#include <semaphore.h> sem_t * sem_open(const char *name, int oflag, ...);
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The sem_open() 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(). When a semaphore is being created, the parameters mode and value must be specified on the call to sem_open(). If a semaphore is created, then the maximum value of the semaphore is set to SEM_VALUE_MAX and the title of the semaphore is set to the last 16 characters of the name.
If sem_open() is called multiple times within the same process using the same name, sem_open() 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 that is used only by named semaphores. 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 following 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.
Authorities
Figure 1-8. Authorization Required for sem_open()
| 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() 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.
A named semaphore exists for the parameter name, but O_CREAT and O_EXCL are both set in oflag.
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 value parameter is greater than SEM_VALUE_MAX.
The name specified on the sem_open() 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 also could 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 if it does not already exist. If the semaphore is created, 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;
my_semaphore = sem_open("/mysemaphore",
O_CREAT, S_IRUSR | S_IWUSR, 10);
}
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