11-01-2001
When programming such IPC's, you should consider 2 things...
1. Handle all signals so that your exiting process cleans up.
2. Use non-blocking techniques to assure no dead-lock.
The semaphore documentation states...
There are two control flags that can be used with semop():
IPC_NOWAIT
-- Can be set for any operations in the array. Makes the function return without changing any semaphore value if any operation for which IPC_NOWAIT is set cannot be performed. The function fails if it tries to decrement a semaphore more than its current value, or tests a nonzero semaphore to be equal to zero.
SEM_UNDO
-- Allows individual operations in the array to be undone when the process exits.
Any process with read permission can test whether a semaphore has a zero value. To increment or decrement a semaphore requires write permission. When an operation fails, none of the semaphores is altered.
The process blocks (unless the IPC_NOWAIT flag is set), and remains blocked until:
the semaphore operations can all finish, so the call succeeds,
the process receives a signal, or
the semaphore set is removed.
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LEARN ABOUT HPUX
sem_trywait
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)