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comp.lang.c FAQ list · Question 1.30

Q: What am I allowed to assume about the initial values of variables and arrays which are not explicitly initialized?
If global variables start out as ``zero'', is that good enough for null pointers and floating-point zeroes?

A: Uninitialized variables with static duration (that is, those declared outside of functions, and those declared with the storage class static), are guaranteed to start out as zero, just as if the programmer had typed ``= 0'' or ``= {0}''. Therefore, such variables are implicitly 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. [footnote]

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.) If they do have initializers, they are initialized each time the function is called (or, for variables local to inner blocks, each time the block is entered at the top[footnote] ).

These rules do apply to arrays and structures (termed aggregates); arrays and structures are considered ``variables'' as far as initialization is concerned. When an automatic array or structure has a partial initializer, the remainder is initialized to 0, just as for statics. [footnote] See also question 1.31.

Finally, dynamically-allocated memory obtained with malloc and realloc is 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
ISO Sec. 6.5.7, Sec., Sec.
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

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