3.7 Example 6

  5336 geterror (abp)
       struct buf *abp;
       {
         register struct buf bp;
         bp = abp;
         if (bp->b flags & B_ERROR)
           if ((u.u_error=bp->b_error)==0)
             u.u_error = EIO;
       }

This procedure simply checks if there has been an error, and if the error indicator “u.u_error” has not been set, sets it to a general error indication

“B_ERROR” has the value 04 (see line 4575) so that, with only one bit set, it can be used as mask to isolate bit number 2. The operator “&” as used in

    bp->b_flags & B_ERROR

is the bitwise logical conjunction (“and”) applied to arithmetic values.

The above expression is greater than one if bit 2 of the element “b_flags” of the “buf” structure pointed to by “bp”, is set.

Thus if there has been an error, the expression

    (u.u_error) = bp->b_error)

is evaluated and compared with zero. Now this expression includes an assignment operator “=”. The value of the expression is the value of “u.u_error” after the value of “bp-b_flags” has been assigned to it.

This use of an assignment as part of an expression is useful and quite common.