Order of evaluation of function arguments is unspecified, from C99 §6.5.2.2p10:
The order of evaluation of the
function designator, the actual
arguments, and subexpressions within
the actual arguments is unspecified,
but there is a sequence point before
the actual call.
Similar wording exists in C89.
Additionally, you are modifying pa multiple times without intervening sequence points which invokes undefined behavior (the comma operator introduces a sequence point but the commas delimiting the function arguments do not). If you turn up the warnings on your compiler it should warn you about this:
$ gcc -Wall -W -ansi -pedantic test.c -o test
test.c: In function ‘main’:
test.c:9: warning: operation on ‘pa’ may be undefined
test.c:9: warning: operation on ‘pa’ may be undefined
test.c:13: warning: operation on ‘pa’ may be undefined
test.c:13: warning: operation on ‘pa’ may be undefined
test.c:17: warning: operation on ‘pa’ may be undefined
test.c:17: warning: operation on ‘pa’ may be undefined
test.c:20: warning: control reaches end of non-void function
