3.1.2. Process Creation

At system boot time, one user level process is created. In Unix, this process is called init. In Windows, it is the System Idle Process. This process is the parent or grand-parent of all other processes.

New Child Processes are created by another process (the Parent Process).

3.1.2.1. Windows Process Creation

Windows process creation specifies a new program to run when the process is created.

3.1.2.2. Unix Process Creation

• Unix fork() creates a process

  • Creates a new address space
  • Copies text, data, & stack into new adress space
  • Provides child with access to open files

• Unix exec() allows a child to run a new program

• Unix wait() allows a parent to wait for a child to terminate

Note

In Unix, process creation and management uses multiple, fairly simple system calls. This provides extra flexability. If needed, the parent process may contain the code for the child process to run, so that exec() is not always needed. The child may also set up inter-process communication with the parent, such as with a pipe before running another program.

The only difference between the parent and process after the call to fork() is the return value from fork().

int pidValue;
...
pidValue = fork();       /* Creates a child process */
if(pidValue == 0) {
/* pidValue is 0 for child, nonzero for parent */
/* The child executes this code concurrently with parent */
    childsPlay(...);      /* A procedure linked into a.out */
    exit(0);
}
/* The parent executes this code concurrently with child */
parentsWork(...);
wait(...);
...

3.1.2.3. Example: Printing using a pipe to the child process

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define MAXLINE 150
#define true 1
#define false 0
int ReadLine(char *,int ,FILE *);

int main(void)
{
char *process="/usr/local/bin/lpr";       /* process to exec */
char *option="-Php126";
char line[MAXLINE];
int fd[2];                     /* pipe file desc. */
FILE *fp;

pipe(fd);
if( fork() == 0 ) {/* child */
    close(fd[1]);
    close(0);
    dup(fd[0]);      /* redirect stdin to be from pipe */
    close(fd[0]);
    execl( process, process, option, 0 );
} else {           /* parent */
    close(fd[0]);
    close(1);
    dup(fd[1]);
    close(fd[1]);    /* redirect stdout to be to pipe */
    fp = stdin;
    while( ReadLine(line,MAXLINE,fp) ) {
        printf( "%s\n", line );
    }
}
return 0;
}


int ReadLine(char *buff,int size,FILE *fp)
{
buff[0] = '\0';
buff[size - 1] = '\0';             /* mark end of buffer */

if (fgets(buff, size, fp) == NULL) {
    *buff = '\0';                   /* EOF */
    return false;
}
else {
    char *tmp;

    /* remove newline */

    if ((tmp = strrchr(buff, '\n')) != NULL) {
        *tmp = '\0';
    }
}
return true;
}