Add getcount system call

This patch enables the getcount systemcall functionality.

syscall.c

@@ -98,6 +98,7 @@ extern int sys_unlink(void);
 extern int sys_wait(void);
 extern int sys_write(void);
 extern int sys_uptime(void);
+extern int sys_getcount(void);
 
 static int (*syscalls[])(void) = {
 [SYS_fork]    sys_fork,
@@ -121,6 +122,7 @@ static int (*syscalls[])(void) = {
 [SYS_link]    sys_link,
 [SYS_mkdir]   sys_mkdir,
 [SYS_close]   sys_close,
+[SYS_getcount] sys_getcount,
 };
 
 void
@@ -130,6 +132,7 @@ syscall(void)
 
   num = proc->tf->eax;
   if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
+    proc->syscall_count[num]++;
     proc->tf->eax = syscalls[num]();
   } else {
     cprintf("%d %s: unknown sys call %d\n",

syscall.h

@@ -20,3 +20,4 @@
 #define SYS_link   19
 #define SYS_mkdir  20
 #define SYS_close  21
+#define SYS_getcount 22

sysproc.c

@@ -88,3 +88,14 @@ sys_uptime(void)
   release(&tickslock);
   return xticks;
 }
+
+int
+sys_getcount(void)
+{
+  int n;
+
+  if(argint(0, &n) < 0)
+    return -1;
+
+  return proc->syscall_count[n];
+}

user.h

@@ -22,6 +22,7 @@ int getpid(void);
 char* sbrk(int);
 int sleep(int);
 int uptime(void);
+int getcount(int);
 
 // ulib.c
 int stat(char*, struct stat*);

usys.S

@@ -29,3 +29,4 @@ SYSCALL(getpid)
 SYSCALL(sbrk)
 SYSCALL(sleep)
 SYSCALL(uptime)
+SYSCALL(getcount)