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shmat - Attach a shared memory region
const void *shmaddr,
int shmflgs );
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 XSH
standards, but may be required on other vendors' systems that conform to
Interfaces documented on this reference page conform to industry standards
shmat(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
Specifies the ID for the shared memory region. The ID is typically
returned by a previous shmget() function.
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.
Specifies the attach options. Possible values are:
SHM_RND If the addr parameter is not 0 (zero), the kernel rounds off
the address, if necessary.
If the calling process has read permission, the kernel attaches
the region for reading only.
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:
Read by user
Write by user
Read by group
Write by group
Read by others
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 option.
The librt library contains alternative interfaces for interprocess
communication. The names of these routines adhere to the format shm_* and
their reference pages are listed in SEE ALSO.
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.
The shmat() function sets errno to the specified values for the following
The calling process does not have the appropriate privilege.
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
[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).
An attempt to attach a shared memory region exceeded the maximum number
of attached regions allowed for any one process.
There was not enough data space available to attach the shared memory
[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.
Functions: exec(2), exit(2), fork(2), shmctl(2), shmdt(2), shmget(2),
Data structures: shmid_ds(4)
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