[Bug libstdc++/60662] simple use of call_once throws a system_error exception, but not if sleep_for is called beforehand

[sambler at alumni dot nd.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60662

--- Comment #2 from Stuart Ambler  ---
Using gdb on my code, it appears that the immediate problem is caused by what
gthr-default.h and/or gthr-posix.h do to detect whether a program is
multi-threaded, which they seem to assume is ok as a precondition for using
call_once.  I think they do this with the code
#ifdef __GLIBC__
__gthrw2(__gthrw_(__pthread_key_create),
 __pthread_key_create,
 pthread_key_create)
# define GTHR_ACTIVE_PROXY__gthrw_(__pthread_key_create)

Just after that code is

static inline int
__gthread_active_p (void)
{
  static void *const __gthread_active_ptr
= __extension__ (void *) >HR_ACTIVE_PROXY;
  return __gthread_active_ptr != 0;
}

This function is called by __gthread_once, which if __gthread_active_p()
returns false, does nothing other than return -1, which results in call_once
throwing a system_error.

__pthread_key_create is a function in libpthread, which my gcc command line
links with.  It's possible to display the value of __gthread_active_ptr in
main, and it's 0 at a point before call_once is called and the error is thrown,
unless the this_thread::sleep_for code is uncommented, in which case
__gthread_active_ptr is not 0 and there is no problem.

It's not necessary for the this_thread::sleep to be executed to avoid the
problem.  Putting that line in a function in the same source file as main also
results in nonzero __gthread_active_ptr and no problem, even if the function is
never called.

I thought it might depend on order of loading the libraries and gave gcc the
-static option to hopefully have more control over that.  Giving gcc also -v
libpthread is listed before libstdc++.  But with static linking,
__gthread_active_ptr is 0 and the problem occurs whether or not the
this_thread::sleep_for is present.  So I gave up on static linking.

this_thread::sleep_for calls nano_sleep, which libpthread exports.  I don't
know much about the linux linker, but it seems that somehow the existence of
this call to nano_sleep, even if not executed, causes libpthread to be loaded
in a way that gives a nonzero value for __gthread_active_ptr when it is
checked.

Though it may work differently on different systems, it seems like the problem
may revolve around an assumption that call_once won't be called except by a
multi-threaded program, and a perhaps somewhat fragile way of determining
whether it's a multi-threaded program.

If you still think I should report it to Ubuntu, I will.

Thanks.

[Bug libstdc++/60662] New: simple use of call_once throws a system_error exception, but not if sleep_for is called beforehand

[sambler at alumni dot nd.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60662

Bug ID: 60662
   Summary: simple use of call_once throws a system_error
exception, but not if sleep_for is called beforehand
   Product: gcc
   Version: 4.8.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: libstdc++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: sambler at alumni dot nd.edu

Created attachment 32457
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32457&action=edit
preprocessed source code, lengthy due to standard C++ headers

file test_unique.cc (misnomer from original guess at cause of problem):

#include 
#include 
#include 

using namespace std;

int main() {
  once_flag once;
  // this_thread::sleep_for(chrono::milliseconds(1));
  call_once(once, [](){});
  return 0;
}

file buildcc11:

g++ -std=c++11 -Wall -Wextra -save-temps $1.cc -lpthread -o $1

execution:

$ ./buildcc11 test_unique
$ ./test_unique
terminate called after throwing an instance of 'std::system_error'
  what():  Unknown error -1
Aborted (core dumped)
$

whereas after uncommenting the sleep_for line:

$ ./buildcc11 test_unique
$ ./test_unique
$

BEGIN PARENTHETICAL NOTE:
Initially discovered in a more complicated context, this problem was quite
robust w.r.t. a number of changes such as for simplification.  For example,
the following less artificial use of call_once has the same problem (file
test_unique_2.cc):

#include 
#include 
#include 
#include 

using namespace std;

once_flag once;
int n = 0;

void f(int n_arg) {
  n = n_arg;
}

int main() {
  // this_thread::sleep_for(chrono::milliseconds(1));
  int m = 1;
  call_once(once, f, m++);
  call_once(once, f, m++);
  call_once(once, f, m++);
  cout << "n " << n << endl;

  return 0;
}

$ ./buildcc11 test_unique_2
$ ./test_unique_2
terminate called after throwing an instance of 'std::system_error'
  what():  Unknown error -1
Aborted (core dumped)
$

whereas after uncommenting the sleep_for:

$ ./buildcc11 test_unique_2
$ ./test_unique_2
n 1
$
END PARENTHETICAL NOTE

compiler & system info:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.8.1-10ubuntu9' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9) 

lenovo ThinkPad SL510 2x Intel Core 2 Duo T6670 2.20 GHz, 8 GB RAM, Ubuntu
13.10 (Lubuntu), Linux 3.11.0-18-generic (x86_64).

preprocessed file from ./buildcc11 test_unique with commented-out sleep_for not
included b/c of 64K char limit on this field; instead attached.

Thanks!