That snippet invokes undefined behavior at runtime (violates C++ aliasing
rules),
so just fix it, rather than bother with bugreports. E.g. look for
-fstrict-aliasing
in GCC documentation, or read the C++ standard. With -fno-strict-aliasing,
which is a workaround for broken code you won't get any warnings about
uninitialized uses.
Jakub
Yes this code has problems with aliasing. But anyway warning messages
are extremely misleading.
And I found that that a version which I beleve mustn't have aliasing
problems still generates same warnings.
The code:
static uint32_t calc_16bit_checksum_part(uint8_t* buf, int len,
uint32_t ret) {
struct ui16{
uint16_t d;
};
ui16 *ptr = (ui16*)buf;
for( int i = 0; i < (len / 2); ++i) {
ret += ptr[i].d;
}
if ( len%2) {
ret += buf[len-1];
}
return ret;
}
static uint32_t calc_ch_udp_pseudo(uint32_t src, uint32_t dst, uint16_t
len) {
struct udp_pseudo {
uint32_t src;
uint32_t dst;
uint8_t z;
uint8_t proto;
uint16_t len;
} tmp;
tmp.src = src;
tmp.dst = dst;
tmp.z = 0;
tmp.proto = UDP_PROTO_NUMBER;
tmp.len = len;
auto ret = calc_16bit_checksum_part((uint8_t*)&tmp,
sizeof(tmp), 0);
return ret;
}
and messages
static uint16_t calc_16bit_checksum(uint8_t* buf, int len) {
^~~~~~~~~~~~~~~~~~~
snippet.cc: In function ‘int main()’:
snippet.cc:66:31: warning: ‘tmp.calc_16bit_checksum_part(uint8_t*, int,
uint32_t)::ui16::d’ is used uninitialized in this function [-Wuninitialized]
ret += ptr[i].d;
~~~~~~~^
snippet.cc:66:31: warning: ‘*((void*)(&
tmp)+2).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(&
tmp)+4).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(&
tmp)+6).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(&
tmp)+8).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(&
tmp)+10).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is
used uninitialized in this function [-Wuninitialized]
[
ui16 is an aggregate type and uint8_t is unsigned char, so there will be
no undefined behaviour.
Kirill.
