SHMOP

Section: Linux Programmer's Manual (2)
Updated: 1993-11-28
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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 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 permissions 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 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 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 shmat call.

On a successful shmdt call the system updates the members of the structure shmid_ds associated to 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 (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 resulting 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. 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 *.)

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Time: 01:18:28 GMT, January 21, 2025