shmget(2)							System Calls Manual							 shmget(2)

NAME

       shmget - Returns (and possibly creates) the ID for a shared memory region

SYNOPSIS

       #include <sys/shm.h>

       int shmget(
	       key_t key,
	       size_t size,
	       int flags);

       If  programs  are  being developed for multiple platforms, application developers may want to specify #include statements for <sys/types.h>
       and <sys/ipc.h> before the one for <sys/shm.h>.	The additional #include statements are not required on Tru64 UNIX systems  or  by  ISO	or
       X/Open standards, but may be required on other vendors' systems that conform to these standards.

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       shmget():  XSH4.2

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

       Specifies  the  key  that identifies the shared memory region.  The value for the key parameter can be IPC_PRIVATE or a random number other
       than zero (0).  If the value of key is IPC_PRIVATE, it can be used to assure the return of a new, unused shared memory  region.	 Specifies
       the  minimum number of bytes to allocate for the region.  Specifies the creation flags.	Possible values are: If the key specified does not
       exist, the shmget() function creates an ID using the specified key.

	      If the specified key does exist, and IPC_EXCL is not set, the ID for the specified key is returned.

	      If the specified key does exist and IPC_EXCL is set, the shmget() function fails and returns an error.  If the key  already  exists,
	      the shmget() function fails and returns an error notification.

DESCRIPTION

       The  shmget()  function returns (and possibly creates) the ID for the shared memory region identified by the key parameter.  If IPC_PRIVATE
       is used for the key parameter, the shmget() function returns the ID for a private (that is, newly created) shared memory region.  The flags
       parameter  supplies  creation options for the shmget() function.  If the key parameter does not already exist, the IPC_CREAT flag instructs
       the shmget() function to create a new shared memory region for the key and return the kernel-assigned ID for the region.

       After creating a new shared memory region ID, the shmget() function initializes the shmid_ds structure associated with the ID  as  follows:
       The  shm_perm.cuid  and	shm_perm.uid  fields  are  set	equal  to  the	effective  user  ID of the calling process.  The shm_perm.cgid and
       shm_perm.gid fields are set equal to the effective group ID of the calling process.  The low-order nine bits of the shm_perm.mode field are
       set  equal  to  the  low-order  nine  bits  of  flags.  The shm_segsz field is set equal to size.  The shm_lpid, shm_nattch, shm_atime, and
       shm_dtime fields are all set equal to 0 (zero).	The shm_ctime field is set equal to the current time.  [Tru64 UNIX]  The shm_cpid field is
       set to the process ID of the calling process.

       [Tru64 UNIX]  To reduce the overhead associated with managing large shared memory regions among many processes, the Tru64 UNIX kernel makes
       use of shared page tables, also referred to as segmented shared memory (SSM).  When the segment size requested by shmget is greater than or
       equal  to  the  value  of  ssm-threshold  (an attribute in /etc/sysconfigtab), the memory region is managed using shared page tables.  When
       attaching to such a segment, the segment must always be aligned and sized correctly.  The alignment and size factor is given by the system-
       defined	SSM_SIZE  value  (see machine/pmap.h).	When the attachment occurs, the segment's memory region is aligned on an SSM_SIZE boundary
       and its size is rounded up to the next SSM_SIZE boundary - thus, the segment size specified by shmget does not need to  be  a  multiple	of
       SSM_SIZE. (Use of segmented shared memory can be disabled by setting the ssm-threshold attribute to zero.)

RETURN VALUES

       Upon successful completion, a shared memory identifier is returned.  If the shmget() function fails, a value of -1 is returned and errno is
       set to indicate the error.

ERRORS

       The shmget() function sets errno to the specified values for the following conditions: A shared memory region ID already exists for the key
       parameter, but operation permission as specified by the low-order nine bits of the flags parameter was not granted.  A shared memory region
       ID already exists for the key parameter, but IPC_CREAT and IPC_EXCL were used for the flags parameter.  The value of the size parameter	is
       less  than the system-defined minimum or greater than the system-defined maximum.  Or, a shared memory region ID already exists for the key
       parameter, but the number of bytes allocated for the region is less than size and size is not equal to 0 (zero).  A shared memory region ID
       does  not  exist  for the key parameter, and IPC_CREAT was not used for the flags parameter.  An attempt was made to create a shared memory
       region ID and its associated shmid_ds structure, but there was not enough physical memory available.  An attempt to  create  a  new  shared
       memory region ID exceeded the system-wide limit on the maximum number of IDs allowed.

RELATED INFORMATION

       Functions: shmat(2), shmctl(2), shmdt(2), sysconfig(8), table(2), ftok(3)

       Files: shmid_ds(4)

       Standards: standards(5) delim off

																	 shmget(2)