What API does the OS provide to the user program.

What API does the OS provide to the user programs?

• API = Application Programming Interface = functions available to write user programs
• API provided by OS is a set of “system calls”,
  – A system call is a function call into OS code that runs at a higher privilege level of the CPU.
  – Sensitive operations (e.g., access to hardware) are allowed only at a higher privilege level
  – Some “blocking” system calls cause the process to be blocked and de scheduled (e.g., read from disk)

So, should we rewrite programs for each OS?

• POSIX API: a standard set of system calls that an OS must implement
  – Programs written to the POSIX API can run on any POSIX compliant OS
  – Most modern OSes are POSIX compliant
  – Ensures program portability
• Program language libraries hide the details of invoking system calls
  – The printf function in the C library calls the writing system call to write to the screen
  – User programs usually do not need to worry about invoking system calls

Process related system calls (in Unix)

• fork() creates a new child process
  – All processes are created by forking from a parent
  – The init process is the ancestor of all processes
• exec() makes a process execute a given executable
• exit() terminates a process
• wait() causes a parent to block until the child terminates
• Many variants exist of the above system calls with different arguments

What happens during a fork?

• A new process is created by making a copy of parent’s memory image
• The new process is added to the OS process list and scheduled
• Parent and child start the execution just after the fork (with different return values)
• Parent and child execute and modify the memory data independently

#include #include #include int main(int argc,char *argv[]){ 

printf("helloworld(pid:%d)\n",(int)getpid());
int rc=fork();
if(rc<0 br="" failed="" fork="" fprintf="" n="" stderr=""> exit(1);}else
if(rc==0) {  int rc=fork(); if(rc<0 br="" failed="" fork="" fprintf="" n="" stderr=""> exit(1);}else
if(rc==0) {  if(rc==0) { printf("hello, I am child (pid:%d)\n",(int)getpid()); }else{ printf("hello,I am parent of %d (pid:%d)\n",rc,(int)getpid());} return 0; }

Waiting for children to die…

• Process termination scenarios
  – By calling exit() (exit is called automatically when end of main is reached)
  – OS terminates a misbehaving process
• Terminated process exists as a zombie
• When a parent calls wait(), zombie child is cleaned up or “reaped”
• wait() blocks in parent until child terminates (non-blocking ways to invoke wait exist)
• What if parent terminates before child? init process adopts orphans and reaps them

More funky things about the shell

• Shell can manipulate the child in strange ways
• Suppose you want to redirect output from a command to a file
• prompt> ls > foo.txt
• Shell spawns a child, rewires its standard output to a file, then calls exec on the child

Calling fork(),wait(),and exec():-

#include

#include

#include

#include

#include

int main(){

printf("hello world (pid=%d)\n",(int)getpid());

int rc=fork();

if(rc<0 exit="" failed="" font="" fork="" nbsp="">

fprintf(stderr,"fork failded\n");

exit(1);}

esle if(rc==0){

//child(new process)

Printf("hello, I am child (pid:%d)\n",(int)getpid());

char *myargs[3];

myargs[0]=strdup("wc"):  // program :"wc:"(word count)

myargs[1]=strdup("p3.c");  //argument: file to count 

myargs[2]=NULL;   //marks end of array

execvp(myargs[0],myargs);  //runs word count

printf("this shouldn't print out:");}

else{          //parent goes down this path (main)

int wc=wait(NULL);

print("hello ,I am parent of %d (wc:%d)(pid:%d)\n",rc,wc,(int)getpid()):

}return 0;

}