1SHMGET(2)                  Linux Programmer's Manual                 SHMGET(2)
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NAME

6       shmget - allocates a shared memory segment
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SYNOPSIS

9       #include <sys/ipc.h>
10       #include <sys/shm.h>
11
12       int shmget(key_t key, size_t size, int shmflg);
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DESCRIPTION

15       shmget() returns the identifier of the shared memory segment associated
16       with the value of the argument key.  A new shared memory segment,  with
17       size  equal to the value of size rounded up to a multiple of PAGE_SIZE,
18       is created if key has the value IPC_PRIVATE or key  isn't  IPC_PRIVATE,
19       no  shared memory segment corresponding to key exists, and IPC_CREAT is
20       specified in shmflg.
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22       If shmflg specifies both IPC_CREAT and IPC_EXCL  and  a  shared  memory
23       segment  already  exists for key, then shmget() fails with errno set to
24       EEXIST.  (This is analogous to the effect of the combination O_CREAT  |
25       O_EXCL for open(2).)
26
27       The value shmflg is composed of:
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29       IPC_CREAT   to  create  a  new segment.  If this flag is not used, then
30                   shmget() will find the  segment  associated  with  key  and
31                   check  to see if the user has permission to access the seg‐
32                   ment.
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34       IPC_EXCL    used with  IPC_CREAT  to  ensure  failure  if  the  segment
35                   already exists.
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37       mode_flags  (least  significant  9  bits)  specifying  the  permissions
38                   granted to the owner, group, and world.   These  bits  have
39                   the same format, and the same meaning, as the mode argument
40                   of open(2).  Presently, the  execute  permissions  are  not
41                   used by the system.
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43       SHM_HUGETLB (since Linux 2.6)
44                   Allocate  the  segment  using "huge pages."  See the kernel
45                   source file  Documentation/vm/hugetlbpage.txt  for  further
46                   information.
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48       SHM_NORESERVE (since Linux 2.6.15)
49                   This  flag serves the same purpose as the mmap(2) MAP_NORE‐
50                   SERVE flag.  Do not reserve swap space  for  this  segment.
51                   When  swap space is reserved, one has the guarantee that it
52                   is possible to modify the segment.  When swap space is  not
53                   reserved  one might get SIGSEGV upon a write if no physical
54                   memory is available.  See also the discussion of  the  file
55                   /proc/sys/vm/overcommit_memory in proc(5).
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57       When  a new shared memory segment is created, its contents are initial‐
58       ized to zero values, and its associated data structure,  shmid_ds  (see
59       shmctl(2)), is initialized as follows:
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61              shm_perm.cuid  and shm_perm.uid are set to the effective user ID
62              of the calling process.
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64              shm_perm.cgid and shm_perm.gid are set to the effective group ID
65              of the calling process.
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67              The  least  significant  9  bits of shm_perm.mode are set to the
68              least significant 9 bit of shmflg.
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70              shm_segsz is set to the value of size.
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72              shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.
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74              shm_ctime is set to the current time.
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76       If the shared memory segment already exists, the permissions are  veri‐
77       fied, and a check is made to see if it is marked for destruction.
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RETURN VALUE

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

83       On failure, errno is set to one of the following:
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85       EACCES The  user  does  not have permission to access the shared memory
86              segment, and does not have the CAP_IPC_OWNER capability.
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88       EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.
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90       EINVAL A new segment was to be created and size < SHMMIN or size > SHM‐
91              MAX,  or  no new segment was to be created, a segment with given
92              key existed, but size is greater than the size of that segment.
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94       ENFILE The system limit on the total number  of  open  files  has  been
95              reached.
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97       ENOENT No segment exists for the given key, and IPC_CREAT was not spec‐
98              ified.
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100       ENOMEM No memory could be allocated for segment overhead.
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102       ENOSPC All possible shared memory IDs  have  been  taken  (SHMMNI),  or
103              allocating  a segment of the requested size would cause the sys‐
104              tem to exceed the system-wide limit on shared memory (SHMALL).
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106       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv‐
107              ileged (did not have the CAP_IPC_LOCK capability).
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CONFORMING TO

110       SVr4, POSIX.1-2001.
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112       SHM_HUGETLB is a non-portable Linux extension.
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NOTES

115       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
116       is used for key, the system call ignores everything but the least  sig‐
117       nificant  9  bits of shmflg and creates a new shared memory segment (on
118       success).
119
120       The following limits on shared  memory  segment  resources  affect  the
121       shmget() call:
122
123       SHMALL System wide maximum of shared memory pages (on Linux, this limit
124              can be read and modified via /proc/sys/kernel/shmall).
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126       SHMMAX Maximum size in bytes for a shared memory segment: policy depen‐
127              dent  (on  Linux,  this  limit  can  be  read  and  modified via
128              /proc/sys/kernel/shmmax).
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130       SHMMIN Minimum size in bytes for a shared memory  segment:  implementa‐
131              tion dependent (currently 1 byte, though PAGE_SIZE is the effec‐
132              tive minimum size).
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134       SHMMNI System wide maximum number of shared memory segments:  implemen‐
135              tation  dependent  (currently 4096, was 128 before Linux 2.3.99;
136              on Linux, this limit can be read and modified via /proc/sys/ker‐
137              nel/shmmni).
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139       The  implementation  has no specific limits for the per-process maximum
140       number of shared memory segments (SHMSEG).
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142   Linux Notes
143       Until version 2.3.30 Linux would return  EIDRM  for  a  shmget()  on  a
144       shared memory segment scheduled for deletion.
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BUGS

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

151       shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabilities(7),  shm_over‐
152       view(7), svipc(7)
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COLOPHON

155       This  page  is  part of release 3.22 of the Linux man-pages project.  A
156       description of the project, and information about reporting  bugs,  can
157       be found at http://www.kernel.org/doc/man-pages/.
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161Linux                             2006-05-02                         SHMGET(2)
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