EmbLogic's Blog

Semaphore Calls and Datatypes
1 union semun
This is a union the semaphore library requires.it’s not specified in the sys/sem.h header file and we’re required to specify it ourselves. It’s needed by semctl()
union semun
{
int val; // value for SETVAL
struct semid_ds* buf; // buffer for IPC_STAT, IPC_SET
unsigned short* array; // array for GETALL, SETALL
struct seminfo* __buf; // buffer for IPC_INFO
};

2 struct sembuf
Defines an operation to be performed by the semop()call. The members you need to know about are:
Sem num is the index of the first semaphore to perform the operation on. Note that the index starts from zero.
Sem op is the operation to perform. Semop() covers this in greater detail.
Sem flg specifies how the operation is supposed to be treated when the process exits. Usually you’ll want this to bem SEM_UNDO so it will be undone when the process exits. Passing the flag IPC NOWAIT will make sure the call to semop() doesn’t block and instead fails if it would have blocked.

3 semget()
Gets a semaphore set. The value returned is its id, for use with other calls.
int semget(key_t key, int n, int flags);
key is the key associated with the semaphore set you want. n is the number of semaphores the set should contain. Flags specifies how how the set should be allocated.

4 semop()
Performs a semaphore operation (i.e. incrementing, decrementing, etc.) on the selected members of a semaphore set.
int semop (int id, struct sembuf* op, unsigned n);
id is the semaphore set’s id.
Op is the operation to perform.
N is the number of semaphores to affect. You’ll nearly always be passing in a value of 1 here.
struct sembuf ’s sem op field is important. It specifies what you want to do to the semaphore, be it incrementing, decrementing, or toasting over an open fire.
A non-zero value will be added to the semaphore’s value. Note that this means negative values indicate subtraction.
A value of zero will make the operation block until the semaphore value becomes zero.

5 semctl()
int semctl(int id, int iSem, int cmd, union semun arg);
id is the semaphore set id.
Isem is the semaphore you want to twiddle. Only valid with some of the commands.
Cmd is the command you want to perform.
Arg is used for fiddling with semaphore values. There are two values for
cmd worth looking at: SETVAL and IPC_RMID. For details on the others,
type
man semctl

5.1 IPC_RMID
Automatically removes the semaphore set and awakens any processes waiting for a semaphore in the set to unlock. This should be done by the last process using the semaphores. Semctl() will return -1 if something goes horribly wrong with this.
5.2 SETVAL
it is used to set the value of semaphore.

Processes communicate with each other and with the kernel to coordinate their activities. Linux supports a number of Inter-Process Communication (IPC) mechanisms. Signals and pipes are two of them but Linux also supports the System V IPC mechanisms named after the Unix release in which they first appeared.Different processes on the same machine/OS use Inter-process Communication (IPC) to communicate with each other. There are several IPC mechanisms, but almost all typically boil down to carefully controlled access to shared memory. The OS will allocate a block of memory specifically for this purpose.Interprocess communication (IPC) is a set of programming interfaces that allow a programmer to coordinate activities among different program processes that can run concurrently in an operating system. This allows a program to handle many user requests at the same time.

Semaphores are another IPC mechanism available when developing on UNIX. They allow different processes to synchronise their access to certain resources. In Computer Language, the most common and simplest kind of semaphore is called a binary semaphore because they have two states
locked or unlocked. These act much like traffic lights.
When a process wants exclusive access to a resource, shared memory being an example, they attempt to lock the semaphore associated with that resource. If the semaphore they are attempting to lock is already locked, the caller is suspended, otherwise they are granted to lock. When you’ve finished doing whatever you wanted to do, you unlock the resource and any processes that have attempted to lock that semaphore in the meantime are woken up again to attempt the lock again. This way only one process can have access to the resource at once.
In addition, semaphores can be used as a signalling mechanism. In the simple chat application. the instance of the application acting as the server locks a semaphore to suspend it until a client application unlocks the semaphore to signal that it’s connected to the server and mapped the
shared memory area onto its own address space

RCS file: create_node.c,v
Working file: create_node.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
creating node in the linklist
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revision 1.1 locked by: root;
date: 2017/04/02 05:11:24; author: root; state: Exp;
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RCS file: delete_beg.c,v
Working file: delete_beg.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
in this function we are deleting node from begining of linklist
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date: 2017/04/07 04:32:28; author: root; state: Exp;
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RCS file: delete_end.c,v
Working file: delete_end.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
in this deletion of node take place from end
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revision 1.1 locked by: root;
date: 2017/04/07 04:40:10; author: root; state: Exp;
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RCS file: delete_key.c,v
Working file: delete_key.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
in this deletion o node take place where key value matches with entered value
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revision 1.1 locked by: root;
date: 2017/04/07 04:34:55; author: root; state: Exp;
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RCS file: deletemenu.c,v
Working file: deletemenu.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
in this menu list is shown from where you want to delete node
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revision 1.1 locked by: root;
date: 2017/04/07 04:35:46; author: root; state: Exp;
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RCS file: delete_nth.c,v
Working file: delete_nth.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
in this the node you want to delete is processing
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revision 1.1 locked by: root;
date: 2017/04/07 04:36:16; author: root; state: Exp;
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RCS file: delete_oper.c,v
Working file: delete_oper.c
head: 1.2
branch:
locks: strict
root: 1.2
access list:
symbolic names:
keyword substitution: kv
total revisions: 2; selected revisions: 2
description:
all delete operations is in this function
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RCS file: insert_beg.c,v
Working file: insert_beg.c
head: 1.4
branch:
locks: strict
root: 1.4
access list:
symbolic names:
keyword substitution: kv
total revisions: 4; selected revisions: 4
description:
insert node in the begining off linklist
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s.e
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se
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insert node in begining of the link list.
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date: 2017/04/02 05:11:44; author: root; state: Exp;
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RCS file: insert_end.c,v
Working file: insert_end.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
insert node at the end of linklist
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revision 1.1 locked by: root;
date: 2017/04/02 05:12:06; author: root; state: Exp;
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=============================================================================

RCS file: insert_key.c,v
Working file: insert_key.c
head: 1.4
branch:
locks: strict
root: 1.4
access list:
symbolic names:
keyword substitution: kv
total revisions: 4; selected revisions: 4
description:
insert node after the key value
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date: 2017/04/07 04:37:15; author: root; state: Exp; lines: +3 -0
s.e
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date: 2017/04/03 06:46:15; author: root; state: Exp; lines: +2 -3
insert after the key value
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revision 1.2
date: 2017/04/03 06:09:09; author: root; state: Exp; lines: +17 -1
insert where key value is matched
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revision 1.1
date: 2017/04/02 05:12:27; author: root; state: Exp;
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RCS file: insertmenu.c,v
Working file: insertmenu.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
displays the inserting option.
in this menulist shown where to insert node
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revision 1.1 locked by: root;
date: 2017/04/02 05:13:10; author: root; state: Exp;
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RCS file: insert_nth.c,v
Working file: insert_nth.c
head: 1.3
branch:
locks: strict
root: 1.3
access list:
symbolic names:
keyword substitution: kv
total revisions: 3; selected revisions: 3
description:
insert node at desired or nth location
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date: 2017/04/07 04:37:30; author: root; state: Exp; lines: +30 -3
S.E
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date: 2017/04/03 06:47:33; author: root; state: Exp; lines: +17 -1
insert node at the given or desired location in linklist
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revision 1.1
date: 2017/04/02 05:14:02; author: root; state: Exp;
Initial revision
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RCS file: insert_oper.c,v
Working file: insert_oper.c
head: 1.6
branch:
locks: strict
root: 1.6
access list:
symbolic names:
keyword substitution: kv
total revisions: 6; selected revisions: 6
description:
all operation off insert are displaying here
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insert nth location and insert at key value function calling
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RCS file: main.c,v
Working file: main.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
this is the main program file
which calls the function
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revision 1.1 locked by: root;
date: 2017/04/02 05:15:21; author: root; state: Exp;
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RCS file: mainmenu.c,v
Working file: mainmenu.c
head: 1.2
branch:
locks: strict
root: 1.2
access list:
symbolic names:
keyword substitution: kv
total revisions: 2; selected revisions: 2
description:
a list of menu displays here
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date: 2017/04/02 05:20:42; author: root; state: Exp; lines: +1 -1
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date: 2017/04/02 05:16:13; author: root; state: Exp;
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RCS file: operation.c,v
Working file: operation.c
head: 1.3
branch:
locks: strict
root: 1.3
access list:
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keyword substitution: kv
total revisions: 3; selected revisions: 3
description:
operation to be implemented are displaying here
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deletion func() called
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date: 2017/04/02 05:16:44; author: root; state: Exp;
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RCS file: traverse.c,v
Working file: traverse.c
head: 1.5
branch:
locks: strict
root: 1.5
access list:
symbolic names:
keyword substitution: kv
total revisions: 5; selected revisions: 5
description:
it displays the all elements of linklist
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ssyntax error
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RCS file: header.h,v
Working file: header.h
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
all header files are included here
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date: 2017/04/02 05:17:47; author: root; state: Exp;
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RCS file: prototype.h,v
Working file: prototype.h
head: 1.2
branch:
locks: strict
root: 1.2
access list:
symbolic names:
keyword substitution: kv
total revisions: 2; selected revisions: 2
description:
all functions are declared here
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all prototype is declared
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For one member

#include”header.h”

#define MYFILE1 “./myfile1″
#define MYFILE2 “./myfile2″

int main()
{
printf(“\n %s : bigen”,__FILE__);

struct pollfd mypol = {STDIN_FILENO,POLLIN|POLLPRI};

int ret,fd1,fd2,noc;
char msgr1[50],msgw[50],msgr2[50];

if(access(MYFILE1,F_OK) != 0)
{

ret= mkfifo(MYFILE1,666);
if(ret== -1)
{
printf(“mkfifo”);
exit(EXIT_FAILURE);
}

printf(“\n %s-> fifo created sucessfully”,__FILE__);
}
else
printf(“\n %s-> fifo already created “,__FILE__);

if(access(MYFILE2,F_OK) != 0)
{

ret= mkfifo(MYFILE2,666);
if(ret== -1)
{
printf(“mkfifo”);
exit(EXIT_FAILURE);
}

printf(“\n %s :1-> fifo created sucessfully”,__FILE__, ret);
}
else
printf(“\n %s2-> fifo already created “,__FILE__);

fd1= open(MYFILE1,O_RDWR|O_NONBLOCK);
if(fd1 == -1)
{
printf(“open”);
exit(EXIT_FAILURE);
}
else
printf(“\n %s fifo opened sucessfully :%d  “,__FILE__,fd1);

fd2= open(MYFILE2,O_RDWR|O_NONBLOCK);
if(fd1 == -1)
{
printf(“open”);
exit(EXIT_FAILURE);
}
else
printf(“\n %s fifo opened sucessfully :%d  “,__FILE__,fd1);

printf(“\n %s welcome to chat …………..\n  “,__FILE__);

while(1)
{
memset(msgr2,”,50);
memset(msgr1,”,50);
memset(msgw,”,50);
if(poll(&mypol,1,500))
{
scanf(” %[^\n]“,msgw);
write(fd1,msgw,50);
write(fd2,msgw,50);
}
else
{
noc=read(fd1,msgr1,50);
noc=read(fd2,msgr2,50);
}
if(strlen(msgr1))
printf(“\n\t\t MEMBER 2 : %s \n  “,msgr1);

if(strlen(msgr2))
printf(“\n\t\t MEMBER 3 : %s \n  “,msgr2);

if(strcmp(msgw,”end”)==0 || strcmp(msgr1,”end”)==0||strcmp(msgr2,”end”)==0)
break;
}

printf(“\n  %s : end”,__FILE__);

return 0;

}
……………………………………………………………………………….end…………………………………………………………………………….

For 2nd  member

#include”header.h”

#define MYFILE1 “./myfile1″
#define MYFILE3 “./myfile3″

int main()
{
printf(“\n %s : bigen”,__FILE__);

struct pollfd mypol = {STDIN_FILENO,POLLIN|POLLPRI};

int ret,fd1,fd2,noc;
char msgr1[50],msgw[50],msgr2[50];

if(access(MYFILE1,F_OK) != 0)
{

ret= mkfifo(MYFILE1,666);
if(ret== -1)
{
printf(“mkfifo”);
exit(EXIT_FAILURE);
}

printf(“\n %s-> fifo created sucessfully”,__FILE__);
}
else
printf(“\n %s-> fifo already created “,__FILE__);

if(access(MYFILE3,F_OK) != 0)
{

ret= mkfifo(MYFILE3,666);
if(ret== -1)
{
printf(“mkfifo”);
exit(EXIT_FAILURE);
}

printf(“\n %s :1-> fifo created sucessfully”,__FILE__, ret);
}
else
printf(“\n %s2-> fifo already created “,__FILE__);

fd1= open(MYFILE1,O_RDWR|O_NONBLOCK);
if(fd1 == -1)
{
printf(“open”);
exit(EXIT_FAILURE);
}
else
printf(“\n %s fifo opened sucessfully :%d  “,__FILE__,fd1);

fd2= open(MYFILE3,O_RDWR|O_NONBLOCK);
if(fd1 == -1)
{
printf(“open”);
exit(EXIT_FAILURE);
}
else
printf(“\n %s fifo opened sucessfully :%d  “,__FILE__,fd1);

printf(“\n %s welcome to chat …………..\n  “,__FILE__);

while(1)
{
memset(msgr2,”,50);
memset(msgr1,”,50);
memset(msgw,”,50);
if(poll(&mypol,1,500))
{
scanf(” %[^\n]“,msgw);
write(fd1,msgw,50);
write(fd2,msgw,50);
}
else
{
noc=read(fd1,msgr1,50);
noc=read(fd2,msgr2,50);
}
if(strlen(msgr1))
printf(“\n\t\t MEMBER 1 : %s \n  “,msgr1);

if(strlen(msgr2))
printf(“\n\t\t MEMBER 3 : %s \n  “,msgr2);

if(strcmp(msgw,”end”)==0 || strcmp(msgr1,”end”)==0||strcmp(msgr2,”end”)==0)
break;
}

printf(“\n  %s : end”,__FILE__);

return 0;

}
……………………………………………………………………….End………………………………………………………………………….

for 3rd  member

#include”header.h”

#define MYFILE2 “./myfile2″
#define MYFILE3 “./myfile3″

int main()
{
printf(“\n %s : bigen”,__FILE__);

struct pollfd mypol = {STDIN_FILENO,POLLIN|POLLPRI};

int ret,fd1,fd2,noc;
char msgr1[50],msgw[50],msgr2[50];

if(access(MYFILE2,F_OK) != 0)
{

ret= mkfifo(MYFILE2,666);
if(ret== -1)
{
printf(“mkfifo”);
exit(EXIT_FAILURE);
}

printf(“\n %s-> fifo created sucessfully”,__FILE__);
}
else
printf(“\n %s-> fifo already created “,__FILE__);

if(access(MYFILE3,F_OK) != 0)
{

ret= mkfifo(MYFILE3,666);
if(ret== -1)
{
printf(“mkfifo”);
exit(EXIT_FAILURE);
}

printf(“\n %s :1-> fifo created sucessfully”,__FILE__, ret);
}
else
printf(“\n %s2-> fifo already created “,__FILE__);

fd1= open(MYFILE2,O_RDWR|O_NONBLOCK);
if(fd1 == -1)
{
printf(“open”);
exit(EXIT_FAILURE);
}
else
printf(“\n %s fifo opened sucessfully :%d  “,__FILE__,fd1);

fd2= open(MYFILE3,O_RDWR|O_NONBLOCK);
if(fd1 == -1)
{
printf(“open”);
exit(EXIT_FAILURE);
}
else
printf(“\n %s fifo opened sucessfully :%d  “,__FILE__,fd1);

printf(“\n %s welcome to chat …………..\n  “,__FILE__);

while(1)
{
memset(msgr2,”,50);
memset(msgr1,”,50);
memset(msgw,”,50);
if(poll(&mypol,1,500))
{
scanf(” %[^\n]“,msgw);
write(fd1,msgw,50);
write(fd2,msgw,50);
}
else
{
noc=read(fd1,msgr1,50);
noc=read(fd2,msgr2,50);
}
if(strlen(msgr1))
printf(“\n\t\t MEMBER 1 : %s \n  “,msgr1);

if(strlen(msgr2))
printf(“\n\t\t MEMBER 2 : %s \n  “,msgr2);

if(strcmp(msgw,”end”)==0 || strcmp(msgr1,”end”)==0||strcmp(msgr2,”end”)==0)
break;
}

printf(“\n  %s : end”,__FILE__);

return 0;

}
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