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- 1. CS330: Operating Systems ProcessAPI: System calls
- 2. Recap: The processabstraction - The OS creates a process when we run an executable a.out Execute $./a.out - When we execute “a.out”on a shell a process control block (PCB) is created - Does it raise some questions related to the exact working? PCB (a.out) CPU state PID Memory state File state ….. OS
- 3. Process creation: Whatand How? - How does OS come into action after typing “./a.out”in a shell? - Who invokes the system calls? - What exact system calls are invoked? - Where is the genesis? What is the first user process?
- 4. System call - CPUexecuting user code can invoke the OS functions using system calls OS Process code CPU OS code
- 5. System call - CPUexecuting user code can invoke the OS functions using system calls - The CPU executes the OS handler for the system call OS Process code CPU OS code
- 6. System call - CPUexecuting user code can invoke the OS functions using system calls - The CPU executes the OS handler for the system call - How system call is different from a function call? OS Process code CPU OS code
- 7. System call - CPUexecuting user code can invoke the OS functions using system calls - The CPU executes the OS handler for the system call - How is system call different from a function call? - Can be thought as an invocation of privileged functions (will revisit) OS Process code CPU OS code
- 8. System calls anduser libraries Applications Library API printf( ) Operating System write( ) Monitor/Console dev_write( ) - Most system calls are invoked through wrapper library functions - However,all system calls can be invoked directly - For example, in Linux systems, syscall( ) wrapper can be used (Refer: man syscall)
- 9. A simple systemcall: getpid( ) pid_t getpid( ) { PCB *current = get_current_process( ); return (current → pid); } main( ) { printf(“%dn”, getpid( )); } OS USER
- 10. Process creation: Whatand How? - How does OS come into action after typing “./a.out”in a shell? - System calls invoked to explicitly give control to the OS - What exact system calls are invoked? - Who invokes the system calls? - Where is the genesis? What is the first user process?
- 11. Process creation -fork( ) - fork( ) system call is weird; not a typical “privileged”function call - fork( ) creates a new process; a duplicate of calling process - On success,fork - Returns PID of child process to the caller (parent) - Returns 0 to the child Parent Process Parent Process Child Process fork( )
- 12. Typical implementation offork Parent Process Syscall handler (fork) fork ( ) Copy process PCB (parent) CPU state PID Memory state File state …..
- 13. Typical implementation offork Parent Process Syscall handler (fork) fork ( ) Fix return value - Child should get ‘0’and parent gets PID of child as return value.How? PCB (parent) CPU state PID Memory state File state ….. PCB (child) CPU state PID Memory state File state …..
- 14. Typical implementation offork Parent Process Syscall handler (fork) fork ( ) Fix return value - Child should get ‘0’and parent gets PID of child as return value.How? - OS returns different values for parent and child PCB (parent) CPU state PID Memory state File state ….. PCB (child) CPU state PID Memory state File state …..
- 15. Typical implementation offork Parent Process Syscall handler (fork) fork ( ) Fix return value - Child should get ‘0’and parent gets PID of child as return value.How? - OS returns different values for parent and child - When does child execute? PCB (parent) CPU state PID Memory state File state ….. PCB (child) CPU state PID Memory state File state …..
- 16. Typical implementation offork Parent Process Syscall handler (fork) fork ( ) Fix return value - Child should get ‘0’and parent gets PID of child as return value.How? - OS returns different values for parent and child - When does child execute? - When OS schedules the child process PCB (parent) CPU state PID Memory state File state ….. PCB (child) CPU state PID Memory state File state …..
- 17. Typical implementation offork Parent Process Syscall handler (fork) OS scheduler Child Process ret = 0 - PC is next instruction after fork( ) syscall,for both parent and child - Child memory is an exact copy of parent - Parent and child diverge from this point PCB (parent) CPU state PID Memory state File state ….. PCB (child) CPU state PID Memory state File state …..
- 18. Load a newbinary - exec( ) - Replace the calling process by a new executable - Code,data etc.are replaced by the new process - Usually,open files remain open Process (1. exe) Process (2.exe) exec (2.exe)
- 19. Typical implementation ofexec Process (1.exe) Syscall handler (exec) exec (“2.exe”) - The calling process commits self destruction! (almost) PCB (1.exe) CPU state PID Memory state File state ….. Code Data 1.exe
- 20. Typical implementation ofexec Process (1.exe) Syscall handler (exec) exec (“2.exe”) - The calling process commits self destruction! (almost) - The calling process is cleaned up and replaced by the new executable - PID remains the same cleanup Load 2.exe from disk
- 21. Typical implementation ofexec Process (1.exe) Syscall handler (exec) return (0) PCB (2.exe) CPU state PID Memory state File state ….. Code Data 2.exe - The calling process commits self destruction! (almost) - The calling process is cleaned up and replaced by the new executable - PID remains the same - On return,new executable starts execution - PC is loaded with the starting address of the newly loaded binary
- 22. Process creation: Whatand How? - How does OS come into action after typing “./a.out”in a shell? - System calls invoked to explicitly give control to the OS - What exact system calls are invoked? - fork( ),exec ( ),wait( ) and exit( ) - Who invokes the system calls? In what order? - - Where is the genesis? What is the first user process?
- 23. wait( ) andexit( ) Parent Child fork( ) Parent - The wait system call makes the parent wait for child process to exit wait( )
- 24. wait( ) andexit( ) Parent Child fork( ) Parent - The wait system call makes the parent wait for child process to exit - On child exit( ),the wait() system call returns in parent wait( ) exit( )
- 25. Shell command line:fork + exec + wait BASH BASH fork( ) BASH - The BASH process calls fork( )
- 26. Shell command line:fork + exec + wait BASH BASH fork( ) BASH exec (“a.out” ) Child Process (a.out) wait( ) - Parent process calls wait( ) to wait for child to finish - Child process invokes exec( )
- 27. Shell command line:fork + exec + wait BASH BASH fork( ) BASH exec (“a.out” ) Child Process (a.out) wait( ) exit( ) - When child exits, parent gets notified - The BASH shell is ready for the next command at this point of time
- 28. Process creation: Whatand How? - How does OS come into action after typing “./a.out”in a shell? - System calls invoked to explicitly give control to the OS - What exact system calls are invoked? - fork( ),exec ( ),wait( ) and exit( ) - Who invokes the system calls? In what order? - The shell process (bash process) - What is the first user process?
- 29. Unix process familyusing fork + exec Parent Process (init) Child Process (init) fork( ) Parent Process (init) exec (/bin/sh ) Child Process (sh) - Fork and exec are used to create the process tree - Commands: ps,pstree - See the /proc directory in linux systems
- 30. Process creation: Whatand How? - How does OS come into action after typing “./a.out”in a shell? - System calls invoked to explicitly give control to the OS - What exact system calls are invoked? - fork( ),exec ( ),wait( ) and exit( ) - Who invokes the system calls? - The shell process (bash process) - What is the first user process? - In Unix systems,it is called the init process - Who creates and schedules the init process?