辅导CIS 548编程、c/c++程序设计调试、c++程序讲解解析Java程序|解析Java程序

项目预算：开发周期：发布时间：要求地区：

CIS 548 - Spring 2021 - Project 3
PennOS: A User-level UNIX-like Operating System
“UNIX is basically a simple operating system, but you have to be a genius to understand the simplicity.”
– Dennis Ritchie
Prof. Boon Thau Loo
Directions
This is a group project. You must work in groups of four or five. You are expected to follow along to
Piazza as updates and clarifications made there will be reflected in the final grading. You may reuse code
from previous projects, but only code you wrote.
You are forbidden to reuse any code obtained from the internet or from previous
semesters. Violators will be sent to the Office of Student Conduct and will receive
a failing grade for the course.
Overview
In this assignment you will implement PennOS, your own UNIX-like operating system. PennOS is designed
around subsystems that model those of standard UNIX. This will include programming a basic priority
scheduler, FAT file system, and user shell interactions. Unlike a real operating system, you will not be
required to actually boot on hardware; rather, your PennOS will run as a guest OS within a single process
running on a host OS.
How to use this document
The following specification provides a road map for completing this project; however, as you develop your
code, you may find it necessary to expand upon the specification. Feel free to add additional shell/user
level functions to support debugging, as long as you adhere to the provided interfaces in this
document. Not only is this encouraged, but it is expected. If you are ever in doubt about a design
decision, ask your friendly TAs, and be sure to document such changes in your README and companion
document.
1
PennOS CIS 548 - Spring 2021
1 Specification
There are two states in which an operating system exists: (1) kernel land and (2) user land. During
execution, an operating system switches between these two states continuously. As an example, consider
what happens when a program issues a system call. First, the system call is executed in user land which
hits a hook; that is, the running process actually calls the system call. Next, the operating system must
handle the system call, switching from user to kernel land. Once the system call completes, the operating
system returns control to the calling process, returning back to user land. Unlike in a real operating
system, however, this separation between user land and kernel land is simply taken on faith. You are not
required to implement a mechanism to enforce this separation.
In the previous projects, you have interacted with the operating system at the user land level, and in this
project, you will take a peek behind the curtain at kernel land. Well, not exactly, but you will simulate
the basic functionality of an operating system by programming your own operating system simulator
PennOS. Using the ucontext library, you will implement a basic priority scheduler; additionally, you
will implement a FAT file system for your operating system to mount, and a basic shell and programming
API for a user to interact with your operating system.
Unlike a real operating system, your PennOS is not required to boot on hardware (or handle devices
in general); instead, it will run as a single process on a host OS. The ucontext library is similar to a
threading API in that it allows one process to split its resources across multiple instances. ucontexts
do not provide a scheduler like you may be used to with traditional threading, and your first task will be
implementing a SIGALRM-based scheduler for context switching.
Another critical part of an operating system is handling file reads and writes. Your operating system
will mount a single file system, PennFAT: a simple file system implementation based on FAT. Your
PennFAT implementation will be stored within a single file on the host file system, and will be mounted by
PennOS in a loopback like manner. Additionally, unlike traditional file systems, PennFAT is only required
to handle files within a single top level directory. You are required to allow the creation, modification,
and removal of files under the top level directory.
The last part of your operating system is providing user land interaction via a simple shell. You will
program this shell using the user land system calls providing by PennOS. Your shell will provide job
control, stdin/stdout redirection, and a functional set of built-in commands for testing and exploring
your operating system.
One last note: this is a long and complex project that will take you many, many hours to
complete. This document can only provide you with a primer of the innumerable challenges you will
face; read it carefully, but be sure to use other resources as well. In particular, read the manual pages,
and ask questions of your friendly TAs. Likely, this will be the largest program you have yet written as a
student, so divide and conquer and plan ahead so that the pieces fit together. Remember, a lot of small,
easy programs equal one large, complex program. Don’t try to conquer the entire project in one go: it is
organized such that splitting up tasks into discrete components and writing modular code are strategies
that will be rewarded.
1.1 Function Terminology
Your operating system will provide a number of different functions and interfaces. The following symbols
indicate how the functions are to be used:
• (K): indicates a kernel level function that may only be called from the kernel side of the operating
system.
2
PennOS CIS 548 - Spring 2021
• (U): indicates a user level function that may only be called from the user side of the operating
system. These are your operating system’s system calls.
• (S): indicates a built in program that can be called directly from the shell.
The function definitions provided are suggestions for you to build upon. You may find it useful to add
additional kernel/system/user level functions as you see fit as long as they are in the spirit of the assignment.
1.2 PennOS Processes/Threads
Your PennOS operating system will run as a single process on the host OS (e.g., Linux on your virtual
machine). That is, each of the “processes” in PennOS is really the same process as PennOS according to
the host operating system, but within PennOS, each process will be separated into context threads that
are independently scheduled by PennOS. To accomplish this task you will be using the ucontext library:
If you are issuing calls to fork(2), exec(2), or wait(2), you are doing something very
wrong. Despite being context threads1
, your operating system will treat them like processes and will
organizing them into process control blocks (or PCB’s).
A PCB is a structure that describes all the needed information about a running process (or thread).
One of the entries will clearly be the ucontext information, but additionally, you will need to store
the thread’s process id, parent process id, children process ids, open file descriptors, priority level, etc.
Refer to Steven’s Advanced Programming in the UNIX Environment and Tanenbaum’s Modern Operating
Systems for more information about data normally stored in a PCB. You must describe your PCB
structure in your companion document.
1.2.1 Process Related Required Functions
Your operating system must provide the following user level functions for interacting with PennOS process
creation:
• pid_t p_spawn(void (*func)(), char *argv[]) (U) forks a new thread that retains most
of the attributes of the parent thread (see k_process_create). Once the thread is spawned, it
executes the function referenced by func with its argument array argv. It returns the pid of the
child thread on success, or -1 on error.
• pid_t p_waitpid(pid_t pid, int *wstatus, bool nohang) (U) sets the calling thread
as blocked (if nohang is false) until a child of the calling thread changes state. It is similar to
Linux waitpid(2). If nohang is true, p_waitpid does not block but returns immediately.
p_waitpid returns the pid of the child which has changed state on success, or -1 on error.
• int p_kill(pid_t pid, int sig) (U) sends the signal sig to the thread referenced by pid.
It returns 0 on success, or -1 on error.
• void p_exit(void) (U) exits the current thread unconditionally.
Additionally, your operating system will have the following kernel level functions:
• k_process_create(Pcb * parent) (K) create a new child thread and associated PCB. The
new thread should retain much of the properties of the parent. The function should return a reference
to the new PCB.
1We use “context” and “thread” interchangeably in this document to describe a PennOS process.
3
PennOS CIS 548 - Spring 2021
• k_process_kill(Pcb * process, int signal) (K) kill the process referenced by process
with the signal signal.
• k_process_cleanup(Pcb * process) (K) called when a terminated/finished thread’s resources
needs to be cleaned up. Such clean-up may include freeing memory, setting the status of the child,
etc.
1.2.2 Zombies and Waiting
As processes complete, it may not be the case that their parent threads can wait on them immediately. If
that is the case, you must queue up these threads so that the parent may wait on them in the future. These
threads are Zombies. You will likely have a zombie queue for each thread, referenced in the PCB. If at any
time the parent thread exits without waiting on zombie children, the zombie children should immediately
die, as well as non-zombie children threads. Note, this is similar to how INIT inherits orphan child
processes and kills them off.
Additionally, as noted above, other child process state changes can cause a p_waitpid() to return.
In UNIX, a child process that transitions from running to stopped would issue a SIGCHLD signal. Your
operating system also should have functionality for parent process to learn of similar state changes. In your
PennOS kernel land implementation of p_waitpid() you will find it useful to maintain other queues of
“waitable” children, not just zombie children.
1.2.3 Signals in PennOS
Your operating system will not have traditional signals and signal handlers. (However, the host operating
system may deliver signals that you must handle.) Instead, signaling a PennOS process indicates to
PennOS that it should take some action related to the signaled thread, such as change the state of a
thread to stopped or running. Your operating system will minimally define the following signals:
• S_SIGSTOP: a thread receiving this signal should be stopped
• S_SIGCONT: a thread receiving this signal should be continued
• S_SIGTERM: a thread receiving this signal should be terminated
If you would like to add additional signals, be sure to document them and their functionality in your
companion document file.
1.2.4 Process Statuses
PennOS will provide at least the following user level functions/macros that will return booleans based on
the status returned from p_waitpid.
• W_WIFEXITED(status): return true if the child terminated normally, that is, by a call to p_exit
or by returning.
• W_WIFSTOPPED(status): return true if the child was stopped by a signal.
• W_WIFSIGNALED(status): return true if the child was terminated by a signal, that is, by a call
to p_kill with the S_SIGTERM signal.
4
PennOS CIS 548 - Spring 2021
1.3 Programming with User Contexts
ucontext is a basic thread-like library provided on most UNIX systems. Essentially, it allows a user to
isolate some portion of code execution within a context and switch between them. On the course website,
we have provided a sample program that demonstrates how to switch between contexts in a round robin
fashion. A high-level description of context creation and execution is provided below, and more information
can be found in the manual.
First, a ucontext structure must be initialized with a call to getcontext(2). The structure will
have the following fields (and more not shown):
typedef struct ucontext {
struct ucontext *uc_link;
sigset_t uc_sigmask;
stack_t uc_stack;
...
} ucontext_t;
You still need to set the structure values above: uc_link is a the next context to run when this
context completes2
; uc_sigmask is a signal mask for blocking signals in this context; and, uc_stack
is the execution stack used by this context. For a description of the uc_stack structure reference the
manual for sigaltstack(2). Setting these values is still insufficient to executed the context, and you
still need to set up the function that will be called when the context is set or swapped. This done by a
call to makecontext(2), and it is well described in the manual. A context is switched in using either
setcontext(2) or swapcontext(2), which either directly sets the context, or sets and also saves
the state of the running context, respectively.
We are leaving much of the details for you to learn on your own, but a good place to start is with
a Hello World program for ucontext. We have provided one in the Appendix (item A). Try editing
that programming and adding new features. Here are some mini-exercise you might want to try: What
happens if you want to switch back to the main program after printing “Hello World”? Can you write a
program that alternates between two functions indefinitely? What happens when a signal is delivered?
How do signals affect the execution of a context? How do you track the current context?
1.4 Priority Scheduler
Perhaps the most critical part of any operating system is the scheduler. This is the part of your operating
system that chooses which program to run next. As described in class, most operating systems, including
Linux, use a priority queue based scheduler. In your PennOS, you will also implement a priority scheduler,
although it will be a much more simplified version with a fixed quanta.
1.4.1 Clock Ticks
You will schedule a SIGALRM signal to be delivered to your operating system every 100 milliseconds.
We refer to this event as a clock tick, and on every clock tick the operating system will switch in and
execute the scheduler, which will then choose which process to run next. This may be any process that is
runnable (i.e., not zombied, blocked, nor stopped) to execute, including the shell. To set an alarm timer
at millisecond granularity, refer to setitimer(2). Note that since your operating system is relying on
SIGALRM, non-kernel functions may not block SIGALRM.
2What might you want to set uc_link to?
5
PennOS CIS 548 - Spring 2021
1.4.2 Priority Queues
Your operating system will have three priority levels: -1, 0, 1. As in standard UNIX, the lower a priority
level, the more heavily the “process” should be scheduled. Child threads created by k_process_create
should have a default priority level of 0, unless otherwise specified. For example, the shell should execute
with priority level -1 because it is interactive.
Your priority queues will be relative. Threads scheduled with level -1 should run 1.5 times more often
as threads scheduled with priority level 0, which run 1.5 times more often as threads scheduled with
priority level 1. Within each priority queue, the threads are selected in a round robin format, and each
queue must be implemented as a linked list. You may reuse your linked list implementation from
previous projects. You must also ensure that no thread is starved.
As an example, consider the scheduling of the following threads with these priority levels: (1) shell,
priority level -1; (2) sleep, priority level 0; and (3), cat, priority level 0. With a 100 millisecond
quanta, after 10 seconds, what is the proportion of quanta for each process? First, there are 10 quanta per
second, which means a total of 100 quanta in 10 seconds. Of the available quanta, 60 quanta will be used
for priority level -1, (that is the shell), since it must be scheduled 1.5 times as often. Of the remaining
40 quanta, 20 quanta will be used for sleep, and 20 quanta for cat.
To verify the correctness of your scheduler, you will implement a detailed logging facility that will
generate a log entry for every clock tick. The specification of the log format is described in Section 1.8.
1.4.3 The Idle Process
When all processes in the scheduler queues are blocked, the scheduler needs to decide what to do. It
cannot just keep waiting in a while loop until a process is ready to be scheduled, since this would spike
the CPU usage of PennOS up to 100%, when it should actually be 0% (since the OS isn’t actually doing
anything). To get past this, you can create an additional "idle" process.
The idle process does nothing and suspends itself. To be clear, the idle process must not run
an infinite while loop, since this would spike CPU usage to 100%. You should use sigsuspend(2)
instead, which will suspend the idle process until a signal is delivered to it, resulting in 0% CPU usage.
The idle process must be scheduled only when all other processes are blocked and the scheduler has
no real process to schedule.
Correct usage of the idle process is as follows: The shell spawns a new process sleep 200. The
child process for sleep will be blocked for 200 clock ticks, and the shell will be blocked waiting for sleep
to terminate. At this point, all the scheduler queues are empty and the idle process can be scheduled until
sleep completes.
Incorrect usage of the idle process is as follows: The shell spawns a new process busy. The shell is
blocked waiting for busy to complete, the busy process is being scheduled, and so is the idle process. So
the resulting CPU usage is 60%. This is incorrect. Since the scheduler has the busy process to schedule,
the idle process must not be scheduled, and the resulting CPU usage of PennOS should increase to 100%.
1.5 Running vs. Stopped vs. Blocked vs. Zombied
Threads can exists in four states: running, stopped, blocked, or zombied. A running thread may be
scheduled; however, a stopped, blocked, or zombied thread should not be. A thread should only become
stopped if it was appropriately signaled via p_kill. A thread should only be blocked if it made a call to
p_waitpid or p_sleep (see below).
6
PennOS CIS 548 - Spring 2021
1.5.1 Required Scheduling Functions
Your operating system will provide at least the following user level functions for interacting with the
scheduler:
• int p_nice(pid_t pid, int priority) (U) sets the priority of the thread pid to priority.
• void p_sleep(unsigned int ticks) (U) sets the calling process to blocked until ticks of
the system clock elapse, and then sets the thread to running. Importantly, p_sleep should not
return until the thread resumes running; however, it can be interrupted by a S_SIGTERM signal.
Like sleep(3) in Linux, the clock keeps ticking even when p_sleep is interrupted.
1.6 PennFAT File System
Your operating system will mount its own file system, PennFAT, based on FAT16. (For references, see
Chapter 4.3.2 in Tanenbaum 4th edition, pages 285-286.) You are only required to implement the
root directory of PennFAT.
1.6.1 FAT File System Regions
A FAT file system has mainly two regions: the FAT region which hosts the File Allocation Table (FAT), and
the data region which stores the files (including directory files). The block numbering of PennFAT’s
data region begins at 1.
1.6.2 FAT File Systems and File System Blocks
Conceptually, you can think of a file as a sequence of fixed-size blocks and a file system as a way to find
the right blocks for a file in the right order. A FAT provides a simple way to do this by storing links
between physical blocks of a file. Placed at the beginning of the file system before any block, the FAT has
a predetermined size and can be easily mapped into memory. This is also its greatest disadvantage: being
of fixed size, the FAT also limits the size of the file system.
The FAT of PennFAT consists of a predetermined number of entries. It is specified when the filesystem
is first formatted using the mkfs command, which dictates how the FAT and your file system as a whole
should be formatted. The least significant byte (LSB) of the first entry of the FAT specifies the size of
each block with the mapping displayed below:
LSB | Size (bytes)
---------+-------------
0 | 256
---------+-------------
1 | 512
---------+-------------
2 | 1024
---------+-------------
3 | 2048
---------+-------------
4 | 4096
The most significant byte (MSB) of the first entry of the FAT will be used to calculate the size of the
FAT, with the MSB ranging from 1-32 (numbers outside of this range will be considered an error). The
size of your FAT will be
7
PennOS CIS 548 - Spring 2021
FAT size = block size * number of blocks in FAT
and with each entry of the FAT consisting of 2 bytes, the number of entries in your FAT will be
# of FAT entries = block size * number of blocks in FAT / 2
In the figure below, each row (table entry) corresponds to a block in the file system and the value
(Link) represents the next block number of the file (or 0xFFFF to denote the last block of the file). In
other words, each table entry is a link to the table entry for the next block of the file.
Index | Link
---------+-------
0 | 0x2004 <--- MSB=0x20 (32 blocks in FAT), LSB=0x04 (4K-byte block size)
---------+-------
1 | 0xFFFF <--- Block 1 is the only block in the root directory file
---------+-------
2 | 5 <--- File A starts with Block 2 followed by Block 5
---------+-------
3 | 4 <--- File B starts with Block 3 followed by Block 4
---------+-------
4 | 0xFFFF <--- last block of File B
---------+-------
5 | 6 <--- File A continues to Block 6
---------+-------
6 | 0xFFFF <--- last block of File A
---------+-------
... | ...
Like other FAT16 variants, 0 denotes a free block, and 0xFFFF the last block of a file. (Thus, the
maximum possible block number for PennFAT is 2
16 − 2).
With each FAT entry representing a file block, there will be a total of
Data region size = block size * (number of FAT entries - 1)
We subtract 1 here because the first entry (fat[0]) in the FAT is used to store the filesystem
formatting information, so it does not have a corresponding block. (This is also why the block numbering
of the data region begins at 1.) Consequently, fat[1] references Block 1 which will always be the first
block of the root directory file (the directory file may encompass multiple blocks as it grows).
A directory file consists of directory entries, each of which stores metadata about another file (including
a directory file). PennFAT has a 64-byte fixed directory entry size. The structure of the directory entry
as stored in the filesystem is as follows:
char name[32];
uint32_t size;
uint16_t firstBlock;
uint8_t type;
uint8_t perm;
time_t mtime;
// The remaining 16 bytes are reserved (e.g., for extra credits)
• char name[32]: null-terminated file name
name[0] also serves as a special marker:
8
PennOS CIS 548 - Spring 2021
– 0: end of directory
– 1: deleted entry; the file is also deleted
– 2: deleted entry; the file is still being used
• uint32_t size: number of bytes in file
• uint16_t firstBlock: the first block number of the file (undefined if size is zero)
• uint8_t type: the type of the file, which will be one of the following:
– 0: unknown
– 1: a regular file
– 2: a directory file
– 4: a symbolic link
• uint8_t perm: file permissions, which will be one of the following:
– 0: none
– 2: write only
– 4: read only
– 5: read and executable (shell scripts)
– 6: read and write
– 7: read, write, and executable
• time_t mtime: creation/modification time as returned by time(2) in Linux
By iterating over the directory files, all files in the file system can be enumerated. To find a block
in the file system using a FAT entry, you will use lseek(2). For example, given a FAT entry for the
value 5, you can find that block in the file containing your PennFAT by the following call to lseek:
lseek(fs_fd, fat_size + block_size * 4, SEEK_SET)
where fat_size is the size of the FAT and block_size is the size of a block.
1.6.3 Standalone PennFAT
Your file system itself will exist as a single binary file on the host OS. In order to interact with the file
system from outside of your operating system implementation (i.e. the shell), you will provide a single
executable that will interact with your file system accordingly.
You will provide a single standalone program, separate from the final PennOS executable, called
pennfat, which doesn’t have any arguments. Within pennfat, there exist a number of commands
similar to their counterparts in bash:
• mkfs FS_NAME BLOCKS_IN_FAT BLOCK_SIZE_CONFIG
Creates a PennFAT filesystem in the file named FS_NAME. The number of blocks in the FAT region
is BLOCKS_IN_FAT (ranging from 1 through 32), and the block size is 512, 1024, 2048, or 4096
bytes corresponding to the value (1 through 4) of BLOCK_SIZE_CONFIG.
9
PennOS CIS 548 - Spring 2021
• mount FS_NAME
Mounts the filesystem named FS_NAME by loading its FAT into memory.
• umount
Unmounts the currently mounted filesystem.
• touch FILE ...
Creates the files if they do not exist, or updates their timestamp to the current system time.
• mv SOURCE DEST
Renames SOURCE to DEST.
• rm FILE ...
Removes the files.
• cat FILE ... [ -w OUTPUT_FILE ]
Concatenates the files and prints them to stdout by default, or overwrites OUTPUT_FILE. If
OUTPUT_FILE does not exist, it will be created. (Same for OUTPUT_FILE in the commands below.)
• cat FILE ... [ -a OUTPUT_FILE ]
Concatenates the files and prints them to stdout by default, or appends to OUTPUT_FILE.
• cat -w OUTPUT_FILE
Reads from the terminal and overwrites OUTPUT_FILE.
• cat -a OUTPUT_FILE
Reads from the terminal and appends to OUTPUT_FILE.
• cp [ -h ] SOURCE DEST
Copies SOURCE to DEST. With -h, SOURCE is from the host OS.
• cp SOURCE -h DEST
Copies SOURCE from the filesystem to DEST in the host OS.
• ls
List all files in the directory, similar to ls -il in bash. For example:
2 -rw 30000 Nov 7 01:00 up-to-thirty-one-byte-file-name
120 -r- 1000 Oct 31 21:59 Posix-file_name.2
The columns above are first block number, permissions, size, month, day, time, and name.
• chmod
Similar to chmod(1) in the VM.
The purpose of the pennfat program is to test your file system’s functionality, independent of your
PennOS. This will be needed for the second milestone as well as the final demo, so make sure you update
it as you make progress or change things.
10
PennOS CIS 548 - Spring 2021
1.6.4 Loading the FAT into Memory
When mounting your PennFAT filesystem, you will likely find it useful to mmap(2) the FAT region directly
into memory. This way you will have copy-on-write for free. Here is a code snippet (minus error checking)
to get you started on this process:
#include
#include
...
int fs_fd = open(fs_name, O_RDWR);
uint16_t *fat = mmap(NULL, fat_size, PROT_READ | PROT_WRITE, MAP_SHARED, fs_fd, 0);
Now, fat references an array.
1.6.5 File System and Operating System Interaction
The role of the operating system is to protect the file system from corruption as well as manipulate
it by reading, writing, and deleting files. Internally, the operating system will store a reference to a file
descriptor (an integer) for each open file, as well as a structure indicating the mode for the file and relevant
file pointers indicating where subsequent reads and writes should take place. The user side will reference
a file by its file descriptor and manipulate the file via the user level interface described below.
Note that the file system-related system calls are abstraction layers and should not care about the
format of the underlying file system. That is, consider what must occur when an operating system has
mounted multiple file systems of different types. When there is a call to read(2) for a particular file
descriptor, the operating system will determine on which file system variant the file lives (e.g., FAT32,
ext3, etc.) and then call the appropriate file system dependent function to perform the read operation
(which usually is provided by module).
PennOS must work in a similar way, particularly when considering how to handle the stdin and
stdout file descriptors. Essentially, PennOS must manipulate two types of file descriptors, those for
PennFAT and those for stdin and stdout. A user level program should be able to write to stdout
using the same interface as it would write to a PennFAT file. If a user level program is calling
read(2), then you are doing something wrong.
1.6.6 Required File System Related System Calls
Your operating system will provide at least the following functions for file manipulation.
• f_open(const char * fname, int mode) (U) open a file name fname with the mode mode
and return a file descriptor. The allowed modes are as follows:
– F_WRITE - writing and reading, truncates if the file exists, or creates it if it does not exist.
Only one instance of a file can be opened in F_WRITE mode; error if PennOS attempts to open
a file in F_WRITE mode more than once
– F_READ - open the file for reading only, return an error if the file does not exist
– F_APPEND - open the file for reading and writing but does not truncate the file if exists;
additionally, the file pointer references the end of the file.
f_open returns a file descriptor on success and a negative value on error.3
3What characters are allowed in filenames? You may follow the POSIX standard, linked here.
11
PennOS CIS 548 - Spring 2021
• f_read(int fd, int n, char * buf) (U) read n bytes from the file referenced by fd. On
return, f_read returns the number of bytes read, 0 if EOF is reached, or a negative number on
error.
• f_write(int fd, const char * str, int n) (U) write n bytes4 of the string referenced
by str to the file fd and increment the file pointer by n. On return, f_write returns the number
of bytes written, or a negative value on error.
• f_close(int fd) (U) close the file fd and return 0 on success, or a negative value on failure.
• f_unlink(const char * fname) (U) remove the file. 5
• f_lseek(int fd, int offset, int whence) (U) reposition the file pointer for fd to the
offset relative to whence. You must also implement the constants F_SEEK_SET, F_SEEK_CUR,
and F_SEEK_END, which reference similar file whences as their similarly named counterparts in
lseek(2).
• f_ls(const char *filename) (U) list the file filename in the current directory. If filename
is NULL, list all files in the current directory.
You may require kernel level functions as well. Be sure to document them in your code and companion
document file.
1.7 Shell
Once PennOS is booted (i.e., executed from the host OS), it will execute a shell. You will program this
shell using the PennOS user interfaces described above. Although there is not strong separation between
user and kernel land, you may only use the user level functions to program your shell and
programs that run in it. That is, you may only use functions indicated with a (U).
The shell is like any other ucontext thread running in PennOS and should be scheduled as such,
except it will always have a priority level of -1. Unlike traditional shells, it is not capable of running
arbitrary programs; instead you will provide built-in programs to execute within a user context. Below
are the following features your shell should provide:
• Synchronous Child Waiting: PennOS does not provide a means to perform asynchronous signal handling;
instead, you will use a synchronous signal

辅导CIS 548编程、c/c++程序设计调试、c++程序讲解 解析Java程序|解析Java程序

辅导CIS 548编程、c/c++程序设计调试、c++程序讲解解析Java程序|解析Java程序