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– TIME:Get elapsed time (return the current time in

sec :only display the starting time eg: Feb. 2, 2010)– STIME:Set elapsed time (to allow it to be read later: )– UTIME:Set a file’s “last access time ”.(to allow owner of

the file to change the time stored)– TIMES: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