sem_open(3)sem_open(3)NAMEsem_open - Opens/creates a named semaphore for use by a process
(P1003.1b)
SYNOPSIS
#include <semaphore.h>
sem_t *sem_open ( const char *name, int ooption, mode_t mode, unsigned
int value);
LIBRARY
Realtime Library (librt.so, librt.a)
PARAMETERS
Points to a string naming the semaphore object. Specifies whether the
semaphore is to be created (O_CREAT option bit set) or only opened
(O_CREAT option bit clear). If O_CREAT is set, the O_EXCL option bit
may additionally be set to specify that the call should fail if a sema‐
phore of the same name already exists. The O_CREAT and O_EXCL options
are defined in the <fcntl.h> header file. Specifies the semaphore's
permission bits. This argument is used only when the semaphore is being
created. Specifies the initial value to be given to the semaphore.
This argument is used only when the semaphore is being created.
DESCRIPTION
Use the sem_open function to establish the connection between a named
semaphore and a process. Subsequently, the calling process can refer‐
ence the semaphore by using the address returned from the call. The
semaphore is available in subsequent calls to sem_wait, sem_trywait,
sem_post, and sem_getvalue functions. The semaphore remains usable by
the process until the semaphore is closed by a successful call to the
sem_close function.
The O_CREAT option bit in the ooption parameter controls whether the
semaphore is created or only opened by the call to sem_open.
A created semaphore's user ID is set to the user ID of the calling
process and its group ID is set to a system default group or to the
group ID of the process. The semaphore's permission bits are set to
the value of the mode argument, except for those set in the file mode
creation mask of the process.
After a semaphore is created, other processes can open the semaphore by
calling sem_open with the same value for the name argument.
RETURN VALUES
On successful completion, this function opens the semaphore and returns
the semaphore's descriptor. Otherwise, the function returns -1 and sets
errno to indicate the error.
ERRORS
The sem_open function fails under the following conditions: The named
semaphore exists and the permissions specified by ooption are denied,
or the named semaphore does not exist and the permissions specified by
ooption are denied. O_CREAT and O_EXCL are set and the named semaphore
already exists. The sem_open operation was interrupted by a signal.
The sem_open operation is not supported for the given name. Or, O_CREAT
was specified in ooption and value was greater than {SEM_VALUE_MAX}.
Too many semaphore descriptors or file descriptors are currently in use
by this process. The length of the name string exceeds {PATH_MAX}, or
a pathname component is longer than {NAME_MAX} while {_POSIX_NO_TRUNC}
is in effect. Too many semaphores are currently open in the system.
O_CREAT is not set and the named semaphore does not exist. Insuffi‐
cient space exists for the creation of a new named semaphore.
SEE ALSO
Functions: sem_close(3), sem_post(3), sem_trywait(3), sem_unlink(3),
sem_wait(3), mkfifo(3)
Guide to Realtime Programming
sem_open(3)