Some constructs cause gcc to warn as always causing a buffer overflow incorrectly. For example, this is a minimalistic version of a warning found in wine-1.3.2:

----------
#include <string.h>
#include <stdlib.h>
#include <stdint.h>

struct T {
	union {
		struct {
			char str[1];
		} x;
	} u;
};

int main() {
	struct T *p = malloc(sizeof(char) * 100);
	strcpy(p->u.x.str, "ABCD");
	return 0;
}
----------

This is a slightly obfuscated version of the struct hack and is clearly not a buffer overflow. Yet compiling with: "gcc -O2 test.c -o test" results in:



In file included from /usr/include/string.h:640:0,
                 from test.c:2:
In function 'strcpy',
    inlined from 'main' at test.c:16:8:
/usr/include/bits/string3.h:107:3: warning: call to __builtin___strcpy_chk will always overflow destination buffer