NAME
shmat, shmdt
— attach or detach shared
memory
LIBRARY
library “libc”
SYNOPSIS
#include
<machine/param.h>
#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
Shmat()
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_RNDis not specified in flag, the segment is attached the specified address. - If addr is specified and
SHM_RNDis specified, addr is rounded down to the nearest multiple ofSHMLBA.
Shmdt()
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.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.
If the USR_SYSVIPC variable is set in a
process' environment, the process and its children will use the userspace
implementation.
ERRORS
Shmat() will fail if:
- [
EINVAL] - No shared memory segment was found corresponding to shmid.
- [
EINVAL] - addr was not an acceptable address.
Shmdt() will fail if:
- [
EINVAL] - addr does not point to a shared memory segment.
SEE ALSO
AUTHORS
The DragonFly specific userspace implementation (see ENVIRONMENT) was written by Larisa Grigore.