[9fans] System call mania

[Pietro Gagliardi]

Hello. I was recently looking in libc when I noticed a few things  
about system calls:

1) brk and sbrk are implemented atop brk_, which is not documented
2) the seek function seems to be a system call, but the alpha folder  
defines a function seek which calls _seek
3) Some items in that file have underscores, like BRK_ and _READ
This leads me to the following question: what exactly are the system  
calls in Plan 9?

Thanks.