The Birth of a Process

A process is born when a program is executed. So let us back-track a little more and start from the program birth. The program is born when there is a need for the programmer. So now I have a need to write a program so that I could create a process out of it and start explaining what happens along the way. The following is the sample code used for explaining various concepts in the rest of the article.

#include <stdio.h>
#include <math.h>

int main()
{
    float d;

    d = cos(20);
    printf("%f\n", d);
}

The source is trivial, which just finds the cosine of a number and prints it. Now compiling the program should give us an executable from which we will start the journey of tracking the process down.

$ cc sample-source.c -lm

which should give us a.out. Note that we are linking with the math library. Now executing this should create a process, on which our study will be based on.

The Program and the Shell

$ ./a.out

When typing ./a.out in the shell, the shell first creates a process of its own using the fork() system call. This fork() system call will create a new process. This new process will overlay itself with the executable image given through the execv() set of system calls. We will go more into each of these system calls in the coming sections. Roughly what the shell will do is in the following listing.

#include <unistd.h>
#include <stdio.h>

int shell_exec (char *command)
{
    pid_t pid;

    pid = fork();
    if (pid == 0) {
        execlp(command, command, NULL);
    }
    /* parent process continues running */
    return 0;
}

int main (int count, char **command)
{
    if (count < 2)
        printf("Need a command to execute\n");

    return shell_exec(command[1]);
}

The above code is a major simplification of what the shell does, which handles pipes, permissions, job control and much more.