#include <semaphore.h>
sem_t * sem_open_np(const char *name, int oflag,
mode_t mode, unsigned int value,
sem_attr_np_t * attr);
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The sem_open_np() function opens a named semaphore, returning a semaphore pointer that may be used on subsequent calls to sem_post(), sem_post_np(), sem_wait(), sem_wait_np(), sem_trywait(), sem_getvalue(), and sem_close(). If a named semaphore is being created, the parameters mode, value, and attr are used to set the permissions, value, and maximum value of the created semaphore.
If sem_open_np() is called multiple times within the same process using the same name, sem_open_np() will return a pointer to the same semaphore, as long as another process has not used sem_unlink() to unlink the semaphore.
Parameters
This parameter is assumed to be represented in the CCSID (coded character set identifier) currently in effect for the job. If the CCSID of the job is 65535, this parameter is assumed to be represented in the default CCSID of the job.
The name is added to a set of names used by named semaphores only. The name has no relationship to any file system path names. The maximum length of the name is SEM_NAME_MAX.
The oflag parameter value is either zero or is obtained by performing an OR operation on one or more of the following constants:
The mode parameter value is either zero or is obtained by performing an OR operation on one or more of the list of constants. For another process to open the semaphore, the process's effective uid must be able to open the semaphore in both read and write mode.
The members of the sem_attr_np_t structure are as follows:
Authorities
Figure 1-9. Authorization Required for sem_open_np()
| Object Referred to | Authority Required | errno |
|---|---|---|
| Named semaphore to be created | None | None |
| Existing named semaphore to be accessed | *RW | EACCES |
Return Value
Error Conditions
If sem_open_np() is not successful, errno usually indicates one of the following errors. Under some conditions, errno could indicate an error other than those listed here.
An attempt was made to access an object in a way forbidden by its object access permissions.
In attempting to use an argument in a call, the system detected an address that is not valid.
While attempting to access a parameter passed to this function, the system detected an address that is not valid.
A function was passed incorrect argument values, or an operation was attempted on an object and the operation specified is not supported for that type of object.
An argument value is not valid, out of range, or NULL.
The maxvalue field of the attr argument is greater than SEM_VALUE_MAX.
The maxvalue field of the attr argument is equal to zero.
The value argument is greater than the maxvalue field of the attr argument.
The reserved fields of the attr argument are not set to zero.
The directory or a component of the path name specified does not exist.
A named file or directory does not exist or is an empty string.
The name specified on the sem_open_np() call does not refer to an existing named semaphore and O_CREAT was not set in oflag.
The requested operations required additional space on the device and there is no space left. This could also be caused by exceeding the user profile storage limit when creating or transferring ownership of an object.
Insufficient space remains to hold the intended file, directory, or link.
System semaphore resources have been exhausted.
Error Messages
None.
Related Information
Example
The following example opens the named semaphore "/mysemaphore" and creates the semaphore with an initial value of 10 and a maxiumum value of 11. The permissions are set such that only the current user has access to the semaphore.
#include <semaphore.h>
main() {
sem_t * my_semaphore;
int rc;
sem_attr_np_t attr;
memset(&attr, 0, sizeof(attr));
attr.maxvalue=11;
my_semaphore = sem_open_np("/mysemaphore",
O_CREAT|O_EXCL,
S_IRUSR | S_IWUSR,
10,
&attr);
}
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