+set}" = set; then :
+  enableval=$enable_libstdcxx_pure_stdio;
+      case "$enableval" in
+       yes|no) ;;
+       *) as_fn_error $? "Argument to enable/disable libstdcxx-pure-stdio must be yes or no" "$LINENO" 5 ;;
+      esac
+
+else
+  enable_libstdcxx_pure_stdio=
+fi
+
+
+  if test $enable_libstdcxx_pure_stdio = yes; then
+
+$as_echo "#define _GLIBCXX_USE_PURE_STDIO 1" >>confdefs.h
+
+  fi
+
+
 # Checks for operating systems support that doesn't require linking.
 
 
-- 
2.29.2