XV6 - usys.s what does this code it do?

Question

I have never seen this assembly syntax.

#include "syscall.h"
#include "traps.h"
#define SYSCALL(name) \
  .globl name; \
  name: \
    movl $SYS_ ## name, %eax; \
    int $T_SYSCALL; \
    ret

SYSCALL(fork)
SYSCALL(exit)
SYSCALL(wait)
SYSCALL(pipe)
SYSCALL(read)
SYSCALL(write)
SYSCALL(close)
SYSCALL(kill)
SYSCALL(exec)
SYSCALL(open)
SYSCALL(mknod)
SYSCALL(unlink)
SYSCALL(fstat)
SYSCALL(link)
SYSCALL(mkdir)
SYSCALL(chdir)
SYSCALL(dup)
SYSCALL(getpid)
SYSCALL(sbrk)
SYSCALL(sleep)
SYSCALL(uptime)

Answer 1

For assembly language file with extension .S, gcc will use a C preprocessor.

In C, \ at the end of line means that "connect the next line to this line". For that reason, the macro becomes

#define SYSCALL(name) .globl name; name: movl $SYS_ ## name, %eax; int $T_SYSCALL; ret

## operator will concatenate the tokens in its left and right.

Therefore, for example, SYSCALL(fork) will be expanded to

.globl fork; fork: movl $SYS_fork, %eax; int $T_SYSCALL; ret

This means

1. make the identifire fork public
2. define a label fork (this will work as a function)
3. in this function
   1. assign an immediate value SYS_fork to register %eax
   2. generate an interrupt with code T_SYSCALL
   3. return from this function