Programming

If say a process creates a semaphore/mutex etc then will this semaphore get created in its address space?
If yes then how an another process which wants to acquire this semaphore(created by the first process) will access the other
process address space to acquire the semaphore?
Where I mean in which memory is the sempahore, mtex etc located?
Anybody can pls explain me this?

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:

Code:

#include <sys/ipc.h>
#include <sys/sem.h>

key_t mySemaphore_key;
int semid;

mySemaphore_key = ftok("/tmp/a_disk_fileName", 'E');
semid = semget(mySemaphore_key, 10, 0666 | IPC_CREAT);

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!!!!

a nice explaination