3191932525 Distributed Software Programming Lecture 2 Week 2

1
31919/32525 Distributed Software
ProgrammingLecture 2 (Week 2)

• UNIX Programming (Part 2)

2
Writing UNIX Programs

• There are two categories of UNIX programs
• User Commands programs users run from shells,
  e.g. cc, ls, vi
• Daemons their execution is usually invisible to
  users

3
Arguments from Command Line

• Main function contains two arguments
• argc - count of arguments
• argv0 - pointer to the first argument
• A program may include error checking
• error message is printed on stderr by fprintf
• On successful exit, return 0 otherwise a nonzero
  value (e.g. 1)

4
getopt Routine

• int getopt(int argc char const argv, const char
  opts)
• extern char optarg
• extern int optind, opterr, optopt
• opts points to a string containing the option
  letters. Each option that has an argument is
  followed by a colon ()

5
getopt Routine (contd)

• When getopt returns a character in opts, optarg
  points to the argument of the option (if there is
  one)
• getopt sets optind to the index of the next
  argument in argv
• set opterr to 0 to suppress the error to be
  printed by getopt

6
Error Handling

• perror prints a string on stderr, followed by a
  description of the error
• Two other public routines for error handling
• error(const char fmt, )
• for nonfatal error report
• fatal(const char fmt, )
• for fatal error report and calls to exit at the
  end

7
Inputs and Outputs (I/O)

• FILE fopen(const char fname, const char type)
• int fclose(FILE fp)
• int open(const char path, int oflag, / mode_t
  mode/)
• returns fd (file descriptor)
• ssize_t read(int fd, void buf, size_t nbyte)
• returns number of bytes read and -1 on error
• ssize_t write(int fd, const void buf, size_t
  nbyte)
• returns number of bytes written and -1 on error
• int close(int fd)
• returns 0 on success and -1 on failure

8
Creating a Log File

• Use open to open a log file
• int fcnt1(int fd, int cmd, / void arg /) gets
  the close-on-exec flag
• int fstat(int fd, struc stat sbuf) reads the
  file stat and check its user and group IDs
• int fchown(int fd, uid_t owner, gid_t group)
  changes the user and group IDs to the proper
  values

9
Signaling

• Signals are the software equivalent of hardware
  interrupts.
• When a signal is sent, it is posted or generated.
  When received, it is delivered. Signal between
  posted and delivered is called pending.
• Way to control signals is called signal
  disposition.

10
Signal Interfaces

• void (sigset(int sig, void (disp)(int)))(int)
• sig is the signal number
• disp is address of a signal-handling or a
  constant such as SIG_IGN, SIG_DEF and SIG_HOLD
• On success
• if sig was held, returns SIG_HOLD
• otherwise, returns previous disposition
• On error, returns SIG_ERR and sets errno

11
Signal Interfaces (contd)

• int sighold(int sig) adds sig to signal mask
• int sigrelse(int sig) removes sig from mask
• int sigignore(int sig) ignores the signal
• int sigpause(int sig) removes sig from mask and
  suspends process till next signal
• They all return 0 on success and -1 on failure

12
Creating Child Processes

• Reasons
• Multi processes can work together to accomplish
  the task
• Child process does not destroy parent process
  context
• Use fork to create another process
• Use exec to replace text and data of parent
  process

13
fork System Call

• pid_t fork(void) returns 0 to the child and
  nonzero values to the parent
• If fork fails, it returns -1 to the parent and
  sets errno to EAGAIN
• Child gets its own copies of fds
• When a child terminates, SIGCHLD is sent to the
  parent

14
wait and waitpid System Calls

• pid_t wait(int status) and
• pid_t waitpid(pid_t pid, int status, int
  options) suspend the calling process until one of
  its children terminates or stops
• If the status of a child is available, they
  return the process ID of the child
• If the parent is uninterested in the status,
  status can be set to NULL

15
exec family

• exec family has functions such as execl, execle,
  execlp, execv, execve and execvp
• They all replace the text and data of current
  process with that of the program specified by a
  path of file
• The first argument is the basename of the command
  and the last one is NULL

16
Creating Daemons

• Change the file creation mask
• Create a log file to record errors
• Obtain max number of open files
• Create new session and process group and ignore
  SIGHUP in the child process
• Close all files except the log file
• Reserve fds 0, 1 2

17
Readings

• Rago, UNIX System V Network Programming,
• Chapter 2