An SBW message indicates that your program is about to write across stack frame boundaries. This is similar to an ABW, but concerns a local variable instead of a malloc'd block.
An SBW error can be caused by any of the following:
Making an automatic array too small, for example failing to account for the terminating NULL in a string
Forgetting to multiply by sizeof(type) when allocating for an array of objects
Using an array index that is too large or negative
Failing to NULL terminate a string
Being off by one in copying elements up or down an array
On SPARC systems, Purify also detects access across stack frames. Purify does not support SBW messages on IRIX or HP-UX.