Even on Linux, userland programs are encouraged to perform syscalls through the vDSO, a mini userland library supplied by the kernel. Performing syscalls direc

Even on Linux, userland programs are encouraged to perform syscalls through the vDSO, a mini userland library supplied by the kernel. Performing syscalls directly is supported, but the vDSO is preferred. For most syscalls, using vDSO simply allows the kernel to pick the fastest available method for entering the kernel. But for a handful, the vDSO provides an optimized implementation that doesn't enter the kernel at all. One of those, ironically, is gettimeofday. That said, the vDSO is much smaller than any libc.