We have tested the reliability of a large collection of basic UNIX utility programs, X-Window applications and servers, and network services. We used a simple testing method of subjecting these programs to a random input stream. Our testing methods and tools are largely automatic and simple to use. We tested programs on nine versions of the UNIX operating system, including seven commercial systems and the freely-available GNU utilities and Linux. We report which programs failed on which systems, and identify and categorize the causes of these failures. The result of our testing is that we can crash (with core dump) or hang (infinite loop) over 40% (in the worst case) of the basic programs and over 25% of the X-Window applications. We were not able to crash any of the network services that we tested nor any of X-Window servers. This study parallels our 1990 study (that tested only the basic UNIX utilities); all systems that we compared between 1990 and 1995 noticeably improved..
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