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SHMOP(2) Linux Programmer’s Manual SHMOP(2)
NAME
shmop - shared memory operations
SYNOPSIS
#include <sys/types.h>
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);
DESCRIPTION
The function 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 asserted in shmflg, the attach occurs at the
address equal to shmaddr rounded down to the nearest 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 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 asserted 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 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.
On a successful shmat call the system updates the members of the shmid_ds structure
associated to 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.
Note that the attach succeeds also if the shared memory segment is marked to be
deleted.
The function shmdt detaches the shared memory segment located at the address speci-
fied 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
its attaching shmat call.
On a successful 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.
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 from the
process.
exit() Upon exit() all attached shared memory segments are detached from the pro-
cess.
RETURN VALUE
On failure both functions return -1 with errno indicating the error. On success
shmat returns the address of the attached shared memory segment, and shmdt returns
0.
ERRORS
When shmat fails, errno is set to one of the following:
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, or
SHM_REMAP was specified and shmaddr was NULL.
ENOMEM Could not allocate memory for the descriptor or for the page tables.
The function shmdt can fail only if there is no shared memory segment attached at
shmaddr, in such a case at return errno will be set to EINVAL.
NOTES
Using 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.
The following system parameter affects 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. In SVID-v4 the
type of the shmaddr argument was changed from char * into const void *, and the
returned type of shmat() from char * into void *. (Linux libc4 and libc5 have the
char * prototypes; glibc2 has void *.)
SEE ALSO
brk(2), ipc(5), mmap(2), shmctl(2), shmget(2)
Linux 2.5 2002-01-05 SHMOP(2)
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