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  

2. 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 2^nd 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.