FreeBSD manual
download PDF document: shmdt.2.pdf
SHMAT(2) FreeBSD System Calls Manual SHMAT(2)
NAME
shmat, shmdt - attach or detach shared memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
void *
shmat(int shmid, const void *addr, int flag);
int
shmdt(const void *addr);
DESCRIPTION
The shmat() system call attaches the shared memory segment identified by
shmid to the calling process's address space. The address where the
segment is attached is determined as follows:
o If addr is 0, the segment is attached at an address selected by the
kernel.
o If addr is nonzero and SHM_RND is not specified in flag, the segment
is attached the specified address.
o If addr is specified and SHM_RND is specified, addr is rounded down
to the nearest multiple of SHMLBA.
If the SHM_REMAP flag is specified and the passed addr is not NULL, any
existing mappings in the virtual addresses range are cleared before the
segment is attached. If the flag is not specified, addr is not NULL, and
the virtual address range contains some pre-existing mappings, the
shmat() call fails.
The shmdt() system call detaches the shared memory segment at the address
specified by addr from the calling process's address space.
RETURN VALUES
Upon success, shmat() returns the address where the segment is attached;
otherwise, -1 is returned and errno is set to indicate the error.
The shmdt() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The shmat() system call will fail if:
[EINVAL] No shared memory segment was found corresponding to
shmid.
[EINVAL] The addr argument was not an acceptable address.
[ENOMEM] The specified addr cannot be used for mapping, for
reached.
The shmdt() system call will fail if:
[EINVAL] The addr argument does not point to a shared memory
segment.
SEE ALSO
shmctl(2), shmget(2)
FreeBSD 14.0-RELEASE-p11 January 14, 2019 FreeBSD 14.0-RELEASE-p11