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(libc.info)Obstacks Data Alignment

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Alignment of Data in Obstacks
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   Each obstack has an "alignment boundary"; each object allocated in
the obstack automatically starts on an address that is a multiple of the
specified boundary.  By default, this boundary is 4 bytes.

   To access an obstack's alignment boundary, use the macro
`obstack_alignment_mask', whose function prototype looks like this:

 - Macro: int obstack_alignment_mask (struct obstack *OBSTACK-PTR)
     The value is a bit mask; a bit that is 1 indicates that the
     corresponding bit in the address of an object should be 0.  The
     mask value should be one less than a power of 2; the effect is
     that all object addresses are multiples of that power of 2.  The
     default value of the mask is 3, so that addresses are multiples of
     4.  A mask value of 0 means an object can start on any multiple of
     1 (that is, no alignment is required).

     The expansion of the macro `obstack_alignment_mask' is an lvalue,
     so you can alter the mask by assignment.  For example, this
     statement:

          obstack_alignment_mask (obstack_ptr) = 0;

     has the effect of turning off alignment processing in the
     specified obstack.

   Note that a change in alignment mask does not take effect until
_after_ the next time an object is allocated or finished in the
obstack.  If you are not growing an object, you can make the new
alignment mask take effect immediately by calling `obstack_finish'.
This will finish a zero-length object and then do proper alignment for
the next object.