COP4600- Discussion Session 3- System Calls

COP4600- Discussion Session 3- System Calls Mahendra Kumar makumar@cise.ufl.edu

Outline • What is a system call? • Pipe • Overview of Process related system calls. • Signals • Files • File related system calls

What is a system call? • Interface between the OS and the application program. • Mechanism used by an application program to request service from the OS.

Pipe • Shared in-memory file between two processes. • This common file is used by the two processes to communicated with each other, i.e. to have interprocess communication. • First process is always supposed to write to the pipe and the second process is supposed to read from the pipe.

Pipe (Contd.) • Command to create a pipe • who | sort • Another example: • who | sort | lpr • How many pipes in the above example? • Which processes read and write to which pipes?

Overview of Process related system calls • Fork()- creates a new process. • Creates an exact duplicate of the original process. • The new process is called the child process. • Return value of fork() • 0 in the child process. • PID(non zero) of child in the parent process.

Fork()- Contd. #include <stdio.h> void ChildProcess(void); /* child process prototype */ void ParentProcess(void); /* parent process prototype */ void main(void) { int pid; int status; pid = fork(); if (pid == 0) ChildProcess(); waitpid(-1,&status,0); else ParentProcess(); } void ChildProcess(void) { int i; for (i = 1; i <= 2; i++) printf("child”); } void ParentProcess(void) { int i; for (i = 1; i <= 2; i++) printf("parent"); }

Some common Process system calls • Fork() • Waitpid() • Brk() • Getpid() • Execve()

Signals • Act as a Software interrupt to a process. • notifications sent to a process in order to notify it of various "important" events. • If a signal arrives at a process and the process has no action for that signal, the process is killed. • To avoid a process being killed unexpectedly, signal must be handled.

Signals-Contd. • Some system calls for signals: • Sigaction(type, &action, &oldact); • Sigaction(SIGINT,SIG_IGN,NULL); • SIGKILL cannot be caught or ignored. • Sigreturn(&context); • Sigprocmask(how,&set,&old); • Sigpending(set); • Sigsuspend(sigmask); • kill

Files • Every file can be accessed with the path from the root directory. • What is the difference between the two commands? • Cd /cise/homes/james • Cd cise/homes/james

Files-Contd. • There are two types of files in minix • General files • Special files • Block special files • Character special files • Special files are kept in /dev directory. • Example: printer, hard disk etc are all represented as files in minix/unix.

File related system calls • Used for creating, reading and writing files. • Creat() : creates a file. • Fd=creat(“abc.txt”,0754); • Creat() is obsolete now. • Open() - used to create and open the file at the same time. • Fd=open(“abc.txt”,O_RDONLY); • Close() - to close the file • Read() – to read from a file • Write() – to write to a file • Lseek() - used to change the value of position pointer

Lseek() • long lseek(int fd,off_t offset,int whence) • Fd: file descriptor • Offset: the added offset which is defined by whence. • Whence: • SEEK_SET: set pointer to value of offset. • SEEK_CUR: set the pointer to its current value plus the value of offset. • SEEK_END: set the pointer to the size of the file plus the value of offset . • Lseek() returns the new value of the position pointer. • How to get the current position of the file?

Dup() system call • Duplicates a file descriptor fd. • Points to the same file table entry as the first file descriptor, fd. • Properties: • Same open file (or pipe) • Same file pointer (that is, both file descriptors share one file pointer) • Same access mode (read, write, or read/write)

Dup() diagram

main() { int fd1, fd2; fd1 = open("file2", O_WRONLY); fd2 = open("file2", O_WRONLY); write(fd1, "Jim\n", strlen("Jim\n")); write(fd2, "Plank\n", strlen("Plank\n")); /*code to print out the contents if file2 */ close(fd1); close(fd2); } main() { int fd1, fd2; fd1 = open("file2", O_WRONLY); fd2 = dup(fd1); write(fd1, "Jim\n", strlen("Jim\n")); write(fd2, "Plank\n", strlen("Plank\n")); /*code to print out the contents if file2 */ close(fd1); close(fd2); } Dup() example

Dup() contd. • Helpful in redirection of standard output to a file. • Example: int main() { int fd; fd = open("foo.bar",O_WRONLY | O_CREAT, S_IREAD | S_IWRITE ); close(1); dup(fd); /* fd will be duplicated into standard out's slot */ close(fd); /* close the extra slot */ printf("Hello, world!\n"); /* should go to file foo.bar */ exit (0); /* exit() will close the files */ }

Ioctl() system call • Used by block device drivers to control CD-ROM devices. • Mainly used with special character file like terminals. • Three modes: • Raw mode • Cooked mode • Cbreak mode

File directory management • Mkdir() – creates a directory • Rmdir() – removes a directory • Link() – used to create a symbolic link in another directory. • I-node of the file remains the same with link command. • The name of the file is only changed. • Every file is identified by a unique I-node number.

I-node diagram

Questions?

COP4600- Discussion Session 3- System Calls