Capturing System Calls

[Charles.Green () RL AF MIL (Green Charles Contr AFRL/IFGB)]

On UNIX Systems, (FreeBSD, Linux, Solaris) is there a way to capture/modify
system calls calls from an application with out modifying the kernel (or
using kernel modules) - preferably in userspace? The reason I ask is that a
group of us are being asked to evaluate a piece of software for my company
but they've put some heavy restrictions on how we do it. One of the
restriction is that we're not allowed to modify the kernel.