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20000926: xsatim.c compiler warning


Scott and Steve,

The OSF/1 C compiler provides the following warning message when compiling
$GEMPAK/source/driver/active/xw/xsatim.c:

cc: Warning: xsatim.c, line 414: In this statement, the expression
"*imgptr++=*(imgptr-xwdth)" 
modifies "imgptr", and fetches its value in a computation that is not used 
to produce the modified value without an intervening sequence point.  
This behavior is undefined. (undefvarfetch)
                *imgptr++ =  *(imgptr - xwdth);

I can't attribute any problems to this message, however, the code may be
brittle on some other compilers with optimization flags.

This made me go back to some conversations I had with Steve Drake and Dave
Himes at COMET a while back regarding some HPUX problems in a similar routine.
Dave Himes had suggested as an alternative to:

       else if ( (row != 0) && (rowtrans[row] == rowtrans[row -1]) ){
            /*
             * replicate line
             */
            for ( col = 0; col < xwdth; col++) {
                *imgptr++ =  *(imgptr - xwdth);
            }
            continue; /* finish the row */
        }


Use memcpy:

        else if ( (row != 0) && (rowtrans[row] == rowtrans[row -1]) ){
            /*
             * replicate line
             */
             memcpy(imgptr, imgptr - xwdth, xwdth);
             imgptr += xwdth;
             continue; /* next row */
        }



Steve Chiswell
Unidata User Support