Skip to main content

Processes

In Linux, threads are treated as standalone processes.

Every process has its own:

  • pid
  • ppid - parent process id
  • pgid - process group id
  • program code, data, variables - static libraries are loaded at compile time. shared libraries (or DLLs) are loaded during run time.
  • file descriptors
  • environment

Process attributes:

  • Program being executed
  • Context/state
  • Permissions - (setuid programs are run with the permissions of the owner)
  • Associated resources

Process states:

  • Running
  • Sleeping
  • Stopped
  • Zombie

init is the first process that runs on a system and is the parent of all other processes except for ones originating from the kernel.

If a parent process dies before a child, the ppid of the child is set to 1 and the process is adopted by init. On newer systems, the pid is actually set to 2 which corresponds to an internal kernel thread (kthresadd).

Processes that have terminated but their parent process has not requested their exit code are called a zombie processes. The init process ensures processes die gracefully.

Processes are started by fork and exec where the parent process creates a child process which inherits its pid and then the parent process terminates.

All processes are scheduled by the system. This is important because it enables the kernel to perform context switching when a program is waiting for a response, the resources can be transferred to another program in need of the resources until the other first program receives the data.

Processes interact with the kernel using system calls which act as interfaces to request/release access to the hardware. They run in their own user space for added security where no other process can access it (Process Resource Isolation).

Controlling Resources​

We can use ulimit to control resource limits for processes. There are 2 types of limits:

  • Hard, the maximum value, can be set by root:

    ulimit -H -n
    1048576

  • Soft, the current limiting value, can be set by any user:

    ulimit -S -n
    1024

We can set the limit for the current session by using:

ulimit -n 1600

To modify system-wide, we can modify the file /etc/security/limits.conf and reboot.

We can also use nice to set the niceness level of a process:

nice -n 5 cat
nice -n -5 cat
renice +5 -p 20003

Execution Modes​

A process can be run in two modes:

  • System/Kernel mode - Ring 0
  • User mode - Ring 3 rings A mode is enforced on the hardware level as it is the state of the processor.

When a process needs access to the hardware, it issues a system call which changes the context switch from user mode to kernel mode.

Daemons​

Processes that run in the background, usually started at boot time and provide security.

Scripts in /etc/init.d run daemons on startup.

Shared Libraries​

Libraries that can be linked to any program at run-time. They provide a means to use code that can be loaded anywhere in the memory. Once loaded, the shared library code can be used by any number of programs.

Use ldd to list the shared libraries of an executable. It displays a list of sonames the name of the object (e.g. linux-vdso.so.1)

ldd /usr/bin/apt
 linux-vdso.so.1 (0x00007fff9339b000)
    ...

The ldconfig binary uses /etc/ld.so.conf to configure where to look for shared libraries. We can also use the LD_LIBRARY_PATH to set up the shared libraries.