Keywords: ISO C++ , library

Concurrency
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## Concurrency

This section discusses issues surrounding the proper compilation of multithreaded applications which use the Standard C++ library. This information is GCC-specific since the C++ standard does not address matters of multithreaded applications.

### Prerequisites

All normal disclaimers aside, multithreaded C++ application are only supported when libstdc++ and all user code was built with compilers which report (via `gcc/g++ -v` ) the same thread model and that model is not single. As long as your final application is actually single-threaded, then it should be safe to mix user code built with a thread model of single with a libstdc++ and other C++ libraries built with another thread model useful on the platform. Other mixes may or may not work but are not considered supported. (Thus, if you distribute a shared C++ library in binary form only, it may be best to compile it with a GCC configured with --enable-threads for maximal interchangeability and usefulness with a user population that may have built GCC with either --enable-threads or --disable-threads.)

When you link a multithreaded application, you will probably need to add a library or flag to g++. This is a very non-standardized area of GCC across ports. Some ports support a special flag (the spelling isn't even standardized yet) to add all required macros to a compilation (if any such flags are required then you must provide the flag for all compilations not just linking) and link-library additions and/or replacements at link time. The documentation is weak. Here is a quick summary to display how ad hoc this is: On Solaris, both -pthreads and -threads (with subtly different meanings) are honored. On OSF, -pthread and -threads (with subtly different meanings) are honored. On Linux/i386, -pthread is honored. On FreeBSD, -pthread is honored. Some other ports use other switches. AFAIK, none of this is properly documented anywhere other than in ``gcc -dumpspecs'' (look at lib and cpp entries).

### Thread Safety

We currently use the SGI STL definition of thread safety.

The library strives to be thread-safe when all of the following conditions are met:

- The system's libc is itself thread-safe,

- The compiler in use reports a thread model other than 'single'. This can be tested via output from `gcc -v`. Multi-thread capable versions of gcc output something like this:

      %gcc -v
      Using built-in specs.
      ...
      Thread model: posix
      gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)

  Look for "Thread model" lines that aren't equal to "single."

- Requisite command-line flags are used for atomic operations and threading. Examples of this include `-pthread` and `-march=native`, although specifics vary depending on the host environment. See Machine Dependent Options.

- An implementation of atomicity.h functions exists for the architecture in question. See the internals documentation for more details.

The user-code must guard against concurrent method calls which may access any particular library object's state. Typically, the application programmer may infer what object locks must be held based on the objects referenced in a method call. Without getting into great detail, here is an example which requires user-level locks:

         library_class_a shared_object_a;

         thread_main () {
           library_class_b *object_b = new library_class_b;
           shared_object_a.add_b (object_b);   // must hold lock for shared_object_a
           shared_object_a.mutate ();          // must hold lock for shared_object_a
         }

         // Multiple copies of thread_main() are started in independent threads.

Under the assumption that object_a and object_b are never exposed to another thread, here is an example that should not require any user-level locks:

         thread_main () {
           library_class_a object_a;
           library_class_b *object_b = new library_class_b;
           object_a.add_b (object_b);
           object_a.mutate ();
         }

All library objects are safe to use in a multithreaded program as long as each thread carefully locks out access by any other thread while it uses any object visible to another thread, i.e., treat library objects like any other shared resource. In general, this requirement includes both read and write access to objects; unless otherwise documented as safe, do not assume that two threads may access a shared standard library object at the same time.

See chapters 17 (library introduction), 23 (containers), and 27 (I/O) for more information.

### Atomics

### IO

I'll assume that you have already read the general notes on library threads, and the notes on threaded container access (you might not think of an I/O stream as a container, but the points made there also hold here). If you have not read them, please do so first.

This gets a bit tricky. Please read carefully, and bear with me.

#### Structure

A wrapper type called `__basic_file` provides our abstraction layer for the `std::filebuf` classes. Nearly all decisions dealing with actual input and output must be made in `__basic_file`.

A generic locking mechanism is somewhat in place at the filebuf layer, but is not used in the current code. Providing locking at any higher level is akin to providing locking within containers, and is not done for the same reasons (see the links above).

#### Defaults

The __basic_file type is simply a collection of small wrappers around the C stdio layer (again, see the link under Structure). We do no locking ourselves, but simply pass through to calls to `fopen`, `fwrite`, and so forth.

So, for 3.0, the question of "is multithreading safe for I/O" must be answered with, "is your platform's C library threadsafe for I/O?" Some are by default, some are not; many offer multiple implementations of the C library with varying tradeoffs of threadsafety and efficiency. You, the programmer, are always required to take care with multiple threads.

(As an example, the POSIX standard requires that C stdio FILE* operations are atomic. POSIX-conforming C libraries (e.g, on Solaris and GNU/Linux) have an internal mutex to serialize operations on FILE*s. However, you still need to not do stupid things like calling `fclose(fs)` in one thread followed by an access of `fs` in another.)

So, if your platform's C library is threadsafe, then your `fstream` I/O operations will be threadsafe at the lowest level. For higher-level operations, such as manipulating the data contained in the stream formatting classes (e.g., setting up callbacks inside an `std::ofstream`), you need to guard such accesses like any other critical shared resource.

#### Future

A second choice may be available for I/O implementations: libio. This is disabled by default, and in fact will not currently work due to other issues. It will be revisited, however.

The libio code is a subset of the guts of the GNU libc (glibc) I/O implementation. When libio is in use, the `__basic_file` type is basically derived from FILE. (The real situation is more complex than that... it's derived from an internal type used to implement FILE. See libio/libioP.h to see scary things done with vtbls.) The result is that there is no "layer" of C stdio to go through; the filebuf makes calls directly into the same functions used to implement `fread`, `fwrite`, and so forth, using internal data structures. (And when I say "makes calls directly," I mean the function is literally replaced by a jump into an internal function. Fast but frightening. *grin*)

Also, the libio internal locks are used. This requires pulling in large chunks of glibc, such as a pthreads implementation, and is one of the issues preventing widespread use of libio as the libstdc++ cstdio implementation.

But we plan to make this work, at least as an option if not a future default. Platforms running a copy of glibc with a recent-enough version will see calls from libstdc++ directly into the glibc already installed. For other platforms, a copy of the libio subsection will be built and included in libstdc++.

#### Alternatives

Don't forget that other cstdio implementations are possible. You could easily write one to perform your own forms of locking, to solve your "interesting" problems.

### Containers

This section discusses issues surrounding the design of multithreaded applications which use Standard C++ containers. All information in this section is current as of the gcc 3.0 release and all later point releases. Although earlier gcc releases had a different approach to threading configuration and proper compilation, the basic code design rules presented here were similar. For information on all other aspects of multithreading as it relates to libstdc++, including details on the proper compilation of threaded code (and compatibility between threaded and non-threaded code), see Chapter 17.

Two excellent pages to read when working with the Standard C++ containers and threads are SGI's http://www.sgi.com/tech/stl/thread_safety.html and SGI's http://www.sgi.com/tech/stl/Allocators.html.

However, please ignore all discussions about the user-level configuration of the lock implementation inside the STL container-memory allocator on those pages. For the sake of this discussion, libstdc++ configures the SGI STL implementation, not you. This is quite different from how gcc pre-3.0 worked. In particular, past advice was for people using g++ to explicitly define _PTHREADS or other macros or port-specific compilation options on the command line to get a thread-safe STL. This is no longer required for any port and should no longer be done unless you really know what you are doing and assume all responsibility.

Since the container implementation of libstdc++ uses the SGI code, we use the same definition of thread safety as SGI when discussing design. A key point that beginners may miss is the fourth major paragraph of the first page mentioned above ("For most clients,"...), which points out that locking must nearly always be done outside the container, by client code (that'd be you, not us). There is a notable exceptions to this rule. Allocators called while a container or element is constructed uses an internal lock obtained and released solely within libstdc++ code (in fact, this is the reason STL requires any knowledge of the thread configuration).

For implementing a container which does its own locking, it is trivial to provide a wrapper class which obtains the lock (as SGI suggests), performs the container operation, and then releases the lock. This could be templatized to a certain extent, on the underlying container and/or a locking mechanism. Trying to provide a catch-all general template solution would probably be more trouble than it's worth.

The STL implementation is currently configured to use the high-speed caching memory allocator. Some people like to test and/or normally run threaded programs with a different default. For all details about how to globally override this at application run-time see here.

There is a better way (not standardized yet): It is possible to force the malloc-based allocator on a per-case-basis for some application code. The library team generally believes that this is a better way to tune an application for high-speed using this implementation of the STL. There is more information on allocators here.

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