NAME
shmop - shared memory operations
SYNOPSIS
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/shm.h>
char *shmat ( int shmid, char *shmaddr, int shmflg )
int shmdt ( char *shmaddr))
DESCRIPTION
The function shmat attaches the shared memory segment iden-
tified by shmid to the data segment of the calling process.
The attaching address is specified by shmaddr with one of
the following criteria:
If shmaddr is 0, the system tries to find an unmapped
region in the range 1 - 1.5G starting from the upper
value and coming down from there.
If shmaddr isn't 0 and SHM_RND is asserted in shmflg,
the attach occurs at address equal to the rounding down
of shmaddr to a multiple of SHMLBA. Otherwise shmaddr
must be a page aligned address at which the attach
occurs.
If SHM_RDONLY is asserted in shmflg, the segment is attached
for reading and the process must have read access permis-
sions to the segment. Otherwise the segment is attached for
read and write and the process must have read and write
access permissions to the segment. There is no notion of
write-only shared memory segment.
The brk value of the calling process is not altered by the
attach. The segment will automatically 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.
On a successful shmat call the system updates the members of
the structure shmid_ds associated to the shared memory seg-
ment as follows:
shm_atime is set to the current time.
shm_lpid is set to the process-ID of the calling pro-
cess.
shm_nattch is incremented by one.
Note that the attach succeeds also if the shared memory seg-
ment is marked as to be deleted.
The function shmdt detaches from the calling process's data
segment the shared memory segment located at the address
specified by shmaddr. The detaching shared memory segment
must be one among the currently attached ones (to the
process's address space) with shmaddr equal to the value
returned by the its attaching shat call.
On a successful shmdt call the system updates the members of
the structure shmid_ds associated to the shared memory seg-
ment as follows:
shm_dtime is set to the current time.
shm_lpid is set to the process-ID of the calling pro-
cess.
shm_nattch is decremented by one. If it becomes 0 and
the segment is marked for deletion, the segment is
deleted.
The occupied region in the user space of the calling process
is unmapped.
SYSTEM CALLS
fork()
After a fork() the child inherits the attached shared
memory segments.
exec()
After an exec() all attached shared memory segments are
detached (not destroyed).
exit()
Upon exit() all attached shared memory segments are
detached (not destroyed).
RETURN VALUE
On a failure both functions return -1 with errno indicating
the error, otherwise shmat returns the address of the
attached shared memory segment, and shmdt returns 0.
ERRORS
When shmat fails, at return errno will be set to one among
the following values:
EACCES The calling process has no access permissions for
the requested attach type.
EINVAL Invalid shmid value, unaligned (i.e., not page-
aligned and SHM_RND was not specified) or invalid
shmaddr value, or failing attach at brk.
ENOMEM Could not allocate memory for the descriptor or
for the page tables.
The function shmdt can fails only if there is no shared
memory segment attached at shmaddr, in such a case at return
errno will be set to EINVAL.
NOTES
On executing a fork(2) system call, the child inherits all
the attached shared memory segments.
The shared memory segments attached to a process executing
an execve(2) system call will not be attached to the result-
ing process.
The following is a system parameter affecting a shmat system
call:
SHMLBA Segment low boundary address multiple. Must be
page aligned. For the current implementation the
SHMBLA value is PAGE_SIZE.
The implementation has no intrinsic limit to the per process
maximum number of shared memory segments (SHMSEG)
CONFORMING TO
SVr4, SVID. SVr4 documents an additional error condition
EMFILE.
SEE ALSO
ipc(5), shmctl(2), shmget(2).