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C struct data types may end with a flexible array member [1] with no specified size: struct vectord { short len ; // there must be at least one other data member double arr []; // the flexible array member must be last // The compiler may reserve extra padding space here, like it can between struct members };
In the C programming language, struct is the keyword used to define a composite, a.k.a. record, data type – a named set of values that occupy a block of memory. It allows for the different values to be accessed via a single identifier, often a pointer.
Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on C dynamic memory allocation for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a null ...
C99 adds support for flexible array members to structures. This form of array declaration is allowed as the last element in structures only, and differs from normal arrays in that no length is specified to the compiler. For a structure named s containing a flexible array member named a, sizeof s is therefore equivalent to offsetof (s, a):
This diagram represents five contiguous memory regions which each hold a pointer and a data block. The List Head points to the 2nd element, which points to the 5th, which points to the 3rd, thereby forming a linked list of available memory regions. A free list (or freelist) is a data structure used in a scheme for dynamic memory allocation.
For every type T, except void and function types, there exist the types "array of N elements of type T". An array is a collection of values, all of the same type, stored contiguously in memory. An array of size N is indexed by integers from 0 up to and including N−1. Here is a brief example:
This order is usually determined by the order in which the elements are added to the structure, but the elements can be rearranged in some contexts, such as sorting a list. For a structure that isn't ordered, on the other hand, no assumptions can be made about the ordering of the elements (although a physical implementation of these data types ...
C – an undefined value always syntactically valid, EMPTY – the only value admissible to VOID, needed for selecting VOID in a UNION, VOID – syntactically like a MODE, but not one, NIL or " " – a name not denoting anything, of an unspecified reference mode, () or specifically [1:0]INT – a vacuum is an empty array (here specifically of ...