Process is a binary program that is currently running in the linux system. It can be viewed by using the ps command.
A binary program can be created just as we create a.out files using the gcc compiler. The program when run, becomes the process.
Each process by default, gets three file descriptors that are attached to it. They are stdin, the standard input pipe, stdout, the standard output pipe, stderr, the standard error pipe.
Linux is a multi-process operating system. Each process gets an illusion that it has the access to the full hardware for itself. Each process is interleaved in execution using the concept called scheduling. The scheduling takes many parameters as input to run one of the scheduling algorithms. There are many scheduling algorithms such as roundrobin, FIFO etc. Each of these scheduling algorithms can be selectable or just built-in directly into the Linux kernel that comes with the Linux OS types such as Fedora or Ubuntu. The scheduler is also a thread or a function that gets a periodic interrupt via the hardware timing pulse. At each pulse the scheduler finds out the next process that is eligible to run on the CPU(s). The scheduler then places the process into the Run queue and wakes it up and places the already running process into wait queue and saves all of the registers that it used and etc. The woken up process will then execute on the CPU. The scheduling is a vast and a large topic to cover and it is only explained here in this paragraph brief.
In linux stdin is described as file descriptor 0, stdout is described as file descriptor 1, and stderr is described as file descriptor 2. Upon the each successful pipe / msgqueue / socket call, the file descriptors will get incremented in number usually (i.e from 3, 4 and so on) most of the time. A file descriptor that is opened must always be closed with a corresponding close system call or the similar one. This is a good programming practice.
Each process is identified by a pid (called process identifier). Process ids are 16 bit numbers (Signed numbers). The program can obtain its pid using getpid() call. The parent process id is obtained by using getppid().
NOTE: In linux the process 1 is always the init process.
The below program gets the pid and parent pid.
#include <stdio.h>
#include <unistd.h>
int main(void)
{
printf("process id %d\n", getpid());
printf("parent process id %d\n", getppid());
return 0;
}
The parent process of the program that we have currently executed is always the shell that we ran the binary.
The below example is for the ps command.
PID TTY TIME CMD
22963 ttys000 0:00.29 -bash
22860 ttys001 0:00.01 -bash
22863 ttys002 0:00.02 -bash
pid 0 is a swapper process and pid 1 is an init process. The init process is the one that is called by the kernel to initialise the userspace.
A process has the following states:
Running - The process is either running or is ready to run.
Waiting - The process is waiting for an event or a resource. Such as socket wait for the recv etc.
Stopped - The process has been stopped with a signal such as Ctrl + Z combination or under a debugger.
When a process is created, the kernel allocates enough resources and stores the information about the process. The kernel also creates an entry about the process in the /proc file system. For ex: for a process such as init with pid 1, it stores the name of the process into /proc/<pidname>/cmdline file, and stores links to the opened files in /proc/<pidname>/fd/ directory and it keeps a lot of other information such as memory usage by this process, scheduling statistics etc. When a process is stopped, the information and all the allocated memory and resources will then automatically be freed up by the kernel.