disastrOS-semaphores

An implementation of binary semaphores for disastrOS, for the course of Operative Systems held by Giorgio Grisetti in 2018-19.

Instructions

To compile just run

make

then

./disastrOS_test

to start the tests.

Usage

Semaphores are system structure that wrap a counter that can be increased or decreased. Whenever it is decreased and its current value is <= 0 the process who decremented it is put in a waiting queue whose head is detached every time the counter is incremented.

To open a semaphore in disastrOS, call disastrOS_semOpen(int semnum). It creates a system semaphore with global id semnum (must be >= 0) if it does not exist in the system and opens it for the calling process, putting it in the list of the opened semaphores of the process. If the semaphore with the given id already exists, this function just opens it for the process. If everything went fine it returns an integer representing the descriptor of the semaphore, otherwise an error code (see section below).

Both disastrOS_wait(int semfd) and disastrOS_semPost(int semfd) are used to change the value of the counter of the semaphore with descriptor semfd (the process should have opened the semaphore before calling these functions), respectively by decreasing and increasing it. On success they return 0, an error code otherwise.

Semaphores are closed by calling disastrOS_semClose(int semfd) by passing an integer representig the descriptor. When all processes using a certain semaphore closed it, it is automatically unlinked.

Error codes

In include/disastrOS_constants.h are defined the following error codes:

• DSOS_ESEMOPEN returned by disastrOS_semOpen when no more semaphore can be created in the system
• DSOS_ESEMNEG returned by disastrOS_semOpen when is opened a semaphore with negative id
• DSOS_ESEMNOFD returned by disastrOS_semOpen when it failed to create the SemDescriptor to the given semaphore
• DSOS_ESEMCLOSE returned by disastrOS_semClose when a process is trying to close a semaphore that does not exist or that it is not opened by the process
• DSOS_ENOSEM returned by disastrOS_semWait and by disastrOS_semPost when a process is trying to wait or post to a semaphore that is not opened by it or that does not exist.

Valgrind test

Tested with valgrind-3.12.0.SVN, this implementation presents no memory leak. Valgrind output on an execution of the tests:

==17200== 
==17200== HEAP SUMMARY:
==17200==     in use at exit: 0 bytes in 0 blocks
==17200==   total heap usage: 2 allocs, 2 frees, 2,048 bytes allocated
==17200== 
==17200== All heap blocks were freed -- no leaks are possible
==17200== 
==17200== For counts of detected and suppressed errors, rerun with: -v
==17200== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)