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 HPUX
sem_wait
sem_wait(2) System Calls Manual sem_wait(2)NAME
sem_wait, sem_trywait - lock a POSIX semaphore
SYNOPSIS DESCRIPTION
is used to lock a semaphore. The calling thread will not return from its call to until one of the following events occur: it successfully
obtains a lock on the semaphore; it is interrupted by a signal or an error condition occurs.
is used to lock a semaphore, if it is available. The value of the semaphore sem is checked at some unspecified time during the call. If
the semaphore is available at the time its value is checked, the calling thread will atomically, with respect to the checking of the value,
lock the semaphore. The thread will now own a lock on the semaphore; the call will return successfully. If the semaphore is unavailable
at the time its value is checked, then the call returns -1 with errno set to EAGAIN.
If the specified semaphore referred to by sem is a named semaphore, then this semaphore must have been opened by the calling process with
The calling process must have both read and write permissions on the semaphore to perform these operations. The semaphore will be locked
upon successful return and will stay locked until it is explicitly released by a call to
To use this function, link in the realtime library by specifying on the compiler or linker command line.
EXAMPLES
The following call to will lock the semaphore sem.
The following call to will lock the semaphore sem, if it is available.
RETURN VALUE
A successful call to will return 0 and the calling thread will then own a lock on the semaphore. Otherwise, the call to will return -1
with errno set to the appropriate value of the error condition.
A successful call to will return 0, if the semaphore was available and the calling thread was able to lock the semaphore. Otherwise, the
call to will return -1 with errno set to the appropriate value of the error condition.
ERRORS
and fail and do not perform the requested operation if any of the following conditions are encountered:
[EPERM] The calling process does not have the privileges necessary to lock the semaphore.
[EAGAIN] The semaphore was not available and hence could not be locked by This error condition only occurs in
[EINVAL] The argument sem does not refer to a valid semaphore.
[EINTR] The function was interrupted by a signal
SEE ALSO sem_post(2), <semaphore.h>.
STANDARDS CONFORMANCE sem_wait(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 .... 