1 3 System Call System Call System calls

1. 3 System Call

System Call • System calls provide the interface between a running program and the operating system. • System call is a method by which a program make a request to the OS • Generally system calls available as assembly -language instructions • System call will use by executing a software interrupt instruction.

Cont. . • Languages defined to replace assembly language for systems programming allow system calls to be made directly (e. g. , C, C++). • Three general methods are used to pass parameters between a running program and the operating system: – Pass parameters in registers. – Store the parameters in a table in memory, and the table address is passed as a parameter in a register. – Push (store) the parameters onto the stack by the program, and pop off the stack by operating system.

System Call Working under Process Management • Calls involve in Process Management – FORK : is used to create a new process. – WAITPID : is used to terminate the process. – PTRACE : is used for debugging – SETSID : create a new session and return its process group id. – BRK : is used to set the size of data segment • System call related to process creation termination

System Call Working under Signaling • Although most interprocess communication are planned, situations exist in which unexpected communication needed.

System Call Working under Signaling • Eg: – when user accidently tell a text editor to list the entire content of long text and then realize the error, some way is needed to interrupt the editor. – User can hit the DEL key on keyboard which send a signal and stop the print – out. Signal also can be used to report certain trap detected by hardware. – When signal is sent to process that has not announced its willingness to accept the signal, the process is simply killed.

System Call Working under Signaling • The calls in Signaling: – SIGACTION: define action to take on signal – SIGRETURN: return from signal – SIGPENDING: get the set of blocked signal – SIGSUSPEND : replace the signal mask and suspend the process • The purpose of SC in signaling is handle the signal

System Call Working under File Management • System call operate on individual files. • Calls in File Management – CREAT : to create a new file fd= creat(“abc”, 0751) ; create a file call abc with mode 0751 octal. . specify the rwx for the owner • In C , a leading by zero means that a constant is in octal • Owner : 7 means read- write- execute permission) • Group : 5 means read-execute • Others : 1 means execute only

System Call Working under File Management – OPEN: Open a file for reading, writing or both – CLOSE : Close an open file – WRITE : Write data from a buffer into a class – RENAME: Give a file a new name • The purpose of SC in File Management is reading and writing file

System Call Working under Directory Management • Example of calls involve in this section are – MKDIR : create empty directories – RMDIR : delete directories – LINK : allow the same file to appear under two or more names, often in different directory. (Eg: Allow several members of the same programming team to share common file, which each of them having the file appear in his own directory) – UNLINK : remove the directory entry • The purpose is to manage the directory

System Call Working under Protection • In MINIX every file has an 11 -bit used for protection. • These bit are read-write-execute bit for owner, group and others. • CHMOD system call is used to change the mode of a file • To a file read-only by everyone except the owner, could execute : chmod(“filename”, 0644)

System Call Working under Protection • The other two protection bits: – 02000 : SETUID : set-user-id • When any user execute a program with the SETUID bit on, for duration of that process the user’s effective uid is change to that file’s owner. – 04000 : SETGID : set –group-id • When a programs execute, the gid is change to the current gid. • The purpose of system call in protection is to protect information

System Call Working under Time Management • Example, MINIX has 4 system call that involve the time-of-day clock – : Get elapsed time (return the current time in sec : only display the starting time eg: Feb. 2, 2010) – : Set elapsed time (to allow it to be read later: ) – : Set a file’s “last access time ”. (to allow owner of the file to change the time stored) – : Get the user and system times used so far (to know total user and system time used) • The purpose of time Management is keeping track of time

Summary • The heart of any operating system is the set of system call that it can handle • Example for MINIX, these call can be divided into sixth group. Which are – – – System call related to process creation termination Handle signal Reading and writing files Directory management Protect information Keeping track of time