Processes of C Under Linux in C programming

Gone are the days when only one job (task) could be executed in memory at any time. Today the modern OSs like Windows and Linux permit execution of several tasks simultaneously. Hence these OSs are aptly called ‘Multitasking’ OSs.

In Linux each running task is known as a ‘process’. Even though it may appear that several processes are being executed by the microprocessor simultaneously, in actuality it is not so. What happens is that the microprocessor divides the execution time equally among all the running processes. Thus each process gets the microprocessor’s attention in a round robin manner. Once the time-slice allocated for a process expires the operation that it is currently executing is put on hold and the microprocessor now directs its attention to the next process. Thus at any given moment if we take the snapshot of memory only one process is being executed by the microprocessor. The switching of processes happens so fast that we get a false impression that the processor is executing several processes simultaneously.
The scheduling of processes is done by a program called ‘Scheduler’ which is a vital component of the Linux OS. This scheduler program is fairly complex. Before switching over to the next thread it stores the information about the current process. This includes current values of CPU registers, contents of System Stack and Application Stack, etc. When this process again gets the time slot these values are restored. This process of shifting over from one thread to another is often called a Context Switch. Note that Linux uses preemptive scheduling, meaning thereby that the context switch is performed as soon as the time slot allocated to the process is over, no matter whether the process has completed its job or not.

Kernel assigns each process running in memory a unique ID to distinguish it from other running processes. This ID is often known as processes ID or simply PID. It is very simple to print the PID of a running process programmatically. Here is the program that achieves this… 

int main( ) 

{ 

            printf ( "Process ID = %d", getpid( ) ) ; 

 }

Here getpid( ) is a library function which returns the process ID of the calling process. When the execution of the program comes to an end the process stands terminated. Every time we run the program a new process is created. Hence the kernel assigns a new ID to the process each time. This can be verified by executing the program several times—each time it would produce a different output.