What can I safely assume about the initial values of variables which are not explicitly initialized? If global variables start out as ``zero,'' is that good enough for null pointers and floating-point zeroes?
Variables with static duration (that is, those declared outside of functions, and those declared with the storage class static), are guaranteed initialized (just once, at program startup) to zero, as if the programmer had typed ``= 0''. Therefore, such variables are initialized to the null pointer (of the correct type; see also section 5) if they are pointers, and to 0.0 if they are floating-point.
Variables with automatic duration (i.e. local variables without the static storage class) start out containing garbage, unless they are explicitly initialized. (Nothing useful can be predicted about the garbage.)
Dynamically-allocated memory obtained with malloc and realloc is also likely to contain garbage, and must be initialized by the calling program, as appropriate. Memory obtained with calloc is all-bits-0, but this is not necessarily useful for pointer or floating-point values (see question 7.31, and section 5).
References:
K&R1 Sec. 4.9 pp. 82-4
K&R2 Sec. 4.9 pp. 85-86
ANSI Sec. 3.5.7, Sec. 4.10.3.1, Sec. 4.10.5.3
ISO Sec. 6.5.7, Sec. 7.10.3.1, Sec. 7.10.5.3
H&S Sec. 4.2.8 pp. 72-3, Sec. 4.6 pp. 92-3, Sec. 4.6.2 pp. 94-5, Sec. 4.6.3 p. 96, Sec. 16.1 p. 386
Read sequentially: prev next up top
This page by Steve Summit // Copyright 1995 // mail feedback