There aren't very many syscalls in Linux - 335 for x86_64[1], 190 for i386[2], etc. I tend to like GUIDs, but AFAIK it would cause a massive backward-compatibility problem to switch to something other than an integer.

If I'm reading the kernel source[3] correctly, the syscall interface itself accepts a 32-bit unsigned integer for the call number, but only the lowest 16 bits can be used because the upper 16 bits are used to represent the version. That would still mean that something like 80% of the effective ID space is unused at present.

[1] http://blog.rchapman.org/posts/Linux_System_Call_Table_for_x... [2] http://asm.sourceforge.net/syscall.html [3] https://github.com/torvalds/linux/blob/master/ipc/syscall.c

In Linux, the system call number is used as an index into an array of function pointers. Making it a GUID won't really help with table lookup performance -- O(1) random access lookup versus O(log n) binary search.