kernel/types.h

Short names for C’s integer types, used everywhere in the kernel, in user programs and in mkfs. xv6 defines its own short names instead of using <stdint.h>. That header would in fact be available (it is one of the few headers a freestanding (vs. hosted) C compiler supplies), but xv6 uses no standard headers at all, and the same file is shared with mkfs, a normal host program.

The names that end in a number say exactly how many bits the type has. That matters in an operating system: a CSR (control and status register), a register and a page-table entry are all 64 bits, and on-disk structures must have the same layout every time. On 64-bit RISC-V (the “LP64” model GCC uses here) char is 8 bits, short 16, int 32 and long 64, which is what these typedefs rely on.

The file has no include guard. Standard C99 does not allow repeating a typedef (GCC accepts it unless -pedantic is given); xv6 keeps it simple by including this file once, first, in every .c file, before headers that use these names.

Read next: kernel/param.h, kernel/riscv.h.