Working With Wait() In UNIX
Wait , Wait p id ()
Wait for process to change state
# include <sys / types.h>
# include <sys / wait.h>
P id – t Wait (int * status)
P id – t Wait (p id – t p id, int * status , int options);
Int wait id (id type id type, id – t id, sigin to – t * in fop, int options);
Wait ()
The wait () system call suspends executor of the current . process until one of its children terminates the call wait (& status ) is equivalent to
Wait p id (- 1, & status , 0);
The wait p id () system call suspends execution of the current a child specified by p id argument has changed state , by default wait p id () waits only for terminated children . but this behavior is modified via the options argument as described below
< -1 - meaning wait for any child process whose process group ID is equal to the absolute value of P id
-1 - meaning wait for any child process
0 - meaning wait for any child process whose process group ID is equal to that of calling process
>0 - meaning wait for the child whose process ID is equal to value of Pid
Wait () - is only for parent to wait and not child to wait () for parent & exited value is calls. wait for child exd, this a voids child to be
Wait () - on success, returns the process ID of the terminated child , on error , -1 is returned.
Wait P id() - on Success , returns the process ID of the child whose state has changed ; on error , -1 is returned if ‘WNO HAN &’ was specified and no child specified by p id has yet changed state then o is returned .
Wait id () - Returns 0 on success or if WNO HAN & Was specified and no child specified by id has yet changed state ; on error , -1 is returned.
Errors
ECHID - (for wait ()) the calling process does not have any un waited – for children
ECHILD - (for wait p id () or wait id ())
[ The process specified by p id (wait p id ()) or id type and id (wait id ()) does not exits or is not a child of the calling process . (This can happen for one’s own child if the action for SIGCHILD is set to SIG. IGN) ]
EINUAL - The options argument was in valid
EINTR - WNDHANG was not set and an unblocked signal or a SIGCHILD was caught.
Copy – on – Write ()
A separate copy of data will be created , whenever parent/ child modifies the common data.
#include <stdio.h>
#include <stdlib.h>
Int g = 0;
Main ()
{
Int l = 0;
P F (“l & g modified to l= 20, g = 10 globally \n ”);
L = 20;
G= 10;
If (fork () )
{
PF (“Parent : l = %d, g = %d\n”, l ,g);
}
Else
{
PF (“Child : l = %d, g = %d\n”, l ,g);
L ++;
G++;
PF (“l ++, g++ done in child \ n”);
PF (“child : l + % d, g = %d \n” l , g);
}
}
Output
L = 0, g =0
L & g modified to l = 20, g = 10 globally
Child : l =20, g =10
L++, g ++ done in child
Child : l =21, g =11
Parent : l =20, g =10
- Parent & child share common code
- For’ Data protection ‘ data modified in one process will not be reflected in another process
- Data protection in done through ‘ copy – on – write ’
So, data modified by in child is not reflected in parent.
Name
-exit, - Exit – terminate the current process
#include <unistd.h> #include <stdlib.h>
Void – exit (int status ) Void – Exit (int status )
- The function - Exit () is equivalent to –exit ()
- The function – exit () terminates the calling process “immediately” . Any open file descriptors belonging to the process are closed; any children of the process are inherited by process in it, and the process’s parent is sent a SIGCHILD signal
- The value status is returned to the parent process as the process’s exit status and can be effected using one of the wait () family of calls.
Return Value
- These functions do not return
- Exit values are in the range of 0 to 255 and any exit value does not effect the process.
- Exit value is short int of 2 bytes and is collected by wait () function in MSB.
#include <stdio. h>
#include <stdlib.h>
Main ()
{
If (fork ())
{
Int st, ret
Pf(“parent executing \ n”);
Pf(“parent Waiting \ n”);
Ret = Wait (& st);
Pf(“child exited with % d as collected in MSB \ n”, st);
St = st >> 8 ; // Hence it is shifted 8 Times
Pf(“child exited with % d \ n”, st);
Pf(“parent about to exit \ n”);
Exit (0); // this exit value is returned to bash
}
Else
{
Int t ;
Srand (get p id ());
T = rand () % 10 +1 ;
Sleep (t);
Exit (i);
}
}
Output
Parent executing
Parent waiting
Child exited with 256 as collected in MSB
Child exited with 1
Parent about to exit
W. a. p to input two small integers (in between 1 to 50) from command line create a new process by fork code. The sum of two integers by the child process and print the result
Note
These should not be any print statement in child
#include <stdio. h>
#include <stdlib.h>
Main (int argc, char ** arg v)
{
If (arg c ! =3)
{
Pf (“Usage : ./a.out <Integer 1> <Integer 2> ” /n);
Return;
}
If (fork ())
{
Int st, ret;
Ret = wait (& st);
St = st >>8;
Print F(“Sum = % d /n”, st);
Exit (0);
}
Else
{
Int a, b ;
A = a to I (arg V [1]);
b= a to I (arg V [2]);
exit (a + b);
}
}
Output
./a.out 12 14
Sum = 26
- W. a. p to create three children from common parent & all the children should delay for randomly generated time of 1 to 10 sec. modify the parent such a manner none of child become orphan.
#include <stdio. h>
#include <stdlib.h>
Main ()
{
If (fork ())
{
If (fork ())
{
If (fork ())
{
//parent process
Int st , ret;
PF (“ parent : p id = % d, p pid = % d/n” get p id (), get p id ());
While (ret ! = -1)
{
Ret = wait (& st ),
St >>=8;
If (st = = 1)
PF (“child 1 exited with % d /n ”, st);
If (st = = 2)
PF (“child 2 exited with % d /n ”, st);
If (st = = 3)
PF (“child 3 exited with % d /n ”, st);
}
Exit (0);
}
Else
{
//child 3 process
Int t;
Srand (get p id ());
T = rand () % 10 +1;
Sleep (t);
PF (“child 3 : p id = % d, p pid = %d \ n ”, get p id (), get p pid ());
Exit (3);
}
}
Else
{
Int t;
Srand (get p id ());
T = rand () % 10 +1;
Sleep (t);
PF (“child 2 : p id = % d, p pid = %d \ n ”, get p id (), get p pid ());
Exit (2);
}
}
Else
{
//child 1 process
Int t;
Srand (get p id ());
T = rand () % 10 +1;
Sleep (t);
PF (“child 3 : p id = % d, p pid = %d \ n ”, get p id (), get p pid ());
Exit (1);
}
}
OUTPUT
Here no child is exited after execution. Hence no child becomes orphan.
(ii) Make the parent wait only for 2nd child . If exit states is ‘2’, then wait ()
#include <stdio.h>
#include <stdlib.h>
Main ()
{
If (fork ())
{
If (fork ())
{
If (fork ())
{
//parent process
Int st, ret;
PF(“Parent : p id = % d, p pid = % d\n ”, get p id (), get p pid ());
While (ret ! =1)
{
Ret = wait (& st)
St>>8;
If (st = = 2)
{
Pf (“child 2 exited with % d \n ” , st);
Exit (0);
}
Else
{
//child 3 process
Int t ;
Srand (get p id ());
T = rand () % 10 +1;
Sleep (t);
Pf(“Child : p id = % d, p pid = % d\n ”, get p id (), get p pid ());
Exit (3);
Else
{
//child 2 process
Int t ;
Srand (get p id ());
T = rand () % 10 +1;
Sleep (t);
Pf(“Child 2 : p id = % d, p pid = % d\n ”, get p id (), get p pid ());
}
}
Else
{
//child 1process
Int t ;
Srand (get p id ());
T = rand () % 10 +1;
Sleep (t);
Pf(“Child 1 : p id = % d, p pid = % d\n ”, get p id (), get p pid ());
}
}
Output:
Hence is child 2 is executed , then child 1 & child 3 becomes orphan.
Depending on, random generator of child 2 execution the process that are executed after child 2 becomes orphan.