The variable of the type pthread_mutex_t are treated like any other normal variable of the process. They even follow the same scop rules and life times as any other in built variable would if decleraded / defined in the same scope (as the pthread_mutex_t vars) of the program. Its, however, the pthread lib which gives them the special treatment to make them at par (to an extent) with semaphores and lets the programmer use them (through pthread lib APIs) to synchronise the threads created within the same process spece.
The case, however, is different with the semaphores. They are kernel entities and the Linux kernel remains aware of all the semaphores created in the system. They are not created with the user process memory space rather in a seperate space created by the kernel, for each of the semaphores created. Semaphores can never be accessible directly but always through syscalls.
Now the question of how they are accessed by different process; its the semid through which each process are able to refer to a perticular semaphore. Now how you get the semid? Its another kernel entity help us getting hold of a perticular semaphore , having of type key_t and you get a key_t type object through a call to ftok(). ftok() takes a path of any file name and an id to generate and return a unique key_t varable.
Look at the below code snippet to get an idea of semaohores:
Now try writing two programs and use the above code (as it is) into both of your programs and try synchronize them (as you do in mutext_t types of program) ; but hey do some searching yourself to find out what are all the operations possible on semaphores using it's id (the semid). As a hint semop() is one such thing, you might be interested ....
Also explore the semid, which is a structure defined like below:
Code:
struct semid_ds {
struct ipc_perm sem_perm; /* Ownership and permissions
time_t sem_otime; /* Last semop time */
time_t sem_ctime; /* Last change time */
unsigned short sem_nsems; /* No. of semaphores in set */
};
Happy Learnig!!!!
This User Gave Thanks to Praveen_218 For This Post:
Hi,
I am looking to use a semaphore for the first time in one of my scripts. I am just wondering if there are any simple examples or tutorials around?
I am a beginner so the simpler the better :)
Thanks
-Jaken (2 Replies)
In my server code there is a thread per client...
The server call accept() and after that start the thread.
So there is a thread for client that save in RAM the client's message, that will be send to other clients.
Now in RAM I have created a shared memory in which thread read and write(save)... (2 Replies)
Control two exclusively shared resources(semaphore). The two resources are two files. The producer will write even numbers to one file, and odd numbers to another one. The consumer respectively reads from each file until it gets 5 even numbers and 5 odd numbers.
Can any one help me with the... (0 Replies)
If I create a semaphore and then I fork a number of child processes then all the child process use that same semaphore.
Since the process address spaces are different rfom each other then how all the child process are able to access the same semaphore?
I understand that semaphore/mutex is at os... (0 Replies)
Can anyone tell me abt the Semaphore concept and what is semaphore???
is semaphore id is associated in terms as in resources like semaphore id 1 indicates cpu share unit is given and semaphore id 2 will indicate abt the memore or semaphore id 3 will tell us the i/o components (1 Reply)
I was asked to add this piece of code to a c program which I will execute through the shell:
for(long i = 0; i < NITER; i++)
{ sem_wait( &sema);
count++;
sem_post( &sema); }
I didn't get it, which is the critical section ? if it's "count++" how would a thread wake up in order to enter it... (1 Reply)
Discussion started by: uniran
1 Replies
LEARN ABOUT LINUX
semget
SEMGET(2) Linux Programmer's Manual SEMGET(2)NAME
semget - get a semaphore set identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
DESCRIPTION
The semget() system call returns the semaphore set identifier associated with the argument key. A new set of nsems semaphores is created
if key has the value IPC_PRIVATE or if no existing semaphore set is associated with key and IPC_CREAT is specified in semflg.
If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore set already exists for key, then semget() fails with errno set to EEXIST.
(This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)
Upon creation, the least significant 9 bits of the argument semflg define the permissions (for owner, group and others) for the semaphore
set. These bits have the same format, and the same meaning, as the mode argument of open(2) (though the execute permissions are not mean-
ingful for semaphores, and write permissions mean permission to alter semaphore values).
The values of the semaphores in a newly created set are indeterminate. (POSIX.1-2001 is explicit on this point.) Although Linux, like
many other implementations, initializes the semaphore values to 0, a portable application cannot rely on this: it should explicitly ini-
tialize the semaphores to the desired values.
When creating a new semaphore set, semget() initializes the set's associated data structure, semid_ds (see semctl(2)), as follows:
sem_perm.cuid and sem_perm.uid are set to the effective user ID of the calling process.
sem_perm.cgid and sem_perm.gid are set to the effective group ID of the calling process.
The least significant 9 bits of sem_perm.mode are set to the least significant 9 bits of semflg.
sem_nsems is set to the value of nsems.
sem_otime is set to 0.
sem_ctime is set to the current time.
The argument nsems can be 0 (a don't care) when a semaphore set is not being created. Otherwise nsems must be greater than 0 and less than
or equal to the maximum number of semaphores per semaphore set (SEMMSL).
If the semaphore set already exists, the permissions are verified.
RETURN VALUE
If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicat-
ing the error.
ERRORS
On failure errno will be set to one of the following:
EACCES A semaphore set exists for key, but the calling process does not have permission to access the set, and does not have the
CAP_IPC_OWNER capability.
EEXIST A semaphore set exists for key and semflg specified both IPC_CREAT and IPC_EXCL.
EINVAL nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL), or a semaphore set corre-
sponding to key already exists, and nsems is larger than the number of semaphores in that set.
ENOENT No semaphore set exists for key and semflg did not specify IPC_CREAT.
ENOMEM A semaphore set has to be created but the system does not have enough memory for the new data structure.
ENOSPC A semaphore set has to be created but the system limit for the maximum number of semaphore sets (SEMMNI), or the system wide maximum
number of semaphores (SEMMNS), would be exceeded.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the least
significant 9 bits of semflg and creates a new semaphore set (on success).
The following limits on semaphore set resources affect the semget() call:
SEMMNI System wide maximum number of semaphore sets: policy dependent (on Linux, this limit can be read and modified via the fourth field
of /proc/sys/kernel/sem).
SEMMSL Maximum number of semaphores per semid: implementation dependent (on Linux, this limit can be read and modified via the first field
of /proc/sys/kernel/sem).
SEMMNS System wide maximum number of semaphores: policy dependent (on Linux, this limit can be read and modified via the second field of
/proc/sys/kernel/sem). Values greater than SEMMSL * SEMMNI makes it irrelevant.
BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.
The semaphores in a set are not initialized by semget(). In order to initialize the semaphores, semctl(2) must be used to perform a SETVAL
or a SETALL operation on the semaphore set. (Where multiple peers do not know who will be the first to initialize the set, checking for a
nonzero sem_otime in the associated data structure retrieved by a semctl(2) IPC_STAT operation can be used to avoid races.)
SEE ALSO semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7), svipc(7)COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2004-05-27 SEMGET(2)
06-21-2010
do some searching yourself to find out what are all the operations possible on semaphores using it's id (the semid). As a hint semop() is one such thing, you might be interested .... 