An lvalue was defined as an expression to which a value can be assigned. It was also explained
that an lvalue appears on the left side of an assignment statement. Therefore, an rvalue can be defined as an
expression that can be assigned to an lvalue. The rvalue appears on the right side of an assignment statement.
Unlike an lvalue, an rvalue can be a constant or an expression, as shown here:
int x, y;
x = 1; /* 1 is an rvalue; x is an lvalue */
y = (x + 1); /* (x + 1) is an rvalue; y is an lvalue */
As stated in FAQ I.9, an assignment statement must have both an lvalue and an rvalue. Therefore, the
following statement would not compile because it is missing an rvalue:
int x;
x = void_function_call() /* the function void_function_call()
returns nothing */
If the function had returned an integer, it would be considered an rvalue because it evaluates into something
that the lvalue, x, can store.
Is left-to-right or right-to-left order guaranteed for
operator precedence?
The simple answer to this question is neither. The C language does not always evaluate left-to-right or rightto-
left. Generally, function calls are evaluated first, followed by complex expressions and then simpleexpressions. Additionally, most of today’s popular C compilers often rearrange the order in which the
expression is evaluated in order to get better optimized code. You therefore should always implicitly define
your operator precedence by using parentheses.
For example, consider the following expression:
a = b + c/d / function_call() * 5
The way this expression is to be evaluated is totally ambiguous, and you probably will not get the results you
want. Instead, try writing it by using implicit operator precedence:
a = b + (((c/d) / function_call()) * 5)
Using this method, you can be assured that your expression will be evaluated properly and that the compiler
will not rearrange operators for optimization purposes.
