Description
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(25 pts) The Boolean type is typically represented on a byte in memory, although it can have only two possible values. Why not represent the Boolean type on a single bit?
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(25 pts) Write a small fragment of code that shows how unions can be used in C to interpret the bits of a value of one type as if they represented a value of some other type (non-converting type cast).
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(25 pts) Consider the following:
typedef struct
{
int x;
char y;
} Rec1;
typedef Rec1 Rec2;
typedef struct
{
int x;
char y;
} Rec3;
Rec1
Rec2
Rec3
a, b;
c;
d;
Specify which of the variables a,b,c,d are type equivalent under (a) structural equivalence, (b) strict name equivalence, and (c) loose name equivalence.
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(25 pts) Consider the following program, written in C:
typedef struct
{
int x; int y;
} Foo;
void allocate_node (Foo * f)
{
f = (Foo *) malloc ( sizeof(Foo) );
}
void main ()
{
Foo * p;
allocate_node (p);
p->x = 2;
p->y = 3;
free(p);
}
Although the program compiles, it produces a run-time error. Why?
Rewrite the two functions allocate_node and main so that the program runs correctly.
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(Extra Credit – 10 pts) A compiler for a language with static scoping typically uses a LeBlanc-Cook symbol table to track the bindings of various names encountered in a program. Describe the mechanism that should be used by a compiler to ensure that the names of record fields used inside a with statement are bound to the correct objects. Specify what should be placed in (a) the symbol table, (b) the scope stack, and (c) the run-time program stack.
