CS 355 - Systems Programming class notes

Reference: Molay, Understanding Unix/Linux Programming, Chapter 6

Software tools vs. device-specific program

Software tools: read stdin or files, write to stdout

$ sort > outputfile
$ sort x.txt > /dev/lp
$ who | tr '[a-z]' '[A-Z]'

Device-specific programs: control device for particular application

User programs is a common example of a device-specific program that interacts with the terminal.

Terminal modes

Canonical mode a.k.a. cooked mode: Buffered until the user presses Enter. Buffering enables the driver to perform basic editing with the erase and word-erase keys.
Noncanonical mode a.k.a. crmode: No buffering, but the the driver handles Ctrl-C and translating between newline and carriage return.
Non-anything mode a.k.a. raw mode: The driver passes the input directly into the program.

Setting a specific terminal mode: use the tcsetattr() function.

Signals

A signal is a kernel mechanism that can send a message to a running process. Requests for signals to be sent can come from:

users
kernel
processes

Signals caused by something a process does (e.g. division by zero) are called synchronous signals. Signals caused by events outside the process (e.g. user pressing Ctrl-C) are called asynchronous signals.

A list of signals is available in /usr/include/signal.h:

#define   SIGHUP    1   /* hangup */
#define   SIGINT    2   /* interrupt */
#define   SIGQUIT   3   /* quit */
#define   SIGILL    4   /* illegal instruction (not reset when caught) */
#define   SIGTRAP   5   /* trace trap (not reset when caught) */
#define   SIGABRT   6   /* abort() */
#define   SIGEMT    7   /* EMT instruction */
#define   SIGFPE    8   /* floating point exception */
#define   SIGKILL   9   /* kill (cannot be caught or ignored) */
#define   SIGBUS   10   /* bus error */
#define   SIGSEGV  11   /* segmentation violation */
#define   SIGSYS   12   /* bad argument to system call */
#define   SIGPIPE  13   /* write on a pipe with no one to read it */
#define   SIGALRM  14   /* alarm clock */
#define   SIGTERM  15   /* software termination signal from kill */

What can be done about a signal (e.g. SIGINT):

Accept the default action

This usually terminates the process. As shown below, this can be changed. To restore the original action:

signal(sig_num, SIG_DFL);

Ignore the signal

signal(sig_num, SIG_IGN);

Call a function

signal(sig_num, function_name);

How to use signal()

Using the signal() to handle signals:

result = signal(int sig_num,void (*action)(int));

where sig_num is the signal to respond to and action is either a function pointer, SIG_IGN to ignore the signal, or SIG_DFL to reset the signal to its default action.

Example:

#include <stdio.h>
#include <signal.h>

main() {
   void  f(int);             /* declare the handler     */
   int   i;

   signal(SIGINT, f);        /* install the handler     */
   for(i=0; i<5; i++ ){      /* do something else       */
      printf("hello\n");
      sleep(1);
   }
}

void f(int signum) {         /* this function is called */
   printf("OUCH!\n");
}