[libc++] Ensure streams are initialized early

When statically linking libc++ on some systems, the streams are not
initialized early enough, which causes all kinds of issues. This was
reported e.g. in http://llvm.org/PR28954, but also in various open
source projects that use libc++.

Fixes http://llvm.org/PR28954.

Differential Revision: https://reviews.llvm.org/D31413

libcxx/src/iostream.cpp

@@ -77,7 +77,7 @@ __asm__("?wclog@" _LIBCPP_ABI_NAMESPACE_STR "@std@@3V?$basic_ostream@_WU?$char_t
 #endif
 ;
 
-_LIBCPP_HIDDEN ios_base::Init __start_std_streams;
+_LIBCPP_HIDDEN ios_base::Init __start_std_streams __attribute__((init_priority(101)));
 
 // On Windows the TLS storage for locales needs to be initialized before we create
 // the standard streams, otherwise it may not be alive during program termination

libcxx/test/std/input.output/iostream.objects/init.pass.cpp

@@ -0,0 +1,88 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-has-no-stdin, libcpp-has-no-stdout
+
+// Make sure that the iostreams are initialized before everything else.
+// This has been an issue when statically linking libc++ in some contexts.
+// See https://llvm.org/PR28954 for details.
+//
+// This test works by checking that std::{cin,cout,cerr} is the same in a
+// static object constructor and in the main function. It dumps the memory of
+// each stream in the static object constructor and compares it with the memory
+// in the main function.
+//
+// The assumption is that if there are no uses of the stream object (such as
+// construction), then its memory must be the same. In the case where the test
+// "fails" and we are actually accessing an uninitialized object when we perform
+// the memcpy, the behavior is technically undefined (so the test could still
+// pass).
+
+#include <cassert>
+#include <cstring>
+#include <iostream>
+
+struct Checker {
+    char *cerr_mem_dump;
+    char *cin_mem_dump;
+    char *cout_mem_dump;
+    char *clog_mem_dump;
+
+    char *wcerr_mem_dump;
+    char *wcin_mem_dump;
+    char *wcout_mem_dump;
+    char *wclog_mem_dump;
+
+    Checker()
+        : cerr_mem_dump(new char[sizeof(std::cerr)])
+        , cin_mem_dump(new char[sizeof(std::cin)])
+        , cout_mem_dump(new char[sizeof(std::cout)])
+        , clog_mem_dump(new char[sizeof(std::clog)])
+
+        , wcerr_mem_dump(new char[sizeof(std::wcerr)])
+        , wcin_mem_dump(new char[sizeof(std::wcin)])
+        , wcout_mem_dump(new char[sizeof(std::wcout)])
+        , wclog_mem_dump(new char[sizeof(std::wclog)])
+     {
+        std::memcpy(cerr_mem_dump, (char*)&std::cerr, sizeof(std::cerr));
+        std::memcpy(cin_mem_dump, (char*)&std::cin, sizeof(std::cin));
+        std::memcpy(cout_mem_dump, (char*)&std::cout, sizeof(std::cout));
+        std::memcpy(clog_mem_dump, (char*)&std::clog, sizeof(std::clog));
+
+        std::memcpy(wcerr_mem_dump, (char*)&std::wcerr, sizeof(std::wcerr));
+        std::memcpy(wcin_mem_dump, (char*)&std::wcin, sizeof(std::wcin));
+        std::memcpy(wcout_mem_dump, (char*)&std::wcout, sizeof(std::wcout));
+        std::memcpy(wclog_mem_dump, (char*)&std::wclog, sizeof(std::wclog));
+    }
+
+    ~Checker() {
+        delete[] cerr_mem_dump;
+        delete[] cin_mem_dump;
+        delete[] cout_mem_dump;
+        delete[] clog_mem_dump;
+
+        delete[] wcerr_mem_dump;
+        delete[] wcin_mem_dump;
+        delete[] wcout_mem_dump;
+        delete[] wclog_mem_dump;
+    }
+};
+
+static Checker check;
+
+int main() {
+    assert(std::memcmp(check.cerr_mem_dump, (char const*)&std::cerr, sizeof(std::cerr)) == 0);
+    assert(std::memcmp(check.cin_mem_dump, (char const*)&std::cin, sizeof(std::cin)) == 0);
+    assert(std::memcmp(check.cout_mem_dump, (char const*)&std::cout, sizeof(std::cout)) == 0);
+    assert(std::memcmp(check.clog_mem_dump, (char const*)&std::clog, sizeof(std::clog)) == 0);
+
+    assert(std::memcmp(check.wcerr_mem_dump, (char const*)&std::wcerr, sizeof(std::wcerr)) == 0);
+    assert(std::memcmp(check.wcin_mem_dump, (char const*)&std::wcin, sizeof(std::wcin)) == 0);
+    assert(std::memcmp(check.wcout_mem_dump, (char const*)&std::wcout, sizeof(std::wcout)) == 0);
+    assert(std::memcmp(check.wclog_mem_dump, (char const*)&std::wclog, sizeof(std::wclog)) == 0);
+}