obtain a semaphore id
Standard C Library (libc, -lc)
Based on the values of key
() returns the identifier of a newly
created or previously existing set of semaphores. The key is analogous to a
filename: it provides a handle that names an IPC object. There are three ways
to specify a key:
- IPC_PRIVATE may be specified, in which case a new IPC object will be
- An integer constant may be specified. If no IPC object corresponding to
key is specified and the IPC_CREAT bit is
set in flag, a new one will be
function may be used to generate a key from a pathname.
The mode of a newly created IPC object is determined by ORing these constants
into the flag
- Read access for user.
- Alter access for user.
- Read access for group.
- Alter access for group.
- Read access for other.
- Alter access for other.
If a new set of semaphores is being created,
is used to indicate the number of
semaphores the set should contain. Otherwise,
may be specified as 0.
() system call returns the id of a
semaphore set if successful; otherwise, -1 is returned and
is set to indicate the error.
() system call will fail if:
- Access permission failure.
- IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding
to key already exists.
- The number of semaphores requested exceeds the system imposed maximum per
- A semaphore set corresponding to key
already exists and contains fewer semaphores than
- A semaphore set corresponding to key does
not exist and nsems is 0 or
- Insufficiently many semaphores are available.
- The kernel could not allocate a struct
- No semaphore set was found corresponding to
key, and IPC_CREAT was not