跳转至

Lab1

primes

  • How to stop program
    • -1 作为终结符
    • 注意在父进程 最后 加上 wait(0);
  • when you are confused about the output, pint out the pid would be convenient

find

先看ls的实现细节： - 首先根据参数个数判断输入是否合法 - 若输入的参数个数为0 直接退出 - 否则，分别对每一个参数 执行 ls 函数

ls 函数的实现细节 some struct and function api - struct dirent 

| name | type    | description       |
|------|---------|-------------------|
| inum | ushort  | number of files ? |
| name | char [] | file names array  |

  • struct stat

    name type description
    dev int file system disk divice
    ino uint file poniters ponits to the array
    type short type of a file
    nlink short number of links to the file
    size size size of a file

  • system call close

    arugments description
    const char* pathname the name of the file
    int flag specify the way to open a file
    flag des
    O_RDONLY, O_WRONLY, or O_RDWR. 只读，只写，读写
    return Value des
    int fd file descripnator

  • fstat (file state)

    arugments description
    int fd file descripnator
    struct stat* the info retrived from file fd points to was saved here
    return Value des
    status code 0 success, -1 failed

  • details about the ls

    • first we open the file according to the file name (string) , and we get fd
    • using fd, we call fstat ,to retrieve the metadata from it,
    • according to the file, we have the following operation
      • if it is a T_FILE we print it
      • if it is a T_DIR we
  • persudo code for "find"
takes two argument, 
    1. target directory
    2. find target
open target directory, iterate through the catalogue
    if it is a file, we compare it to file target
    if it is a direc, we do it recursively

problem

  • how to iterate through a direc in unix
  • what is ., .. be represented for
  • cannot open file, maybe file name wrongs
    • do not close the file descripnator, so new file cannot be open

xargs

problem

  • how can we know we are excecuting xargs command
  • how can I get the arguments from the pipe
    • we do not need to know, just read from standard input. the os provide us with abstraction using pipe.
  • how can pass char** arg as the second argument for exec, as we need to increase the number of arugments, instead of copy value, we just assign poniters

How to implement

  • is the second paramter == '-n'
    • yes, then we store the specified maximum arguments
    • no, we read all the input, replace \n -> ' '

颜色主题调整

评论区~