2. Structures, Unions, and Enumerations

2.1 What's the difference between these two declarations?

	struct x1 { ... };
	typedef struct { ... } x2;

2.2 Why doesn't

struct x { ... };
x thestruct;

2.3 Can a structure contain a pointer to itself?

2.4 How can I implement opaque (abstract) data types in C?

2.4b Is there a good way of simulating OOP-style inheritance, or other OOP features, in C?

2.5 Why does the declaration

extern int f(struct x *p);

2.6 I came across some code that declared a structure like this:

struct name {
	int namelen;
	char namestr[1];
};

2.7 I heard that structures could be assigned to variables and passed to and from functions, but K&R1 says not.

2.8 Is there a way to compare structures automatically?

2.9 How are structure passing and returning implemented?

2.10 How can I pass constant values to functions which accept structure arguments? How can I create nameless, immediate, constant structure values?

2.11 How can I read/write structures from/to data files?

2.12 Why is my compiler leaving holes in structures, wasting space and preventing ``binary'' I/O to external data files? Can I turn this off, or otherwise control the alignment of structure fields?

2.13 Why does sizeof report a larger size than I expect for a structure type, as if there were padding at the end?

2.14 How can I determine the byte offset of a field within a structure?

2.15 How can I access structure fields by name at run time?

2.16 Does C have an equivalent to Pascal's with statement?

2.17 If an array name acts like a pointer to the base of an array, why isn't the same thing true of a structure?

2.18 This program works correctly, but it dumps core after it finishes. Why?

	struct list {
		char *item;
		struct list *next;
	}

	/* Here is the main program. */

	main(argc, argv)
	{ ... }

2.19 What's the difference between a structure and a union, anyway?

2.20 Can I initialize unions?

2.21 Is there an automatic way to keep track of which field of a union is in use?

2.22 What's the difference between an enumeration and a set of preprocessor #defines?

2.23 Are enumerations really portable?
Aren't they Pascalish?

2.24 Is there an easy way to print enumeration values symbolically?

2.25 I came across some structure declarations with colons and numbers next to certain fields, like this:

struct record {
	char *name;
	int refcount : 4;
	unsigned dirty : 1;
};

2.26 Why do people use explicit masks and bit-twiddling code so much, instead of declaring bit-fields?

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