Help needed; lowering priorities

[Jeff Parke]

I am looking for a way using c to implement lower priorities for certain
commands, other than nice or calls that spawn new shells.  This would be
used in lieu of the following example:

mv /usr/bin/asm /usr/bin/asm.bin

(edit /usr/bin/asm to read) nice -10 /usr/bin/asm.bin !*

Is there a c program I can use to do same that would run faster? (Our system
is needing some automatic nicing).