SHMGET
Return the identifier of the shared memory segment associated with the value of the argument ecx. If a new shared memory segment is created, then its associated data structure shmid_ds (see SHMCTL) is initialized as follows: shm_perm.cuid and shm_perm.uid are set to the effective user ID of the calling process. shm_perm.cgid and shm_perm.gid are set to the effective group ID of the calling process. The least significant 9 bits of shm_perm.mode are set to the least significant 9 bit of shmflg. shm_segsz is set to the value of size. shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0. shm_ctime is set to the current time. If the shared memory segment already exists, the permissions are verified, and a check is made to see if it is marked for destruction. Arguments
ecx | Key value. If this value is IPC_PRIVATE then a new shared memory segment, with size equal to the value of edx rounded up to a multiple of PAGE_SIZE, is created and the system call ignores everything but the least significant 9 bits of esi.
edx | Shared memory segment size (used when creating new segment).
esi | Flags. It is should be compsed of least significant 9 bits specifying the permissions granted to the owner, group, and world. (These bits have the same format, and the same meaning, as the mode argument of sys_open. Presently, the execute permissions are not used by the system.) and the following flags:
`SHM_HUGETLB` | `Allocate HUGETLB pages for shared memory. IPC_CREAT to create a new segment. If this flag is not used, then SHMGET will find the segment associated with ecx and check to see if the user has permission to access the segment.`
`SHM_NORESERVE` | `(since Linux 2.6.15) This flag serves the same purpose as the sys_mmap MAP_NORESERVE flag. Do not reserve swap space for this segment. When swap space is reserved, one has the guarantee that it is possible to modify the segment. When swap space is not reserved one might get SIGSEGV upon a write if no physical memory is available.`
`SHM_DEST` | `Destroy segment on last detach.`
`SHM_LOCKED` | `Segment wont be swapped.`
`IPC_CREAT` | `Create a new segment. If this flag is not used, then SHMGET will find the segment associated with ecx, check to see if the user has permission to receive the shmid associated with the segment, and ensure the segment is not marked for destruction.`
`IPC_EXCL` | `Used with IPC_CREAT to ensure failure if the segment exists.`
If esi specifies both IPC_CREAT and IPC_EXCL and a shared memory segment already exists for ecx, then SHMGET fails with -EEXIST (This is analogous to the effect of the combination "O_CREAT or O_EXCL" for sys_open).
Return values If the function succeeds the return value is a valid segment identifier. If the function fails the return value is one of the following errno values:
`-EACCES` | `The user does not have permission to access the shared memory segment, and does not have the CAP_IPC_OWNER capability.`
`-EEXIST` | `"IPC_CREAT or IPC_EXCL" was specified and the segment exists.`
`-EINVAL` | `A new segment was to be created and size < SHMMIN or size > SHMMAX, or no new segment was to be created, a segment with given key existed, but edx is greater than the size of that segment.`
`-ENFILE` | `The system limit on the total number of open files has been reached.`
`-ENOENT` | `No segment exists for the given ecx, and IPC_CREAT was not specified.`
`-ENOMEM` | `No memory could be allocated for segment overhead.`
`-ENOSPC` | `All possible shared memory IDs have been taken (SHMMNI), or allocating a segment of the requested size would cause the system to exceed the system-wide limit on shared memory (SHMALL).`
`-EPERM` | `The SHM_HUGETLB flag was specified, but the caller was not privileged (did not have the CAP_IPC_LOCK capability).`
Remarks The following limits on shared memory segment resources affect the SHMGET call:
`SHMALL` | `System wide maximum of shared memory pages (this limit can be read and modified via /proc/sys/kernel/shmall).`
`SHMMAX` | `Maximum size in bytes for a shared memory segment: policy dependent (this limit can be read and modified via /proc/sys/kernel/shmmax).`
`SHMMIN` | `Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though PAGE_SIZE is the effective minimum size).`
`SHMMNI` | `System wide maximum number of shared memory segments: implementation dependent (currently 4096, this limit can be read and modified via /proc/sys/kernel/shmmni).`
After a sys_fork the child inherits the attached shared memory segments. After a sys_exec all attached shared memory segments are detached (not destroyed). Upon sys_exit all attached shared memory segments are detached (not destroyed). See samples/ipc/shmem.asm for an example.