sem_open()--Open Named Semaphore

 #include <semaphore.h>

 sem_t * sem_open(const char *name, int oflag, ...);  
  

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.

If sem_open() is called from a program using data model LLP64, the returned semaphore pointer must be declared as a sem_t *__ptr128.

Parameters

name

(Input) A pointer to the null-terminated name of the semaphore to be opened. The name should begin with a slash ('/') character. If the name does not begin with a slash ('/') character, the system adds a slash to the beginning of the name.

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.

See QlgSem_open()--Open Named Semaphore (using NLS-enabled path name) for a description and an example of supplying the name in any CCSID.

oflag

(Input) Option flags.

The oflag parameter value is either zero or is obtained by performing an OR operation on one or more of the following constants:

'0x0008' or O_CREAT	Creates the named semaphore if it does not already exist.
'0x0010' or O_EXCL	Causes sem_open() to fail if O_CREAT is also set and the named semaphore already exists.

mode

(input) Permission flags.

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 UIDd must be able to open the semaphore in both read and write mode.

'0x0100' or S_IRUSR	Permits the creator of the named semaphore to open the semaphore in read mode.
'0x0080' or S_IWUSR	Permits the creator of the named semaphore to open the semaphore in write mode.
'0x0020' or S_IRGRP	Permits the group associated with the named semaphore to open the semaphore in read mode.
'0x0010' or S_IWGRP	Permits the group associated with the named semaphore to open the semaphore in write mode.
'0x0004' or S_IROTH	Permits others to open the named semaphore in read mode.
'0x0002' or S_IWOTH	Permits others to open the named semaphore in write mode.

value

(Input) Initial value of the named semaphore.

Authorities

Authorization required for sem_open()

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.

[EACCES]

Permission denied.

An attempt was made to access an object in a way forbidden by its object access permissions.

[EEXIST]

Semaphore exists.

A named semaphore exists for the parameter name, but O_CREAT and O_EXCL are both set in oflag.

[EINVAL]

The value specified for the argument is not correct.

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.

[ENAMETOOLONG]

The name is too long. The name is longer than the SEM_NAME_MAX characters.

[ENOENT]

No such path or directory.

The name specified on the sem_open() call does not refer to an existing named semaphore and O_CREAT was not set in oflag.

[ENOSPC]

No space available.

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

• The <semaphore.h> file (see Header Files for UNIX-Type Functions)
  
  
• QlgSem_open() Open Named Semaphore (using NLS-enabled path name)
  
  
• sem_close()--Close Named Semaphore
  
  
• sem_getvalue()--Get Semaphore Value
  
  
• sem_open_np()--Open Named Semaphore with Maximum Value
  
  
• sem_post()--Post to Semaphore
  
  
• sem_post_np()--Post Value to Semaphore
  
  
• sem_trywait()--Try to Decrement Semaphore
  
  
• sem_unlink()--Unlink Named Semaphore
  
  
• sem_wait()--Wait for Semaphore
  
  
• sem_wait_np()--Wait for Semaphore with Timeout

Example

See Code disclaimer information for information pertaining to code examples.

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);

}