Linux processes

Process is a running user space program. Kernel starts the first process /sbin/init in function run_init_process _idusing kernel_execve _id. Processes occupy system resources, like memory, CPU time. System calls sys_fork _id and sys_execve _id are used to create new processes from user space. The process exit with an sys_exit _id system call.

Linux inherits from Unix its basic process management system calls (⚲ API ↪ ⚙️ implementations):

man 2 fork ↪ kernel_clone _id creates a new process by duplicating the process invoking it.

man 2 _exit ↪ do_exit _id terminates the calling process "immediately". Any open file descriptors belonging to the process are closed.

man 2 wait ↪ kernel_waitid _id suspends the execution of the calling process until one of its children processes terminates.

Linux enhances the traditional Unix process API with its own system calls man 2 clone. Clone creates a child process that may share parts of its execution context with the parent. It is often used to implement threads (though programmers will typically use a higher-level interface such as man 7 pthreads, implemented on top of clone).

PID - Process identifier defined as pid_t _id is unique sequential number. man 1 ps -A lists current processes.

⚲ API

unistd.h

sys/types.h

sys/wait.h

⚙️ Internals

task_struct _id

pid_type _id

kernel/fork.c _src

syscalls:

man 2 set_tid_address – set pointer to thread ID

man 2 fork – create a child process

man 2 vfork – create a child process and block parent

man 2 clone – create a child process

man 2 unshare – disassociate parts of the process execution context

kernel/sys.c _src

syscalls:

man 2 prctl – operations on a process or thread

kernel/pid.c _src

syscalls:

man 2 pidfd_open – obtain a file descriptor that refers to a process

man 2 pidfd_getfd – obtain a duplicate of another process's file descriptor

syscalls:

man 2 pidfd_open – obtain a file descriptor that refers to a process

man 2 pidfd_getfd – obtain a duplicate of another process's file descriptor

kernel/exit.c _src

syscalls:

man 2 exit – terminate the calling process

man 2 exit_group – exit all threads in a process

man 2 waitid – wait for process to change state

man 2 waitpid – wait for process to change state

fs/exec.c _src

📖 References

fork (system call)

exit (system call)

wait (system call)

exec (system call)