shmget(2) -- Linux man page



shmget - allocates a shared memory segment  


#include <sys/ipc.h>

#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);  


shmget() returns the identifier of the shared memory segment associated with the value of the argument key. A new shared memory segment, with size equal to the value of size rounded up to a multiple of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no shared memory segment corresponding to key exists, and IPC_CREAT is asserted in shmflg (i.e. shmflg&IPC_CREAT isn't zero).

The value shmflg is composed of:

used for allocating 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 key and check to see if the user has permission to access the segment.
used with IPC_CREAT to ensure failure if the segment already 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 has the following form:

struct shmid_ds {
    struct ipc_perm shm_perm;    /* operation perms */
    size_t          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 */

When creating a new shared memory segment, the system call initializes the shmid_ds 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.



After a fork() the child inherits the attached shared memory segments.
After an exec() all attached shared memory segments are detached (not destroyed).
Upon exit() all attached shared memory segments are detached (not destroyed).



A valid segment identifier, shmid, is returned on success, -1 on error.  


On failure, errno is set to one of the following:
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.
if IPC_CREAT | IPC_EXCL was specified and the segment exists.
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).
if no segment exists for the given key, and IPC_CREAT was not specified.
if the user does not have permission to access the shared memory segment.
if no memory could be allocated for segment overhead.


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:

System wide maximum of shared memory pages: policy dependent.
Maximum size in bytes for a shared memory segment: implementation dependent (currently 4M).
Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though PAGE_SIZE is the effective minimum size).
System wide maximum number of shared memory segments: implementation dependent (currently 4096, was 128 before Linux 2.3.99).

The implementation has no specific limits for the per process maximum number of shared memory segments (SHMSEG).  


The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.  


SVr4, SVID. SVr4 documents an additional error condition EEXIST. Until version 2.3.30 Linux would return EIDRM for a shmget on a shared memory segment scheduled for deletion.  


ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2)