clang – LnRSoft

[C++] ThreadSanitizer

27th January 2018 16th March 2018 / By Roland Ihasz

ThreadSanitizer is a data race detector for C/C++. Data races are one of the most common and hardest to debug types of bugs in concurrent systems.
Here is an example of a data race that can lead to crashes and memory corruptions:

#include <pthread.h>
#include <stdio.h>

int Global;

void *Thread1(void *x) {
  Global++;
  return NULL;
}

void *Thread2(void *x) {
  Global--;
  return NULL;
}

int main() {
  pthread_t t[2];
  pthread_create(&t[0], NULL, Thread1, NULL);
  pthread_create(&t[1], NULL, Thread2, NULL);
  pthread_join(t[0], NULL);
  pthread_join(t[1], NULL);
}

#include <pthread.h>

#include <stdio.h>

int Global;

void *Thread1(void *x) {

Global++;

return NULL;

}

void *Thread2(void *x) {

Global--;

return NULL;

}

int main() {

pthread_t t[2];

pthread_create(&t[0], NULL, Thread1, NULL);

pthread_create(&t[1], NULL, Thread2, NULL);

pthread_join(t[0], NULL);

pthread_join(t[1], NULL);

}

$ clang++ simple_race.cpp -fsanitize=thread -fPIE -pie -g
$ ./a.out

==================
WARNING: ThreadSanitizer: data race (pid=36804)
  Write of size 4 at 0x0001062b7068 by thread T2:
  * #0 Thread2(void*) simple_race.cpp:12 (a.out:x86_64+0x100000d8a)

  Previous write of size 4 at 0x0001062b7068 by thread T1:
  * #0 Thread1(void*) simple_race.cpp:7 (a.out:x86_64+0x100000d2a)

  Issue is caused by frames marked with "*".

  Location is global 'Global' at 0x0001062b7068 (a.out+0x000100001068)

  Thread T2 (tid=817154, running) created by main thread at:
    #0 pthread_create <null>:1600736 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2a34d)
    #1 main simple_race.cpp:19 (a.out:x86_64+0x100000e07)

  Thread T1 (tid=817153, finished) created by main thread at:
    #0 pthread_create <null>:1600736 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2a34d)
    #1 main simple_race.cpp:18 (a.out:x86_64+0x100000de8)

SUMMARY: ThreadSanitizer: data race simple_race.cpp:12 in Thread2(void*)
==================
ThreadSanitizer: reported 1 warnings
Abort trap: 6

$ clang++ simple_race.cpp -fsanitize=thread -fPIE -pie -g

$ ./a.out

==================

WARNING: ThreadSanitizer: data race (pid=36804)

Write of size 4 at 0x0001062b7068 by thread T2:

* #0 Thread2(void*) simple_race.cpp:12 (a.out:x86_64+0x100000d8a)

Previous write of size 4 at 0x0001062b7068 by thread T1:

* #0 Thread1(void*) simple_race.cpp:7 (a.out:x86_64+0x100000d2a)

Issue is caused by frames marked with "*".

Location is global 'Global' at 0x0001062b7068 (a.out+0x000100001068)

Thread T2 (tid=817154, running) created by main thread at:

#0 pthread_create <null>:1600736 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2a34d)

#1 main simple_race.cpp:19 (a.out:x86_64+0x100000e07)

Thread T1 (tid=817153, finished) created by main thread at:

#0 pthread_create <null>:1600736 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2a34d)

#1 main simple_race.cpp:18 (a.out:x86_64+0x100000de8)

SUMMARY: ThreadSanitizer: data race simple_race.cpp:12 in Thread2(void*)

==================

ThreadSanitizer: reported 1 warnings

Abort trap: 6

[C++] cxx_status and support in Clang

14th November 2017 9th December 2017 / By Roland Ihasz

Here is the list of current C++ implementation status and support in Clang as per Aug 2017.
You can find the complete list of features and minimum Clang version with support on the source page: https://clang.llvm.org/cxx_status.html

C++11 implementation status
Clang 3.3 and later implement all of the ISO C++ 2011 standard. By default, Clang builds C++ code according to the C++98 standard, with many C++11 features accepted as extensions. You can use Clang in C++11 mode with the -std=c++11 option. Clang’s C++11 mode can be used with libc++ or with gcc’s libstdc++.

C++14 implementation status
Clang 3.4 and later implement all of the ISO C++ 2014 standard.

You can use Clang in C++14 mode with the -std=c++14 option (use -std=c++1y in Clang 3.4 and earlier).

C++17 implementation status
Clang 5 and later implement all the features of the C++ 2017 Draft International Standard.
You can use Clang in C++17 mode with the -std=c++17 option (use -std=c++1z in Clang 4 and earlier).

C++2a implementation status
Clang has experimental support for some proposed features of the C++ standard following C++17, provisionally named C++2a. Note that support for these features may change or be removed without notice, as the draft C++2a standard evolves.
You can use Clang in C++2a mode with the -std=c++2a option.

More technical specifications and standing documents on the source page: https://clang.llvm.org/cxx_status.html