Re: Strict aliasing errors

Hi Orion,
I have created ticket (NCL-1515 for reference) for this issue. Thanks for the report.
 -dave

On Jul 13, 2012, at 10:13 AM, Orion Poplawski wrote:

> Compiling ncl 6.0.0 on Fedora yields the following strict-aliasing warnings,
> which can lead to unexpected code behavior:
>
> NclCCM.c:312:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:320:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:326:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:347:2: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:351:2: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:724:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:725:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:729:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:312:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:320:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:326:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:347:2: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:351:2: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:724:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:725:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> NclCCM.c:729:3: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
>
> comes from (int *) casts:
> static int extractCCM(int which,char buf[8])
> {
> char tmp[8];
>
> switch(which){
> case 0:
> return((int)buf[0]>>4);
> case 1:
> tmp[BYTE0] = 0;
> tmp[BYTE1] = (buf[0]<<4)>>4;
> tmp[BYTE2] = buf[1];
> tmp[BYTE3] = buf[2];
> return(*(int*)tmp);
> case 2:
> return((int)buf[3]>>7);
> case 3:
> tmp[BYTE0] = 0;
> tmp[BYTE1] = buf[4]>>1;
> tmp[BYTE2] = buf[5]>>1 | (buf[4]&(char)0001)<<7;
> tmp[BYTE3] = buf[6]>>1 | (buf[5]&(char)0001)<<7;
> return(*(int*)tmp);
> case 4:
> tmp[BYTE0] = 0;
> tmp[BYTE1] = 0;
> tmp[BYTE2] = buf[6] & (char)0001;
> tmp[BYTE3] = buf[7];
> return(*(int*)tmp);
> }
>
>
> BuiltInFuncs.c:3110:4: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> BuiltInFuncs.c:3119:4: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> BuiltInFuncs.c:3263:5: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> BuiltInFuncs.c:3275:5: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> BuiltInFuncs.c:3464:4: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> BuiltInFuncs.c:3480:4: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> BuiltInFuncs.c:3501:4: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
>
> From statements like:
> ind1 = *(int*)control_word;
>
> craybin.c:112:17: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> craybin.c:120:17: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
> craybin.c:126:17: warning: dereferencing type-punned pointer will break
> strict-aliasing rules [-Wstrict-aliasing]
>
> Comes from the (int*) casts:
> static int extract(int which,char buf[8])
> {
> char tmp[8];
>
> switch(which){
> case 0:
> return((int)buf[0]>>4);
> case 1:
> tmp[BYTE0] = 0;
> tmp[BYTE1] = (buf[0]<<4)>>4;
> tmp[BYTE2] = buf[1];
> tmp[BYTE3] = buf[2];
> return(*(int*)tmp);
> case 2:
> return((int)buf[3]>>7);
> case 3:
> tmp[BYTE0] = 0;
> tmp[BYTE1] = buf[4]>>1;
> tmp[BYTE2] = buf[5]>>1 | (buf[4]&(char)0001)<<7;
> tmp[BYTE3] = buf[6]>>1 | (buf[5]&(char)0001)<<7;
> return(*(int*)tmp);
> case 4:
> tmp[BYTE0] = 0;
> tmp[BYTE1] = 0;
> tmp[BYTE2] = buf[6] & (char)0001;
> tmp[BYTE3] = buf[7];
> return(*(int*)tmp);
> }
> return 0;
> }
>
> These seem to arise from trying to convert arrays of character's into integers
> by casting them from (char *) to (int *), which while seemingly
> straightforward does not conform to modern C standard requirements so there is
> really no guarantee that it will do what you expect. Probably better to
> construct the integer explicitly from the component bytes.
>
> --
> Orion Poplawski
> Technical Manager 303-415-9701 x222
> NWRA, Boulder Office FAX: 303-415-9702
> 3380 Mitchell Lane orion@nwra.com
> Boulder, CO 80301 http://www.nwra.com
>
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Received on Fri Jul 13 14:32:51 2012