http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47932
Summary: __typeof__ prevents VLA from being evaluated
Product: gcc
Version: 4.4.4
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: [email protected]
ReportedBy: [email protected]
The program below shows that the __typeof__ macro prevents the evaluation of
the variable-length array in a context where one would expect it to be
evaluated according to the documentation:
The operand of typeof is evaluated for its side effects if and only if it
is an expression of variably modified type or the name of such a type.
$ cat <<EOF | gcc -xc - && ./a.out
extern int printf(const char*, ...);
#define A(a) (sizeof (a) / sizeof ((a)[0]))
#define B(a) (sizeof (a) / sizeof (__typeof__((a)[0])))
int i;
int f() { printf("%s\n", __func__); return i++; }
void g(int n) {
int a[n][n][n];
i = 0;
printf("%zu\n", A (a[f()]));
printf("i = %d\n", i);
i = 0;
printf("%zu\n", B (a[f()]));
printf("i = %d\n", i);
}
int main () {
g(3);
}
EOF
f
f
3
i = 2
f
3
i = 1
