minishell inkwan yu iyu@cise.ufl.edu. topics unix system calls waitpid() pipe() dup2() c function...

Minishell

InKwan Yuiyu@cise.ufl.edu

Topics Unix System calls

waitpid() pipe() dup2()

C function calls strtok() strcmp()

Minishell Software Enginnering tip Evolutionary & Iterative

Unix System calls waitpid()

For out purpose: waitpid(pid, 0, 0); Blocking wait for a child until it

terminates For Project 1, every child is waited

until they all terminate

Unix System calls Ex

#include <unistd.h>

main()

{

int pid;

if ((pid = fork()) ==0) {

print("This is the child\n");

// execvp() can be here.

}

else {

waitpid(pid, 0, 0); // wait until the child terminates

printf("This is the parent\n");

}

}

Unix System calls pipe()

Creates pipes dup2()

Duplicates an open file descriptor Next slides will show how they

work pipe() and dup2() should be used to

implement ‘|’ operator in the shell

File Descriptor/Table/i-node

fd 0fd 1fd 2fd 3fd 4

Descriptor table[one table per process]

Open file table [shared by all processes]

i-node table[shared by all processes]

File pos

refcnt==1

...

File pos

Refcnt==1

...

stderrstdout

stdin File access

...

File size

File type

File access

...

File size

File type

File A (terminal)

File B (disk)

pipe() Call int pfd[2]; pipe(pfd);

fd 0fd 1fd 2fd 3fd 4

Descriptor table(one table

per process)

Open file table (shared by

all processes)

i-node table(shared by

all processes)

File pos

Refcnt==1...

File pos

Refcnt==1

...

File access

...

File size

File type

Pipe 0

Pipe 1

pfd[1] pfd[0]

After the First fork()

fd 0fd 1fd 2fd 3fd 4

Descriptor tables Open file table i-node table

File pos

refcnt==2

...

File pos

refcnt==2

...

Parent's table

fd 0fd 1fd 2fd 3fd 4

Child's table

File access

...

File size

File type

Pipe 0

Pipe1

fork();

pfd[1]pfd[0]

pfd[1]pfd[0]

File access

...

File size

File type

Child dup2()

fd 0fd 1fd 2fd 3fd 4

Descriptor tables Open file table i-node table

File pos

refcnt=2

...

File pos

refcnt=2

...

Parent's table

fd 0fd 1fd 2fd 3fd 4

Child 1's table

File access

...

File size

File type

File access

...

File size

File type

Pipe 0

Pipe1

dup2(pdf[1], STDOUT_FILENO); // in forked child // STDOUT_FILENO == 1 close(pdf[1]);

Finally after Two fork()s, dup2()s, and close()s

fd 0fd 1fd 2fd 3fd 4

File pos

refcnt==1

...

File pos

refcnt==1

...

Child 1's table

fd 0fd 1fd 2fd 3fd 4

Child 2's table

File access

...

File size

File type

pipe 0

pipe 1

fd 0fd 1fd 2fd 3fd 4

Parent's table

stdoutstdin

stdoutstdin

C Function Calls strtok()

Stg type variables are strings terminating with zero (0)

typedef of Stg in arabildr.h and stgbildr.h should be factored out into main.h

strtok() can be used to generate a parsed argument to execvp()

Takes two arguments The first argument is a string to be parsed The second argument is a string of

delimiters

C Function Calls strtok()

The string in the first argument is altered after strtok() call

The first argument should be in write-able memory

For exmaple, Stg cmd = “Hello, World”; may not work

Initialized strings can be placed in BSS region (read-only)

C Function Calls strtok()

// Stg cmd has “Hello,I am here”

Arabildr ar_bildr = arabildr_init();

Stg tok = strtok(cmd, " ,"); // tok has “Hello”

arabildr_append(arr_bildr, tok);

tok = Stg strtok(NULL, " ,"); // tok has “I”

arabildr_append(arr_bildr, tok);

tok = Stg strtok(NULL, " ,"); // tok has “am”

arabildr_append(arr_bildr, tok);

tok = Stg strtok(NULL, " ,"); // tok has “here”

arabildr_append(arr_bildr, tok);

tok = Stg strtok(NULL, " ,"); // tok == NULL; The end

AraOfStgs cmd_array = arabildr_array(arr_bildr);

“Hello”“I”

“am”“Here”

0

cmd_array[0]

cmd_array[4]

C Function Calls strcmp()

Compares two zero terminated strings If they are exact match, returns 0 Else returns nonzero

C Function Calls strcmp()

Arabildr ar_bildr = arabildr_init();

arabildr_append(arr_bildr, “exit”);

AraOfStgs cmd_array = arabildr_array(arr_bildr); ;

// cmd_array[0] is assigned to “exit”

int match = strcmp(cmd_array[0], “exit”);

printf(“match %d\n”, match);

Minishell SE tip 1st step

Read user input & display : getchar() 2nd step

Parse the input : strtok() 3rd step

Run the program : fork(), exec(), waitpid(), strcmp()

4th step Run the piped programs : pipe(), dup2()

Minishell SE tip 1st step

While(1) { Read a user key stoke using getchar() Use Stgbildr APIs to accumulate the user

input Display the input when ‘\n’ is

encountered Destroy the Stgbildr object

}

Minishell SE tip 2nd step

While(1) { Get the user input Parse the input using strtok()

Use Arabildr APIs Display Arabildr objects

AraOfStgs type variable is useful in displaying

}

Minishell SE tip 3rd step

While(1) { Get the user input Parse the input Use strcmp() to “exit”, “logout” or “quit” ; if yes

then exit(); Run the non-piped program

fork() Parent: waitpid() for the child Child: execvp() using AraOfStgs type argument

Destroy objects }

Minishell SE tip 4th step

While(1) { Get the user input Parse the input strcmp() to “exit” Check if ‘|’ exist in the input If yes run the piped programs

Two children are created; pipe() and dup2() are helpful for ‘|’

strtok(), pipe(), dup2(), 2*fork(), 2*execvp(), 2*waitpid() Else run the non-piped program

strtok(), fork(), execvp(), waitpid() Destroy objects

}

Further Information Advanced Programming in the UNIX

Environment, Addison-Wesley, 1992, ISBN: 0-201-56317-7

Design of the UNIX Operating System, Prentice Hall, 1986, ISBN: 0132017997