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shmat(2)
NAME
  shmat - Attaches a shared memory region

SYNOPSIS
  #include <sys/shm.h>

  void *shmat(
	  int shmid,
	  const void *shmaddr,
	  int shmflg);

  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:

  shmat(): XSH4.2

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

PARAMETERS
  shmid	    Specifies the ID for the shared memory region.  The ID is
	    typically returned by a previous shmget() function.

  addr	    Specifies the virtual address at which the process wants to
	    attach the shared memory region.  The process can also specify 0
	    (zero) to have the kernel select an appropriate address.

  flags	    Specifies the attach flags.	 Possible values are:

	    SHM_RND   If the addr parameter is not 0 (zero), the kernel
		      rounds off the address, if necessary.

	    SHM_RDONLY
		      If the calling process has read permission, the kernel
		      attaches the region for reading only.

DESCRIPTION
  The shmat() function attaches the shared memory region identified by the
  shmid parameter to the virtual address space of the calling process.	For
  the addr parameter, the process can specify either an explicit address or 0
  (zero), to have the kernel select the address.  If an explicit address is
  used, the process can set the SHM_RND flag to have the kernel round off the
  address, if necessary.

  Access to the shared memory region is determined by the operation
  permissions in the shm_perm.mode member in the region's shmid_ds structure.
  The low-order bits in shm_perm.mode are interpreted as follows:

  00400	    Read by user

  00200	    Write by user

  00040	    Read by group

  00020	    Write by group

  00004	    Read by others

  00002	    Write by others

  The calling process is granted read and write permissions on the attached
  region if at least one of the following is true:

    ·  The effective user ID of the process is superuser.

    ·  The effective user ID of the process is equal to shm_perm.cuid or
       shm_perm.uid and bit 0600  in shm_perm.mode is set.

    ·  The effective group ID of the process is equal to shm_perm.cgid or
       shm_perm.gid and bit 0060  in shm_perm.mode is set.

    ·  Bit 0006	 in shm_perm.mode is set.

  If the process has read permission, it can attach the region as read only
  by setting the SHM_RDONLY flag.

RETURN VALUES
  Upon successful completion, shmat() increments the value of shm_nattach in
  the data structure associated with the shared memory ID of the attached
  shared memory segment and returns the segment's start address.  If the
  shmat() function fails, the shared memory region is not attached, a value
  of -1 is returned, and errno is set to indicate the error.

ERRORS
  The shmat() function sets errno to the specified values for the following
  conditions:

  [EACCES]  The calling process does not have the appropriate privilege.

  [EINVAL]  The shmid parameter does not specify a valid shared memory region
	    ID; the addr parameter is not 0 (zero) and not a valid address;
	    or the addr parameter is not 0 (zero) and not a valid address,
	    and SHM_RND is not set.

	    [Tru64 UNIX]  The shmid refers to a segmented shared memory
	    region and the addr parameter is not aligned at SSM_SIZE (that
	    is, (addr % SSM_SIZE) != 0).

  [EMFILE]  An attempt to attach a shared memory region exceeded the maximum
	    number of attached regions allowed for any one process.

  [ENOMEM]  There was not enough data space available to attach the shared
	    memory region.

	    [Tru64 UNIX]  The shmid refers to a segmented shared memory
	    region whose size is not a multiple of SSM_SIZE and where not
	    enough data space is available for the rounded-up size.




RELATED INFORMATION
  Commands: sysconfig(8)

  Functions: exec(2), exit(2), fork(2), shmctl(2), shmdt(2), shmget(2)

  Data structures: shmid_ds(4)

  Standards: standards(5)

Index for Section 2

Alphabetical listing for S

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