Exam 3 practice questions

A. Unix I/O

A1) Consider the following server main loop:

int server_fd = Open_listenfd(port);

while (1) {
  int client_fd = Accept(server_fd, NULL, NULL);
  int pid = Fork();
  if (pid == 0) {
    // in child
    chat_with_client(client_fd);
    exit(0);
  }
}

Assume that

Open_listenfd, Accept, and Fork are functions from csapp.h and csapp.c
the chat_with_client function correctly handles all details of communicating with the remote client, including closing the client socket file descriptor
the main server process has a handler for SIGCHLD to wait for child processes to exit
the server intentionally does not place any limit on how many client sub-processes can be running
the server intentionally does not have any mechanism for handling shutdown requests

Describe a bug in this main loop, and how to fix it.

Answer: The problem is that after the fork, the client file descriptor (client_fd) is duplicated by the child process, but is also still open in the parent process. To avoid a file descriptor leak, the parent process should close its copy of client_fd immediately after the fork. So:

int server_fd = Open_listenfd(port);

while (1) {
  int client_fd = Accept(server_fd, NULL, NULL);
  int pid = Fork();
  if (pid == 0) {
    // in child
    chat_with_client(client_fd);
    exit(0);
  }
  close(client_fd); // <-- add this
}

B. Network communication

B1) Complete the following function, called chat_with_client. It should implement the server side of a network protocol in which the client will repeatedly send lines of text, and for each line of text that is not quit, the server will send back the reversal of the line sent by the client. When the client sends the line quit, then the client session has ended, and the connection should be closed by the server.

For example, if the client sends

Hello, world
Colorless green sheep sleep furiously
A smell of petroleum prevails throughout
quit

then the server should send back

dlrow ,olleH
ylsuoiruf peels peehs neerg sselroloC
tuohguorht sliaverp muelortep fo llems A

You may implement and call helper functions as necessary. You may use the functions defined in csapp.h and csapp.c.

void chat_with_client(int client_fd) {
  // TODO: complete this function

Answer:

#include <assert.h>
#include <string.h>
#include "csapp.h"

#define MAX_LINE 1024

void reverse_string(char s[]) {
  size_t len = strlen(s);
  for (size_t i = 0; i < len/2; i++) {
    size_t j = len - i - 1;
    char tmp = s[i];
    s[i] = s[j];
    s[j] = tmp;
  }
}

void trim_line_terminator(char s[]) {
  char *p = strchr(s, '\r');
  if (p != NULL) {
    // line was terminated by \r\n
    *p = '\0';
  } else {
    // line is (probably) terminated by \n
    p = strchr(s, '\n');
    if (p != NULL) {
      *p = '\0';
    }
  }
}

void chat_with_client(int client_fd) {
  char buf[MAX_LINE];
  int done = 0;

  rio_t in;
  Rio_readinitb(&in, client_fd);

  while (!done) {
    ssize_t n = Rio_readlineb(&in, buf, MAX_LINE);
    if (n <= 0) {
      done = 1;
    } else {
      trim_line_terminator(buf);

      if (strcmp(buf, "quit") == 0) {
        done = 1;
      } else {
        reverse_string(buf);
        Rio_writen(client_fd, buf, strlen(buf));
      }
    }
  }

  close(client_fd);
}

C. Concurrency

C1) Consider the following IntStack data type.

The header file:

// intstack.h

#ifndef INTSTACK_H
#define INTSTACK_H

#define MAX 256

struct IntStack {
  int contents[MAX];
  int top;
};

void intstack_init(struct IntStack *s);
void intstack_push(struct IntStack *s, int val);
int intstack_pop(struct IntStack *s);

#endif // INTSTACK_H

The implementation file:

// intstack.c

void intstack_init(struct IntStack *s) {
  s->top = 0;
}

void intstack_push(struct IntStack *s, int val) {
  assert(s->top < MAX);
  s->contents[s->top] = val;
  s->top++;
}

int intstack_pop(struct IntStack *s) {
  assert(s->top > 0);
  s->top--;
  return s->contents[s->top];
}

Modify the IntStack data type so that it can be safely used by multiple threads.

When the intstack_push function is called, and the stack is full, the function should wait until the stack is not full, and then push the specified value.

When the intstack_pop function is called, and the stack is empty, the function should wait until the stack is not empty, and then pop the top value.

Answer:

Header file:

// intstack.h

#ifndef INTSTACK_H
#define INTSTACK_H

#include <pthread.h>
#include <semaphore.h>

#define MAX 256

struct IntStack {
  int contents[MAX];
  int top;
  pthread_mutex_t lock;
  sem_t slots, items;
};

void intstack_init(struct IntStack *s);
void intstack_push(struct IntStack *s, int val);
int intstack_pop(struct IntStack *s);
#endif // INTSTACK_H

Implementation file:

// intstack.c

void intstack_init(struct IntStack *s) {
  s->top = 0;
  pthread_mutex_init(&s->lock, NULL);
  sem_init(&s->slots, 0, MAX); // initially, all slots available
  sem_init(&s->items, 0, 0);   // initially, no items available
}

void intstack_push(struct IntStack *s, int val) {
  // wait for slot to be available
  sem_wait(&s->slots);

  // push the item
  pthread_mutex_lock(&s->lock);
  assert(s->top < MAX);
  s->contents[s->top] = val;
  s->top++;
  pthread_mutex_unlock(&s->lock);

  // item is now available
  sem_post(&s->items);
}

int intstack_pop(struct IntStack *s) {
  // wait for item to be available
  sem_wait(&s->items);

  // pop an item
  pthread_mutex_lock(&s->lock);
  assert(s->top > 0);
  s->top--;
  int result = s->contents[s->top];
  pthread_mutex_unlock(&s->lock);

  // a slot is now available
  sem_post(&s->slots);

  return result;
}