Limitations

In most cases, thread safety does not imply that the same object can be shared between threads. In particular, when objects have user visible state, it would not make sense to share them between threads. Consider the following:

void f(ostream &out, int x, int y) {

out << setw(3) << x << setw(10) << y;

}

This function would not be thread safe if called from multiple threads with the same object, since the width in the shared object could be changed at any time. Therefore, such objects are not protected from interactions between multiple threads, and the result of sharing such an object between threads is undefined.

If the same object is shared between threads, a runtime crash, abort, or intermingled output may occur. With the Rogue Wave Standard C++ Library 2.2.1, output may be intermingled but no aborts will occur.

Using -D_THREAD_SAFE with the cfront Compatible libstream

There is an exception to the above rule for the cfront compatible libstream. For the frequently used objects cout, cin, cerr, and clog, you can specify the -D_THREAD_SAFEcompile time flag for any file that includes <iostream.h>. In this case, a new instance of the object is transparently created for each thread that uses it. All instances share the same file descriptor. The f function in the above example will now work, because it receives one new out object per thread. However, the results of two simultaneous executions of f will be mixed in any order in the output.

Using -D_THREAD_SAFEwith the global scope operator is not supported for cout, cin, cerr, and clog. For example, the following code would generate an error:

::cout << endl;

NOTE: If you use locks, you need not use the -D_THREAD_SAFEcompile time flag since you are now responsible for ensuring thread safety.

Differences between Standard iostreams and cfront Compatible libstream

The cfront compatible libstream supports locking for each insertion. Rogue Wave Standard C++ Library 1.2.1 and Tools.h++ 7.0.6 do not support locking but do provide a thread private buffer.

Visible differences would be as follows. In the case of standard iostreams, there is intermingling of each component being inserted. With cfront compatible iostreams, there is intermingling of complete buffers (depending on when endl or flush is called).

Using -D__HPACC_THREAD_SAFE_RB_TREE

The Rogue Wave Standard C++ Library 1.2.1 (libstd) and Tools.h++ 7.0.6 (librwtool) are not thread safe if the underlying implementation rb_tree class is involved. In other words, if the tree header file (which includes tree.cc) under /opt/aCC/include/ is used, these libraries are not thread safe. Most likely, it is indirectly referenced by including the standard C++ library container class map or set headers, or by including a RogueWave tools.h++ header like tvset.h, tpmset.h, tpmset.h, tvset.h, tvmset.h, tvmset.h, tpmap.h, tpmmap.h, tpmmap.h, tvmap.h, and tvmmap.h. Since changing the rb_tree implementation to make it thread safe would break binary compatibility, the preprocessing macro, __HPACC_THREAD_SAFE_RB_TREE,

must be defined. The macro is automatically defined in the Itanium® based environment. A new object file compiled with the macro defined should not be linked with older ones that were compiled without the macro defined. Library providers whose library is built with the macro defined may need to notify their users to also compile their source with the macro defined when the tree header is included.

Using Threads 165