A program that performs short burst of work and pause for input is considered better behaved than the one that hog the processor by continually calculating/querying the system. Usually, several programs are competing for the same resources. Linux kernel uses a process scheduler to decide which process will get the next time slice based on the process priority. That's why we may see two such processes listed in ps output. The R+ indicates that the process is in foreground not waiting for other processes to finish nor waiting for input or output to complete.
The indicator shows that the program is ready to run, and is not necessarily running. The STAT R indicates the process 23603 is in a run state. Let's look at the ps STAT output for ps ax itself: Makes a range of services accessible to programs via the system call application That the CPU can access protected kernel memory. Request that the kernel perform some action for the process.Īctually, a system call changes the processor state from user mode to kernel mode, so We can do the same thing within our own programs with the system calls fork(), exec(), and wait().Ī system call is a controlled entry point into the kernel, allowing a process to The ability to start new processes and to wait for them to finish is fundamental to the system. When the user shell exits, init starts another getty process. The getty processes wait for activity at the terminal, prompt the user with the login prompt, and then pass control to the login program, which sets up the user environment, and starts a shell. The init starts the getty program once for each terminal that we can use to log in, and it is shown in the ps as below: The login procedure is one of the example. Then, other system processes are started by init or by other processes started by init. This is OS process manager, and it's the prime ancestor of all processes. When linux starts, it runs a single program, init, with process #1. The process is in the foreground process group.Įach child process is started by parent process. Usually stopped by shell job control or the process is under the control of a debugger. Usually waiting for an event to occur, such as a signal or input to become available. Uninterruptible sleep (waiting) - usually waiting for input or output to complete. Running or runnable (either executing or about to run).
LINUX LIST PROCESSES BY SESSION CODE
The meaning of the code is in the table below: STAT Code Here is the STAT output from ps: $ ps -axģ428 ? SLs 0:00 ntpd -u ntp:ntp -p /var/run/ntpd.pid -g A process should also has its own program counter, a record of where it has gotten to in its execution (execution thread - more on linux pthread). It also has its own environment variables which are maintained by each process. In other words, a process has its own stack space, used for local variables. When we run two programs, there are variables unique to each programs, unlike the shared libraries, these are in separate data space of each process, and usually can't be shared. Therefore, there need be only one copy of printf() in memory, even if there are many programs calling it. The area is for holding the program code so that the code can be loaded into memory as read-only (so, it can be safely shared). In general, a linux process can't write to the memory area. When we run a program, the code that will be executed is stored in a disk file. When a process is started, the numbers restart from 2, and the number 1 is typically reserved for the init process as shown in the above example. Root 24765 1 0 Apr08 ? 00:00:01 /usr/sbin/httpdĮach process is allocated a unique number, process identifier (PID).
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