Building Multithreaded Programs

All libraries shipped with the C++ compiler are multithreading-safe. If you want to build a multithreaded application, or if you want to link your application to a multithreaded library, you must compile and link your program with the -mt option. This option passes -D_REENTRANT to the preprocessor and passes -lthread in the correct order to ld. For -compat=4, the -mt option ensures that libthread is linked before libC. You should not link your application directly with -lthread as this causes libthread to be linked in an incorrect order.

The following example shows the correct way of building a multithreaded application:

demo% CC -c -mt myprog.cc
demo% CC -mt myprog.o

The following example shows the wrong way of building a multithreaded application:

demo% CC -c -mt myprog.o
demo% CC myprog.o -lthread

Indicating Multithreaded Compilation

You can check whether an application is linked to libthread or not by using the ldd command:

demo% CC -mt myprog.cc
demo% ldd a.out
libm.so.1 =>      /usr/lib/libm.so.1
libCrun.so.1 =>   /usr/lib/libCrun.so.1
libw.so.1 =>      /usr/lib/libw.so.1
libthread.so.1 => /usr/lib/libthread.so.1
libc.so.1 =>      /usr/lib/libc.so.1
libdl.so.1 =>     /usr/lib/libdl.so.1

Using libC With Threads and Signals

The C++ support libraries, libCrun, libiostream, libCstd, and libC are multithread safe but are not async safe. This means that in a multithreaded application, functions available in the support libraries should not be used in signal handlers. Doing so can result in a deadlock situation.

It is not safe to use the following in a signal handler in a multithreaded application:

• Iostreams

• new and delete

• Exceptions