Duplication Of Side Effects - Red Hat ENTERPRISE LINUX 3 - USING CPP Using Instructions

Chapter 3. Macros
if (*p != 0)
SKIP_SPACES (p, lim);
else ...
The presence of two statements--the compound statement and a null statement--in between the
condition and the
else
The definition of the macro
statement. Here is how:
while
#define SKIP_SPACES(p, limit)
do { char *lim = (limit);
while (p
if (*p++ != ' ') {
p--; break; }}}
while (0)
Now
SKIP_SPACES (p, lim);
do {...} while (0);
which is one statement. The loop executes exactly once; most compilers generate no extra code for it.

3.9.4. Duplication of Side Effects

Many C programs define a macro
#define min(X, Y)
When you use this macro with an argument containing a side effect, as shown here,
next = min (x + y, foo (z));
it expands as follows:
next = ((x + y)
where has been substituted for
x + y
The function is used only once in the statement as it appears in the program, but the expression
foo
has been substituted twice into the macro expansion. As a result,
foo (z)
times when the statement is executed. If it has side effects or if it takes a long time to compute, the
results might not be what you intended. We say that
The best solution to this problem is to define
once. The C language offers no standard way to do this, but it can be done with GNU extensions as
follows:
makes invalid C code.
SKIP_SPACES
lim) {
expands into
, for "minimum", like this:
min
((X) (Y) ? (X) : (Y))
(foo (z)) ? (x + y) : (foo (z)));
and
X
can be altered to solve this problem, using a
\
\
\
\
\
for .
foo (z) Y
is an unsafe macro.
min
in a way that computes the value of
min
do ...
might be called two
foo
only
foo (z)
29
if