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SHMGET (2) | System calls | Unix Manual Pages | :man▋

NAME

shmget - obtain a shared memory identifier

CONTENTS

Library
Synopsis
Description
Return Values
Errors

LIBRARY


.Lb libc

SYNOPSIS


.In machine/param.h
.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:

SHM_R Read access for user.
SHM_W Write access for user.
( SHM_R>>3)
Read access for group.
( SHM_W>>3)
Write access for group.
( SHM_R>>6)
Read access for other.
( SHM_W>>6)
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), ftok(3)

 
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