Process Creation and Termination in the LINUX Environment, страница 2

Type the following C program to see how shell invocation of a command works:

#include  <stdio.h>

main ( )


int i;

            /* fork another process */


            if (i < 0 ) {        /* error occured */

                     fprintf(stderr, "Fork Failed");



             else if (i==0){     /* child process */

                  execlp("/bin/ls","ls", NULL);


            else {                   /* parent process */

              /* parent will wait for the child to complete */


              printf("\n\n\n\n\nChild Complete\n\n\n\n");




Call the above program p4.c, and compile it as follows: gcc -o p4 p4.c <Enter>.

Then execute p4 and report the output.


4.1 [2] When does the wait system call returns in this program?

4.2 [4] Explain the shell invocation of a command?

4.3 [2] Compare this program with shell invocation of a command?

5. LINUX Signals

A signal is a notification to a process that an event has occurred. Signals are also called Software Interrupts. Signals usually occur asynchronously. By this we mean that the process doesn’t know ahead of time exactly when a signal occur. Signals can be sent

  • By one process to another process (or to itself).
  • By the kernel to a process.

Every signal has a name, specified in the header file <signal.h>. There are five conditions that generates signals:

  • The kill() system call allows a process to send a signal to another process or to itself. The name kill is somewhat a misnomer, since a signal does not always terminate a process. Some signals do indeed kill the receiving process, while others are used to inform the process of some condition that the receiving process then handles. This is the format of kill() system call: int kill(int pid, int sig); The kill() system call does many different things, depending on the values of its arguments. As expected, a process is not able to send a signal to any other arbitrary process. To send a signal, the sending process and the receiving process must belong to the same user, or the sending process must be the superuser.
  • The kill command is also used to send signals. This command is a program that takes its command line arguments and issue a kill() system call. For example the kill –STOP <PID> <Enter> send a SIGSTP signal  to the process with the process ID of <PID>.
  • Certain terminal characters generate signals. For example, every interactive terminal has associated with it an interrupt character character. The interrupt character (typically Control-C or Delete) terminates a process that is running (it generate a SIGINT signal). A terminal suspend character (typically Control-Z) generates a SIGSTP signal immediately.
  • Certain hardware conditions generate signals. For example referencing an address outside a process’ address space generates a SIGSEGV signal.
  • Certain software conditions that are noticed by the kernel cause signals to be generated.

Having described what signals are and how they are generated, what can a process do with a signal?

  • A process can provide a function that is called whenever a specific type of signal occurs. The function, called a signal handler, can do whatever the process wants to handle the condition. This is called catching the signal.
  • A process can choose to ignore a signal. All signals other than SIGKILL, can be ignored. The SIGKILL signal is special, since the system administrator should have a guaranteed way of terminating any process.
  • A process can allow the default to happen. You can find the Default actions in the Figure 2.4 of the Stevens book (UNIX network programming) that I will put on reserve at the library. Also in this book, you will find complete information about the system calls that we used in this lab.

Programming Assignment:

Include <signal.h> header file in your program, when you are writing the following program. Use the SIGKILL signal in kill() system call to kill all the processes in the parent process group. Use the kill(0,SIGKILL); system call in your program.

[18] Develop a small program in C using system calls that does the following:

  • A parent process creates a child process that prints it’s PID.
  • The child creates a grandchild and sleeps 120 seconds.
  • The grandchild immediately exists.
  • The parent sleeps for 60 seconds and then kills all the processes.