### Step 1: Define the Interface of System Call and Name the Function in `LibOS/shim/src/shim_syscalls.c`

For example, assume we are implementing `sched_setaffinity`. You must find the definition of
`sched_setaffinity` in `shim_syscalls.c`, which will be the following code:

```
SHIM_SYSCALL_PASSTHROUGH(sched_setaffinity, 3, int, pid_t, pid, size_t, len,
                         __kernel_cpu_set_t*, user_mask_ptr)
```

Change this line to `DEFINE_SHIM_SYSCALL(...)` to name the function that implements this system
call: `shim_do_sched_setaffinity` (this is the naming convention, please follow it).

```
DEFINE_SHIM_SYSCALL(sched_setaffinity, 3, shim_do_sched_setaffinity, int, pid_t, pid, size_t, len,
                    __kernel_cpu_set_t*, user_mask_ptr)
```


### Step 2: Add Definitions to `LibOS/shim/include/shim_table.h`

To implement system call `sched_setaffinity`, three functions need to be defined in `shim_table.h`:
`__shim_sched_setaffinity`, `shim_sched_setaffinity`, and `shim_do_sched_setaffinity`. The first
two should already be defined. Add the third in respect to the system call you are implementing,
with the same prototype as defined in `shim_syscalls.c`.

```
int shim_do_sched_setaffinity(pid_t pid, size_t len, __kernel_cpu_set_t* user_mask_ptr);
``` 

### Step 3: Implement the System Call under `LibOS/shim/src/sys`

You can add the function body of `shim_do_sysinfo` (or the function name defined earlier) in a new
source file or any existing source file in `LibOS/shim/src/sys`.

For example, in `LibOS/shim/src/sys/shim_sched.c`:
```
int shim_do_sched_setaffinity(pid_t pid, size_t len, __kernel_cpu_set_t* user_mask_ptr) {
   /* code for implementing the semantics of sched_setaffinity */
}
```

### Step 4 (Optional): Add New PAL Calls if Necessary for the System Call

The concept of Graphene library OS is to keep the PAL interface as simple as possible. So, you
should not add new PAL calls if the features can be fully implemented inside the library OS using
the existing PAL calls. However, sometimes the OS features needed involve low-level operations
inside the host OS and cannot be emulated inside the library OS. Therefore, you may have to add a
few new PAL calls to the existing interface.

To add a new PAL call, first modify `Pal/src/pal.h`. Define the PAL call in a platform-independent way.

```
PAL_BOL DkThreadSetCPUAffinity(PAL_NUM cpu_num, PAL_IDX* cpu_indexes);
```

Make sure you use the PAL-specific data types, including `PAL_BOL`, `PAL_NUM`, `PAL_PTR`,
`PAL_FLG`, `PAL_IDX`, and `PAL_STR`. The naming convention of a PAL call is to start functions
with the `Dk` prefix, followed by a comprehensive name describing the purpose of the PAL call.

### Step 5 (Optional): Export the new PAL call from the PAL binaries

For each directory in `PAL/host/`, there is a `pal.map` file. This file lists all the symbols
accessible to the library OS. The new PAL call needs to be listed here in order to be used by
your system call implementation.

### Step 6 (Optional): Implementing the New PAL Call in `PAL/src`

(Not finished...)