NAME
shmat, shmdt
— map/unmap shared
memory
SYNOPSIS
#include
<sys/shm.h>
void *
shmat(int shmid,
const void *shmaddr, int
shmflg);
int
shmdt(const void *shmaddr);
DESCRIPTION
shmat()
maps the shared memory segment associated with the shared memory identifier
shmid into the address space of the calling process.
The address at which the segment is mapped is determined by the
shmaddr parameter. If it is equal to 0, the system
will pick an address itself. Otherwise, an attempt is made to map the shared
memory segment at the address shmaddr specifies. If
SHM_RND is set in shmflg, the system will round the
address down to a multiple of SHMLBA bytes (SHMLBA is defined in
⟨sys/shm.h⟩ ). A shared memory segment
can be mapped read-only by specifying the SHM_RDONLY flag in
shmflg.
shmdt()
unmaps the shared memory segment that is currently mapped at
shmaddr from the calling process' address space.
shmaddr must be a value returned by a prior
shmat() call. A shared memory segment will remain
existant until it is removed by a call to
shmctl(2)
with the IPC_RMID command.
RETURN VALUES
shmat() returns the address at which the
shared memory segment has been mapped into the calling process' address
space when successful, shmdt() returns 0 on
successful completion. Otherwise, a value of -1 is returned, and the global
variable errno is set to indicate the error.
ERRORS
The shmat() system call will fail if:
- [
EACCES] - The calling process has no permission to access this shared memory segment.
- [
EINVAL] - shmid is not a valid shared memory identifier. shmaddr specifies an illegal address.
- [
EMFILE] - The number of shared memory segments has reached the system-wide limit.
- [
ENOMEM] - There is not enough available data space for the calling process to map the shared memory segment.
The shmdt() system call will fail if:
- [
EINVAL] - shmaddr is not the start address of a mapped shared memory segment.
LEGACY SYNOPSIS
#include
<sys/types.h> #include
<sys/ipc.h> #include
<sys/shm.h>
The include files
<sys/types.h> and
<sys/ipc.h> are necessary
for both functions.