Re: C Interpreter and sizeof operator

ozbear wrote: 
If one were writing a C interpreter, is there anything in the standard
standard that requires the sizeof operator to yield the same value for
two different variables of the same type?

Yes. (I know this because I looked up the exact same thing lately.) If x and y have the same type, sizeof x = sizeof y must hold without exception.

6.5.3.4: "The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant."

Note that the *value* of the operand is irrelevant. Only its *type* matters. Therefore the size of an object may *not* depend on its value, but only on its type. If x and y are of type 'int', then sizeof x = sizeof(int) and sizeof y = sizeof(int), therefore sizeof x = sizeof y. Bad Things happen if you violate this.

Let's assume that the interpreter does conform to the range values
for, say, type int, but allocates storage for the variables based
on their value.  So, for two variables foo and bar

  int foo = 0;          /* interpreter allocates two bytes */
  int bar = 200000000;  /* interpreter allocates four bytes */

Does the standard require that sizeof foo  == sizeof bar  thereby
making this allocation scheme broken, unless hidden in some way?


Yes.

You can try and fudge this in cases where the application cannot possibly trip over the wrong size, but it's tricky to do this without violating the standard, and in an interpreter it's very unlikely to be of any value. Just pick a constant size for integers (4 bytes happens to be a very common one).

Or is it perfectly acceptable for the sizeof operator to different
results?

No, it's not.

S.
.

Relevant Pages

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