Question

I am trying to solve readers-writers problem. The following functions is supposed to create named semaphores.

void createSems(void){

if((sem_counter = sem_open("counter", O_CREAT, 0660, 1)) == SEM_FAILED)
    printErrorAndKill("sem_open_counter");
if((sem_reader = sem_open("reader", O_CREAT, 0660, 1)) == SEM_FAILED)
    printErrorAndKill("sem_open_reader");    
if((sem_writer = sem_open("writer", O_CREAT, 0660, 1)) == SEM_FAILED)
    printErrorAndKill("sem_open_writer");  
if((sem_writer = sem_open("mutex1", O_CREAT, 0660, 1)) == SEM_FAILED)
    printErrorAndKill("mutex1");
if((sem_writer = sem_open("mutex2", O_CREAT, 0660, 1)) == SEM_FAILED)
    printErrorAndKill("mutex2");
if((sem_writer = sem_open("mutex3", O_CREAT, 0660, 1)) == SEM_FAILED)
    printErrorAndKill("mutex3");

}

void printErrorAndKill (const char *functionName){    
perror(functionName);
printf("%s: %s\n",functionName, strerror(errno));
exit(1);
}

Everything is working as expected on my MBP 10.7.3. But when I test it on school server I get the following output.

sem_open_reader: Permission denied
sem_open_reader: Illegal seek

The first semaphore is created successfully every time. I tried to google the error but with no succes or any connection to sem_open.

My question is am I doing something wrong when I am creating semaphores or the problem is somewhere else?

UPDATE

I did more testing and I eventually found out that it has nothing to do with illegal seek. I got rid of the line “perror(functionName);” just to found out the problem is just with permissions. Some names were reserved on our school OS so I just had to come with some more relevant names.

Answer 1

The “Illegal seek” message is probably caused by perror(). The man page for perror on my system says: “Note that errno is undefined after a successful library call: this call may well change this variable, even though it succeeds, for example because it internally used some other library function that failed.”