Shared Memory Functions

Processes and threads can communicate directly with one another by sharing parts of their memory space and then reading and writing the data stored in the shared memory. Synchronization of shared memory is the responsibility of the application program. Semaphores and mutexes provide ways to synchronize shared memory use across processes and threads.

A thread gets a shared memory identifier by calling the shmget() function. Depending on the key and shmflg parameters passed in, either a new shared memory segment is created or an existing shared memory segment is accessed. The size of the shared memory segment is specified by the size parameter. When a new shared memory segment is created, a data structure is also created to contain information about the shared memory segment. This structure is defined in the <sys/shm.h> header file as follows:

typedef struct shmid_ds {
    struct ipc_perm shm_perm;   /* Operation permission struct*/
    int             shm_segsz;  /* Segment size               */
    pid_t           shm_lpid;   /* Process id of last shmop   */
    pid_t           shm_cpid;   /* Process id of creator      */
    int             shm_nattch; /* Current # attached         */
    time_t          shm_atime;  /* Last shmat time            */
    time_t          shm_dtime;  /* Last shmdt time            */
    time_t          shm_ctime;  /* Last change time           */
} shmtablentry_t;

A process gets addressability to the shared memory segment by attaching to it using the shmat() function. The following parameters are passed in:

• Shared memory ID
• Pointer to an address
• Flag specifying how the shared memory segment is to be attached

A process detaches a shared memory segment by calling the shmdt() function. The only parameter passed in is the shared memory segment address. The process implicitly detaches from the shared memory when the process ends.

A thread removes a shared memory ID by calling the shmctl() function. The thread also can use the shmctl() function to change the data structure values associated with the shared memory ID or to retrieve the data structure values associated with the shared memory ID. The following parameters are passed in:

• Shared memory ID
• Command the thread wants to perform (remove ID, set data structure values, receive data structure values)
• Pointer to a buffer from which to set data structure values, or in which to receive data structure values.

Shared Memory Differences and Restrictions

Shared memory segments are created as teraspace-shared memory segments or as nonteraspace-shared memory segments. A teraspace shared memory segment is accessed by adding the shared memory segment to a process's teraspace. A teraspace is a space that has a much larger capacity than other i5/OS spaces and is addressable from only one process. A nonteraspace shared memory segment creates shared memory using i5/OS space objects.

A teraspace shared memory segment is created if SHM_TS_NP is specified on the shmflag parameter of shmget() or if a shared memory segment is created from a program that was compiled using the TERASPACE(*YES *TSIFC) option of CRTBNDC or CRTCMOD. The following capabilities and restrictions apply for teraspace shared memory segments.

• Teraspace shared memory objects may be attached in read-only mode.
  
  
• The address specified by shmaddr is only used when shmat() is called from a program that uses data model LLP64 and attaches to a teraspace shared memory segment. Otherwise it is not possible to specify the address in teraspace at which the shared memory is to be mapped. The shmaddr parameter on the shmat() function is ignored.
  
  
• After a teraspace shared memory segment is detached, it cannot be addressed through a pointer saved by the process.
  
  
• The maximum size of a teraspace shared memory segment is 4 294 967 295 bytes (4 GB minus 1).
  
  
• The maximum number of shared memory segments that can be created (system-wide) is 2 147 483 646.
  
  
• A teraspace shared memory segment may be created such that its size can be changed after it is created. The maximum size of this type of shared memory segment is 268 435 456 bytes (256 MB).

The i5/OS nonteraspace shared memory differs from the shared memory definition in the Single UNIX Specification in the following ways:

• The nonteraspace shared memory segments are i5/OS space objects and can be attached only in read/write mode, not in the read-only mode that the Single UNIX Specification allows. If the SHM_RDONLY flag is specified in the shmflg parameter on a shmget() call, the call fails and the errno variable is set to [EOPNOTSUPP].
  
  
• A nonteraspace shared memory segment can be attached only at the actual address of the i5/OS space object, not at an address specified by the thread. The shmaddr parameter on the shmat() function is ignored.
  
  
• After a nonteraspace shared memory segment is detached from a process, it still can be addressed through a pointer saved by the process. For nonteraspace shared memory segments, i5/OS does not "map" and "unmap" regions of storage to the address space of a process.
  
  
• The maximum size of a nonteraspace shared memory segment is 16 776 960 bytes. Although the maximum size of a shared memory segment is 16 776 960 bytes, shared memory segments larger than 16 773 120 bytes should be created as teraspace shared memory segments. When the operating system accesses a nonteraspace shared memory segment that has a size larger than 16 773 120 bytes, a performance degradation may be observed.
  
  
• The maximum number of shared memory segments that can be created (system-wide) is 2 147 483 646.
  
  
• The size of a nonteraspace shared memory segment may be changed using the SHM_RESIZE command of shmctl(), up to a maximum size of 16 773 120 bytes.

The shared memory functions are:

• shmat() (Attach Shared Memory Segment to Current Process) returns the address of the shared memory segment associated with the specified shared memory identifier.
• shmctl() (Perform Shared Memory Control Operations) provides shared memory control operations as specified by cmd on the shared memory segment specified by shmid.
• shmdt() (Detach Shared Memory Segment from Calling Process) detaches the shared memory segment specified by shmaddr from the calling process.
• shmget() (Get ID of Shared Memory Segment with Key) returns the shared memory ID associated with the specified shared memory key.

See also IPC Key Generation Functions for additional shared memory functions.