Question

1. Which system call should you use to create get the child process’ exit status?

2. What do operating systems use to allow multiple processes load from and store to the same address without interfering with each other?

Answer 1

System Call :

A system call is the programmatic way in which a computer program requests a service from the kernel of the operating system it is executed on. A system call is a way for programs to interact with the operating system. A computer program makes a system call when it makes a request to the operating system’s kernel. System call provides the services of the operating system to the user programs via Application Program Interface(API). It provides an interface between a process and operating system to allow user-level processes to request services of the operating system. System calls are the only entry points into the kernel system. All programs needing resources must use system calls.

Services Provided by System Calls :

1. Process creation and management

2. Main memory management

3. File Access, Directory and File system management

4. Device handling(I/O)

5. Protection

6. Networking, etc.

Types of System Calls : There are 5 different categories of system calls –

1. Process control: end, abort, create, terminate, allocate and free memory.

2. File management: create, open, close, delete, read file etc.

3. Device management

4. Information maintenance

5. Communication

System call use to create get the child process exit status :

It is known that fork() system call is used to create a new process which becomes child of the caller process.
Upon exit, the child leaves an exit status that should be returned to the parent. So, when the child finishes it becomes a zombie.

Whenever the child exits or stops, the parent is sent a SIGCHLD signal.
The parent can use the system call wait() or waitpid() along with the macros WIFEXITED and WEXITSTATUS with it to learn about the status of its stopped child.

// C code to find the exit status of child process

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <sys/types.h>

#include <sys/wait.h>

  

// Driver code

int main(void)

{

    pid_t pid = fork();

      

    if ( pid == 0 )

    {

       /* The pathname of the file passed to execl()

          is not defined   */

       execl("/bin/sh", "bin/sh", "-c", "./nopath", "NULL");

    }

  

    int status;

      

    waitpid(pid, &status, 0);

  

    if ( WIFEXITED(status) )

    {

        int exit_status = WEXITSTATUS(status);        

        printf("Exit status of the child was %d\n",

                                     exit_status);

    }

    return 0;

}