Modifying a variable through a pointer of an incompatible type can lead to unpredictable results. This is often caused by a violation of aliasing rules. C99C11, Section 6.5, paragraph 7 [ISO/IEC 9899:1999] specifies those circumstances in which an object may or may not be aliased.
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union a_union {
int i;
double d;
};
int f() {
a_union t;
int *ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
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union a_union {
int i;
double d;
};
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
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union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
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short a[2];
a[0]=0x1111;
a[1]=0x1111;
*(int *)a = 0x22222222; /* violation of aliasing rules */
printf("%x %x\n", a[0], a[1]);
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union {
short a[2];
int i;
} u;
u.a[0]=0x1111;
u.a[1]=0x1111;
u.i = 0x22222222;
printf("%x %x\n", u.a[0], u.a[1]);
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