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union example program in C

By: Grenfel in C Tutorials on 2007年09月26日 [フレーム]

A union is a variable that may hold (at different times) objects of different types and sizes, with the compiler keeping track of size and alignment requirements. Unions provide a way to manipulate different kinds of data in a single area of storage, without embedding any machine-dependent information in the program. They are analogous to variant records in pascal.

As an example such as might be found in a compiler symbol table manager, suppose that a constant may be an int, a float, or a character pointer. The value of a particular constant must be stored in a variable of the proper type, yet it is most convenient for table management if the value occupies the same amount of storage and is stored in the same place regardless of its type. This is the purpose of a union - a single variable that can legitimately hold any of one of several types. The syntax is based on structures:

 union u_tag {
 int ival;
 float fval;
 char *sval;
 } u;

The variable u will be large enough to hold the largest of the three types; the specific size is implementation-dependent. Any of these types may be assigned to u and then used in expressions, so long as the usage is consistent: the type retrieved must be the type most recently stored. It is the programmer's responsibility to keep track of which type is currently stored in a union; the results are implementation-dependent if something is stored as one type and extracted as another.

Syntactically, members of a union are accessed as

union-name.member

union-pointer->member

just as for structures. If the variable utype is used to keep track of the current type stored in u, then one might see code such as

 if (utype == INT)
 printf("%d\n", u.ival);
 if (utype == FLOAT)
 printf("%f\n", u.fval);
 if (utype == STRING)
 printf("%s\n", u.sval);
 else
 printf("bad type %d in utype\n", utype);

Unions may occur within structures and arrays, and vice versa. The notation for accessing a member of a union in a structure (or vice versa) is identical to that for nested structures. For example, in the structure array defined by

 struct {
 char *name;
 int flags;
 int utype;
 union {
 int ival;
 float fval;
 char *sval;
 } u;
 } symtab[NSYM];

the member ival is referred to as

 symtab[i].u.ival

and the first character of the string sval by either of

 *symtab[i].u.sval
 symtab[i].u.sval[0]

In effect, a union is a structure in which all members have offset zero from the base, the structure is big enough to hold the ``widest'' member, and the alignment is appropriate for all of the types in the union. The same operations are permitted on unions as on structures: assignment to or copying as a unit, taking the address, and accessing a member.

A union may only be initialized with a value of the type of its first member; thus union u described above can only be initialized with an integer value.

The storage allocator shows how a union can be used to force a variable to be aligned on a particular kind of storage boundary.

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