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SHMGET !!!SHMGET NAME SYNOPSIS DESCRIPTION SYSTEM CALLS RETURN VALUE ERRORS NOTES BUGS CONFORMING TO SEE ALSO ---- !!NAME shmget - allocates a shared memory segment !!SYNOPSIS __#include __ __#include __ __int shmget(key_t__ ''key''__, int__ ''size''__, int__ ''shmflg''__);__ !!DESCRIPTION __shmget()__ returns the identifier of the shared memory segment associated to the value of the argument ''key''. A new shared memory segment, with size equal to the round up of ''size'' to a multiple of __PAGE_SIZE__, is created if ''key'' has value __IPC_PRIVATE__ or ''key'' isn't __IPC_PRIVATE__, no shared memory segment is associated to ''key'', and __IPC_CREAT__ is asserted in ''shmflg'' (i.e. ''shmflg''____ isn't zero). The presence in ''shmflg'' is composed of: __IPC_CREAT__ to create a new segment. If this flag is not used, then __shmget()__ will find the segment associated with ''key'', 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. __mode_flags (lowest 9 bits)__ specifying the permissions granted to the owner, group, and world. Presently, the execute permissions are not used by the system. If a new segment is created, the access permissions from ''shmflg'' are copied into the ''shm_perm'' member of the ''shmid_ds'' structure that defines the segment. The ''shmid_ds'' structure: struct shmid_ds { struct ipc_perm shm_perm; /* operation perms */ int shm_segsz; /* size of segment (bytes) */ time_t shm_atime; /* last attach time */ time_t shm_dtime; /* last detach time */ time_t shm_ctime; /* last change time */ unsigned short shm_cpid; /* pid of creator */ unsigned short shm_lpid; /* pid of last operator */ short shm_nattch; /* no. of current attaches */ }; struct ipc_perm { key_t key; ushort uid; /* owner euid and egid */ ushort gid; ushort cuid; /* creator euid and egid */ ushort cgid; ushort mode; /* lower 9 bits of ''shmflg'' */ ushort seq; /* sequence number */ }; Furthermore, while creating, the system call initializes the system shared memory segment data structure __shmid_ds__ 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 lowest order 9 bits of __shm_perm.mode__ are set to the lowest order 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 access permissions are verified, and a check is made to see if it is marked for destruction. !!SYSTEM CALLS __fork()__ After a __fork()__ the child inherits the attached shared memory segments. __exec()__ After an __exec()__ all attached shared memory segments are detached (not destroyed). __exit()__ Upon __exit()__ all attached shared memory segments are detached (not destroyed). !!RETURN VALUE A valid segment identifier, ''shmid'', is returned on success, -1 on error. !!ERRORS On failure, __errno__ is set to one of the following: __EINVAL__ is returned if 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 ''size'' is greater than the size of that segment (On Fedora core this condition results in ESPIPE! Illegal Seek). __EEXIST__ is returned if __IPC_CREAT | IPC_EXCL__ was specified and the segment exists. __EIDRM__ is returned if the segment is marked as destroyed, or was removed. __ENOSPC__ is returned if all possible shared memory id's have been taken (__SHMMNI__) or if allocating a segment of the requested ''size'' would cause the system to exceed the system-wide limit on shared memory (__SHMALL__). __ENOENT__ is returned if no segment exists for the given ''key'', and __IPC_CREAT__ was not specified. __EACCES__ is returned if the user does not have permission to access the shared memory segment. __ENOMEM__ is returned if no memory could be allocated for segment overhead. !!NOTES __IPC_PRIVATE__ isn't a flag field but a __key_t__ type. If this special value is used for ''key'', the system call ignores everything but the lowest order 9 bits of ''shmflg'' and creates a new shared memory segment (on success). The followings are limits on shared memory segment resources affecting a __shmget__ call: __SHMALL__ System wide maximum of shared memory pages: policy dependent. __SHMMAX__ Maximum size in bytes for a shared memory segment: implementation dependent (currently 4M). __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, was 128 in Linux 2.2 kernels). The implementation has no specific limits for the per process maximum number of shared memory segments (__SHMSEG__). !!BUGS Use of __IPC_PRIVATE__ doesn't inhibit to other processes the access to the allocated shared memory segment. As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a shared memory segment. Asserting both __IPC_CREAT__ and __IPC_EXCL__ in ''shmflg'' only ensures (on success) that a new shared memory segment will be created, it doesn't imply exclusive access to the segment. !!CONFORMING TO SVr4, SVID. SVr4 documents an additional error condition EEXIST. Neither SVr4 nor SVID documents an EIDRM condition. !!SEE ALSO ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2) ----
9 pages link to
shmget(2)
:
ipc(2)
Man2s
ftok(3)
ipc(5)
shmat(2)
shmctl(2)
shmdt(2)
shmop(2)
semctl(2)
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