EmbLogic » wait https://www.emblogic.com/blog Embedded System and ARM Training Tue, 03 Mar 2020 13:00:06 +0000 en-US hourly 1 http://wordpress.org/?v=3.9.1 waiting for a process https://www.emblogic.com/blog/01/waiting-for-a-process/ https://www.emblogic.com/blog/01/waiting-for-a-process/#comments Fri, 15 Jan 2016 10:36:05 +0000 http://www.emblogic.com/blog/?p=13160 Continue reading →]]> Waiting For a Process
=====================
- Sometime we would like to find out when a child process has finished.
- It is needed for parent process to wait until the child finishes before continuing by calling wait.
- It has prototype :
pid_t wait(int *stat_loc);
- It is declared in header :
#include<sys/wait.h>
- It has a return type pid_t which is declared in header :
#include<sys/types.h>
- The wait system call causes a parent process to pause until one of its child processes is stopped.
- The wait returns PID of child process which is terminated.
- The status information determine the exit status of child process (i.e  value returned from main or passed to exit)
- If stat_loc is not a null pointer, the status information will be written to location to which it points.
- The status information can be determined by using macros defined in header sys/wait.h

Macro                                        Defination
WIFEXITED(stat_val)          Nonzero if child terminated normally
WEXITSTATUS(stat_val)    If WIFEXITED is nonzero, this returns child exit code

WIFSIGNALED(stat_val)    Nonzero if child is terminated on an uncaught signal
WTERMSIG(stat_val)           If WIFSIGNALED is nonzero, this returns signal number

WIFSTOPPED(stat_val)       Nonzero if the child has stopped
WSTOPSIG(stat_val)            If WIFSTOPPED is nonzero, this returns signal number

- The wait() is used when a result of process done by child is used by the parent for further processing.
- For eg :-
In a program child process write to file1 and parent has to read from (the written data of)file1.
If the scheduler runs read() command of the parent process before the write() command of child process, it reads nothing.
- So, wait() is used before read() statement in parent process to avoid the unnecessory error.

]]> https://www.emblogic.com/blog/01/waiting-for-a-process/feed/ 0 Completed creating a new process using fork, how orphan process works, wait by parent till child exit https://www.emblogic.com/blog/09/completed-creating-a-new-process-using-fork-how-orphan-process-works-wait-by-parent-till-child-exit/ https://www.emblogic.com/blog/09/completed-creating-a-new-process-using-fork-how-orphan-process-works-wait-by-parent-till-child-exit/#comments Mon, 22 Sep 2014 13:03:41 +0000 http://www.emblogic.com/blog/?p=11512 Continue reading →]]> RCS file: fork.c,v
Working file: fork.c
head: 1.4
branch:
locks: strict
access list:
symbolic names:
keyword substitution: kv
total revisions: 4;    selected revisions: 4
description:
program to create a duplicate process using fork
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revision 1.4
date: 2014/09/18 17:37:44;  author: root;  state: Exp;  lines: +2 -0
print child and its parent id
check that orphan process have 1 as a parent id
use sleep command to make sure that child not become orphan
—————————-
revision 1.3
date: 2014/09/18 17:34:40;  author: root;  state: Exp;  lines: +2 -2
get pid of child
print pid of parent
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revision 1.2
date: 2014/09/18 17:30:58;  author: root;  state: Exp;  lines: +11 -1
create a child process using fork
define the body of child and parent using if and else
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revision 1.1
date: 2014/09/18 17:27:22;  author: root;  state: Exp;
Initial revision
=============================================================================

RCS file: fork2.c,v
Working file: fork2.c
head: 1.3
branch:
locks: strict
access list:
symbolic names:
keyword substitution: kv
total revisions: 3;    selected revisions: 3
description:
use multiple fork to create more child and parent process
—————————-
revision 1.3
date: 2014/09/19 12:49:25;  author: root;  state: Exp;  lines: +5 -0
*** empty log message ***
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revision 1.2
date: 2014/09/19 12:03:53;  author: root;  state: Exp;  lines: +2 -3
create more process using multiple fork statement
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revision 1.1
date: 2014/09/19 11:11:06;  author: root;  state: Exp;
Initial revision
=============================================================================

RCS file: orphan.c,v
Working file: orphan.c
head: 1.4
branch:
locks: strict
access list:
symbolic names:
keyword substitution: kv
total revisions: 4;    selected revisions: 4
description:
program to make a process orphan
—————————-
revision 1.4
date: 2014/09/20 09:36:08;  author: root;  state: Exp;  lines: +4 -2
make child orphan and verify with output
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revision 1.3
date: 2014/09/19 13:54:25;  author: root;  state: Exp;  lines: +19 -10
use if else to notify when the child became orphan
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revision 1.2
date: 2014/09/19 13:42:45;  author: root;  state: Exp;  lines: +4 -3
get child id
print parent id
—————————-
revision 1.1
date: 2014/09/19 13:16:57;  author: root;  state: Exp;
Initial revision
=============================================================================

RCS file: wait.c,v
Working file: wait.c
head: 1.1
branch:
locks: strict
access list:
symbolic names:
keyword substitution: kv
total revisions: 1;    selected revisions: 1
description:
parent will wait till the child exit()
—————————-
revision 1.1
date: 2014/09/20 14:26:58;  author: root;  state: Exp;
Initial revision
=============================================================================

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