eastig created this revision.
eastig added a subscriber: cfe-commits.
When an executable (e.g. for bare metal environment) is statically linked with
libc++ the following code might not work as expected:
```
#include <iostream>
class Foo {
public:
Foo(int n) {
std::cout << "Hello World - " << n << std::endl;
}
};
Foo f(5);
int main() {
return 0;
}
```
The program hangs or crashes because 'std::cout' is not initialized before 'Foo
f(5)'.
The standard says:
> 27.3 Standard iostream objects
> Header <iostream> synopsis
> (2) ... The objects are constructed, and the associations are established at
> some time prior to or during first
> time an object of class basic_ios<charT,traits>::Init is constructed, and in
> any case before the body of main
> begins execution [264]. The objects are not destroyed during program
> execution [265].
>
And footnote 265 says:
> Constructors and destructors for static objects can access these objects to
> read input from stdin or write output
> to stdout or stderr.
The similar issue was raised more than year ago. A patch was proposed but not
committed. See discussion of it here:
[[
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20130304/075363.html
| initial patch ]]
[[
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20130325/077130.html
| review, suggestion for #ifdef ]]
The proposed patch caused initialization/deinitialization of the std streams as
many times as the static Init objects were created.
The current patch fixes this issue. A number of uses is counted by means of
__shared_count. It is used instead of a 'int' variable because it is
thread-safe. The standard allows multi-threaded initialization of static
objects from different compilation modules.
http://reviews.llvm.org/D12689
Files:
include/ios
src/iostream.cpp
Index: src/iostream.cpp
===================================================================
--- src/iostream.cpp
+++ src/iostream.cpp
@@ -10,6 +10,8 @@
#include "__std_stream"
#include "string"
#include "new"
+#include "cassert"
+#include "memory"
_LIBCPP_BEGIN_NAMESPACE_STD
@@ -41,9 +43,7 @@
_ALIGNAS_TYPE (ostream) _LIBCPP_FUNC_VIS char clog[sizeof(ostream)];
_ALIGNAS_TYPE (wostream) _LIBCPP_FUNC_VIS char wclog[sizeof(wostream)];
-ios_base::Init __start_std_streams;
-
-ios_base::Init::Init()
+static void init_std_streams()
{
#ifndef _LIBCPP_HAS_NO_STDIN
istream* cin_ptr = ::new(cin) istream(::new(__cin) __stdinbuf <char>(stdin, &mb_cin));
@@ -70,7 +70,7 @@
#endif
}
-ios_base::Init::~Init()
+static void fini_std_streams()
{
#ifndef _LIBCPP_HAS_NO_STDOUT
ostream* cout_ptr = reinterpret_cast<ostream*>(cout);
@@ -85,4 +85,51 @@
wclog_ptr->flush();
}
+/// __Init is a utility class that counts the number of ios_base::Init
+/// objects. It automatically deinitializes the std streams when the
+/// last ios_base::Init object is destroyed.
+class __Init: public __shared_count {
+private:
+ void __on_zero_shared() _NOEXCEPT {
+ fini_std_streams();
+ }
+public:
+ __Init(long count = 0): __shared_count(count) {
+ init_std_streams();
+ }
+};
+
+_ALIGNAS_TYPE (__Init) _LIBCPP_FUNC_VIS static char __init_storage[sizeof(__Init)];
+
+__shared_count *ios_base::Init::UseCount = nullptr;
+
+ios_base::Init::Init()
+{
+ //local static object is initialized only once
+ struct __S {
+ __S() {
+ Init::UseCount = ::new(__init_storage) __Init(-1);
+ }
+ };
+ static __S __s;
+ UseCount->__add_shared();
+}
+
+ios_base::Init::~Init()
+{
+ assert(UseCount);
+ UseCount->__release_shared();
+}
+
+#ifdef __APPLE__
+// Apple linker has a guarantee for an initialization order:
+// if A links against B, B's initializer will be run before A's.
+// I.e. if you link to libc++.dylib, then cout et al. are guaranteed
+// to be constructed before your initializers run. This way, initialization
+// of __start_std_streams might remain here in module initializer
+// Without Apple linker, the following definition must be in header <ios>,
+// so its initialization will be explicitly executed before any stream usage.
+ios_base::Init __start_std_streams;
+#endif
+
_LIBCPP_END_NAMESPACE_STD
Index: include/ios
===================================================================
--- include/ios
+++ include/ios
@@ -423,13 +423,30 @@
virtual ~failure() throw();
};
+class __shared_count;
+
class _LIBCPP_TYPE_VIS ios_base::Init
{
+private:
+ static __shared_count *UseCount;
public:
Init();
~Init();
};
+#ifndef __APPLE__
+// Apple linker has a guarantee for an initialization order:
+// if A links against B, B's initializer will be run before A's.
+// I.e. if you link to libc++.dylib, then cout et al. are guaranteed
+// to be constructed before your initializers run. This way, definition
+// of __start_std_streams might remain in src/iostream.cpp, and linker arranges
+// its initialization before any stream usage.
+// Without Apple linker, that definition must be here,
+// so its initialization will be explicitly executed before any stream usage.
+// For construction of cout, cin, cerr etc.
+static ios_base::Init __start_std_streams;
+#endif
+
// fmtflags
inline _LIBCPP_INLINE_VISIBILITY
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