15.6 Exercise: POSIX Named Semaphores

This exercise describes an implementation of POSIX:SEM-like named semaphores in terms of semaphores sets. Represent the named semaphore by a structure of type mysem_t . The mysemn.h file should include the definition of mysem_t and the prototypes of the following functions.

 mysem_t *mysem_open(const char *name, int oflag, mode_t mode,                      unsigned int value); int mysem_close(mysem_t *sem); int mysem_unlink(const char *name); int mysem_wait(mysem_t *sem); int mysem_post(mysem_t *sem); 

The mysem_open function returns NULL and sets errno when there is an error. All the other functions return “1 and set errno when there is an error. To simplify the interface, always call mysem_open with four parameters.

Represent the named semaphore by an ordinary file that contains the semaphore ID of the semaphore set used to implement the POSIX semaphore. First try to open the file with open , using O_CREAT O_EXCL . If you created the file, use fdopen to get a FILE pointer for the file. Allocate the semaphore set and store the ID in the file. If the file already exists, open the file for reading with fopen . In either case, return the file pointer. The mysem_t data type will just be the type FILE .

The mysem_close function makes the semaphore inaccessible to the caller by closing the file. The mysem_unlink function deletes the semaphore and its corresponding file. The mysem_wait function decrements the semaphore, and the mysem_post function increments the semaphore. Each function reads the semaphore ID from the file by first calling rewind and then reading an integer. It is possible to get an end-of-file if the process that created the semaphore has not yet written to the file. In this case, try again.

Put all the semaphore functions in a separate library and treat this as an object in which the only items with external linkage are the five functions listed above. Do not worry about race conditions in using mysem_open to create the file until a rudimentary version of the test program works. Devise a mechanism that frees the semaphore set after the last mysem_unlink but only after the last process closes this semaphore. The mysem_unlink cannot directly do the freeing because other processes may still have the semaphore open. One possibility is to have mysem_close check the link count in the inode and free the semaphore set if the link count becomes 0.

Try to handle the various race conditions by using an additional semaphore set to protect the critical sections for semaphore initialization and access. What happens when two threads try to access the semaphore concurrently? Use the same semaphore for all copies of your library to protect against interaction between unrelated processes. Refer to this semaphore by a filename, which you can convert to a key with ftok .