System News
"Eliminating Nonre-entrant Library Calls in Multithreaded Programs"
by Bruce Chapman
March 11, 2002,
Volume 49, Issue 2

Nonre-entrant library calls are frequently made from multithreaded programs. The potential for problems is often not caught until the code is placed under heavy load on a multiprocessor machine. To make matters worse, calls such as ctime(3C) can corrupt the process heap and cause crashes in unrelated areas. In his Solaris Developer ConnectionSM Program technical article "Eliminating nonre-entrant Libary Calls in Multithreaded Programs," Bruce Chapman provides a list of APIs to avoid, as well as a means of checking to make sure code is not harboring dangerous, nonre-entrant library calls.

With the SolarisTM Operating Environment (Solaris OE), to avoid making the mistake of using nonre-entrant library calls, you must look at the man page for every library call a code makes. If a re-entrant <name>_r version of the call exists, use it; according to Chapman, that is the only safe call to make in a multithreaded (MT) program. However, again according to Chapman, not all engineers have the discipline to make this approach effective.

This kind of insidious bug can creep into MT code in many other ways as well. Chapman provides illustrations of these and other bugs in the article.

The program in Chapman's article was run with up to 1600 threads on a single-processor machine with no problems. It was then run with 100 threads on a 4-CPU machine and also ran fine. Only when run with 200 threads on a 4-CPU machine did the nonre-entrant call to ctime() finally cause a crash. Since what ctime() has done is corrupt the C heap, the crash could have occurred elsewhere in a program that manipulated the C heap on its own.

The tedious way to eliminate this type of problem is to perform source code analysis by referring back to library call man pages for the Solaris OE. Another approach is to use Solaris OE software tools to look at all the libraries a binary uses. This can be done either statically or with a running process. However, in the early stages of code development and testing, these problems may not yet have manifested themselves.

Chapman illustrates some basic pre-deployment checks that can be made online, along with the source code for a simple library, multithreaded_nonreentrant.c, that can interpose all the nonre-entrant library calls for the Solaris OE that have re-entrant equivalents.

Examples of code and a list of nonre-entrant standard library calls to avoid with the Solaris 8 OE in a multithreaded program are available online:

http://soldc.sun.com/articles/multithreaded.html

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Other articles in the Developer section of Volume 49, Issue 2:

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