NAME
shmat, shmdt
— attach or detach shared
memory
LIBRARY
library “libc”
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
Theshmat()
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:
- If addr is 0, the segment is attached at an address selected by the kernel.
- If addr is nonzero and SHM_RND is not specified in flag, the segment is attached the specified address.
- 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 instance due to the amount of available space being smaller than the segment size, or because pre-existing mappings are in the range and no SHM_REMAP flag was provided.
- [
EMFILE] - Failed to attach the shared memory segment because the per-process kern.ipc.shmseg sysctl(3) limit was reached.
The shmdt() system call will fail if:
- [
EINVAL] - The addr argument does not point to a shared memory segment.