SHMOP

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 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 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 process.

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 *.)