NAME
shmop, shmat, shmdt - shared memory operations
SYNOPSIS
#include <sys/types.h>
#include <sys/shm.h>
I void *shmat(int shmid ,
I const void * shmaddr ,
I int shmflg );
I int shmdt(const void * shmaddr );
DESCRIPTION
R shmat ()
attaches the shared memory segment identified by
shmid
to the address space of the calling process.
The attaching address is specified by
shmaddr
with one of the following criteria:
If
shmaddr
is NULL,
the system chooses a suitable (unused) address at which to attach
the segment.
If
shmaddr
isn't NULL
and
SHM_RND
is specified in
R shmflg ,
the attach occurs at the address equal to
shmaddr
rounded down to the nearest multiple of
R SHMLBA .
Otherwise
shmaddr
must be a page-aligned address at which the attach occurs.
If
SHM_RDONLY
is specified in
R shmflg ,
the segment is attached for reading and the process must have
read permission for the segment.
Otherwise the segment is attached for read and write
and the process must have read and write permission for the segment.
There is no notion of a write-only shared memory segment.
The (Linux-specific)
SHM_REMAP
flag may be specified in
shmflg
to indicate that the mapping of the segment should replace
any existing mapping in the range starting at
shmaddr
and continuing for the size of the segment.
(Normally an
EINVAL
error would result if a mapping already exists in this address range.)
In this case,
shmaddr
must not be NULL.
The
brk(2)
value of the calling process is not altered by the attach.
The segment will automatically be detached at process exit.
The same segment may be attached as a read and as a read-write
one, and more than once, in the process's address space.
A successful
R shmat ()
call updates the members of the
shmid_ds
structure (see
shmctl(2))
associated with the shared memory segment as follows:
shm_atime
is set to the current time.
shm_lpid
is set to the process-ID of the calling process.
shm_nattch
is incremented by one.
R shmdt ()
detaches the shared memory segment located at the address specified by
shmaddr
from the address space of the calling process.
The to-be-detached segment must be currently
attached with
shmaddr
equal to the value returned by the attaching
R shmat ()
call.
On a successful
R shmdt ()
call the system updates the members of the
shmid_ds
structure associated with the shared memory segment as follows:
shm_dtime
is set to the current time.
shm_lpid
is set to the process-ID of the calling process.
shm_nattch
is decremented by one.
If it becomes 0 and the segment is marked for deletion,
the segment is deleted.
After a
fork(2)
the child inherits the attached shared memory segments.
After an
execve(2)
all attached shared memory segments are detached from the process.
Upon
_exit(2)
all attached shared memory segments are detached from the process.
RETURN VALUE
On success
R shmat ()
returns the address of the attached shared memory segment; on error
(void *) -1
is returned, and
errno
is set to indicate the cause of the error.
On success
R shmdt ()
returns 0; on error -1 is returned, and
errno
is set to indicate the cause of the error.
ERRORS
When
R shmat ()
fails,
errno
is set to one of the following:
EACCES
The calling process does not have the required permissions for
the requested attach type, and does not have the
CAP_IPC_OWNER
capability.
EINVAL
Invalid
shmid
value, unaligned (i.e., not page-aligned and SHM_RND was not
specified) or invalid
shmaddr
value, or can't attach segment at
R shmaddr ,
or
SHM_REMAP
was specified and
shmaddr
was NULL.
ENOMEM
Could not allocate memory for the descriptor or for the page tables.
When
R shmdt ()
fails,
errno
is set as follows:
EINVAL
There is no shared memory segment attached at
R shmaddr ;
or,
shmaddr
is not aligned on a page boundary.
CONFORMING TO
SVr4, POSIX.1-2001.
In SVID 3 (or perhaps earlier)
the type of the shmaddr argument was changed from
char *
into
R const void * ,
and the returned type of
R shmat ()
from
char *
into
R void * .
(Linux libc4 and libc5 have the
char *
prototypes; glibc2 has
R void * .)
NOTES
Using
R shmat ()
with
shmaddr
equal to NULL
is the preferred, portable way of attaching a shared memory segment.
Be aware that the shared memory segment attached in this way
may be attached at different addresses in different processes.
Therefore, any pointers maintained within the shared memory must be
made relative (typically to the starting address of the segment),
rather than absolute.
On Linux, it is possible to attach a shared memory segment even if it
is already marked to be deleted.
However, POSIX.1-2001 does not specify this behavior and
many other implementations do not support it.
The following system parameter affects
R shmat ():
SHMLBA
Segment low boundary address multiple.
Must be page aligned.
For the current implementation the
SHMLBA
value is
R PAGE_SIZE .
The implementation places no intrinsic limit on the per-process maximum
number of shared memory segments
(SHMSEG).
SEE ALSO