|SHMAT(2)||System Calls Manual||SHMAT(2)|
LIBRARY¶Standard C Library (libc, -lc)
shmid, const void
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:
- 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
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.
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.
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.
shmat() system call will fail if:
- No shared memory segment was found corresponding to shmid.
- The addr argument was not an acceptable address.
- 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.
- Failed to attach the shared memory segment because the per-process kern.ipc.shmseg sysctl(3) limit was reached.
shmdt() system call will fail if:
- The addr argument does not point to a shared memory segment.
SEE ALSO¶shmctl(2), shmget(2)
|January 14, 2019||Linux 4.19.0-13-amd64|