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Man Pages


Manual Reference Pages  -  SHMGET (2)

NAME

shmget - obtain a shared memory identifier

CONTENTS

Library
Synopsis
Description
Return Values
Errors

LIBRARY


.Lb libc

SYNOPSIS


.In sys/types.h
.In sys/ipc.h
.In sys/shm.h int shmget key_t key size_t size int flag

DESCRIPTION

Based on the values of key and flag, shmget returns the identifier of a newly created or previously existing shared memory segment. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key:
  • IPC_PRIVATE may be specified, in which case a new IPC object will be created.
  • An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created.
  • The ftok(3) may be used to generate a key from a pathname.

The mode of a newly created IPC object is determined by OR ’ing the following constants into the flag argument:
S_IRUSR Read access for owner.
S_IWUSR Write access for owner.
S_IRGRP Read access for group.
S_IWGRP Write access for group.
S_IROTH Read access for other.
S_IWOTH Write access for other.

When creating a new shared memory segment, size indicates the desired size of the new segment in bytes. The size of the segment may be rounded up to a multiple convenient to the kernel (i.e., the page size).

RETURN VALUES

Upon successful completion, shmget returns the positive integer identifier of a shared memory segment. Otherwise, -1 is returned and errno set to indicate the error.

ERRORS

The shmget system call will fail if:
[EINVAL]
  Size specified is greater than the size of the previously existing segment. Size specified is less than the system imposed minimum, or greater than the system imposed maximum.
[ENOENT]
  No shared memory segment was found matching key, and IPC_CREAT was not specified.
[ENOSPC]
  The kernel was unable to allocate enough memory to satisfy the request.
[EEXIST]
  IPC_CREAT and IPC_EXCL were specified, and a shared memory segment corresponding to key already exists.

SEE ALSO

shmat(2), shmctl(2), shmdt(2), stat(2), ftok(3)
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