Re: [PATCH] C/C++: Add -Waddress-of-packed-member

[Martin Sebor]

On 01/14/2018 07:29 AM, H.J. Lu wrote:

When address of packed member of struct or union is taken, it may result
in an unaligned pointer value.  This patch adds -Waddress-of-packed-member
to warn it:

$ cat x.i
struct pair_t
{
  char c;
  int i;
} __attribute__ ((packed));

extern struct pair_t p;
int *addr = &p.i;
$ gcc -O2 -S x.i
x.i:8:13: warning: initialization of 'int *' from address of packed member of 
'struct pair_t' may result in an unaligned pointer value 
[-Waddress-of-packed-member]
 int *addr = &p.i;
             ^
$

This warning is enabled by default.

I like this enhancement.  It would be useful for data types,
packed or not, such as casting int* to long*.

I noticed some differences from Clang for the test case below.
It seems that GCC should warn on all the cases Clang does.

Also, since converting the address of a struct to that of its
first member is common (especially in C and when the member
itself is a struct) I wonder if the warning should trigger
for those conversions as well.

struct A {
  int i;
} __attribute__ ((packed));

long* f8 (struct A *p) { return &p->i; }   // Clang only
int* f4 (struct A *p) { return &p->i; }    // Clang, GCC
short* f2 (struct A *p) { return &p->i; }  // Clang only
char* f1 (struct A *p) { return &p->i; }
void* f0 (struct A *p) { return &p->i; }

struct B { int i; };
struct C { struct B b; } __attribute__ ((packed));

long* g8 (struct C *p) { return p; }        // should warn?
int* g4 (struct C *p) { return &p->b; }     // Clang only

int* h4 (struct C *p) { return &p->b.i; }   // Clang only


Martin