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Home/Magazine Archive/April 2017 (Vol. 60, No. 4)/Uninitialized Reads/Abstract

Practice

Uninitialized Reads

By Robert C. Seacord
Communications of the ACM, April 2017, Vol. 60 No. 4, Pages 40-44
10.1145/3024920
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Uninitialized Reads, illustration — Credit: Moviestore / Rex / Shutterstock

Most developers understand that reading uninitialized variables in C is a defect, but some do it anyway—for example, to create entropy. What happens when you read uninitialized objects is unsettled in the current version of the C standard (C11).³ Various proposals have been made to resolve these issues in the planned C2X revision of the standard. Consequently, this is a good time to understand existing behaviors as well as proposed revisions to the standard to influence the evolution of the C language. Given the behavior of uninitialized reads is unsettled in C11, prudence dictates eliminating uninitialized reads from your code.

This article describes object initialization, indeterminate values, and trap representations and then examines sample programs that illustrate the effects of these concepts on program behavior.

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