.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993 .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified Wed Jul 28 10:57:35 1993, Rik Faith .\" Modified Sun Nov 28 16:43:30 1993, Rik Faith .\" with material from Giorgio Ciucci .\" Portions Copyright 1993 Giorgio Ciucci .\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond .\" .TH SHMGET 2 "November 28, 1993" "Linux 0.99.11" "Linux Programmer's Manual" .SH NAME shmget \- allocates a shared memory segment .SH SYNOPSIS .ad l .B #include .sp .B #include .sp .BI "int shmget(key_t " key ", int " size ", int " shmflg ); .ad b .SH DESCRIPTION .B shmget() returns the identifier of the shared memory segment associated to the value of the argument .IR key . A new shared memory segment, with size equal to the round up of .I size to a multiple of .BR PAGE_SIZE , is created if .I key has value .B IPC_PRIVATE or .I key isn't .BR IPC_PRIVATE , no shared memory segment is associated to .IR key , and .B IPC_CREAT is asserted in .I shmflg (i.e. .IB shmflg &IPC_CREAT isn't zero). The presence in .PP .I shmflg is composed of: .TP 12 .B IPC_CREAT to create a new segment. If this flag is not used, then .B shmget() will find the segment associated with \fIkey\fP, check to see if the user has permission to receive the \fIshmid\fP associated with the segment, and ensure the segment is not marked for destruction. .TP .B IPC_EXCL used with \fBIPC_CREAT\fP to ensure failure if the segment exists. .TP .B 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. .PP If a new segment is created, the access permissions from .I shmflg are copied into the .I shm_perm member of the .I shmid_ds structure that defines the segment. The \fIshmid_ds\fP structure: .PP .in +0.5i .nf 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 */ }; .fi .in -0.5i .PP .in +0.5i .nf 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 \fIshmflg\fP */ ushort seq; /* sequence number */ }; .fi .PP Furthermore, while creating, the system call initializes the system shared memory segment data structure .B shmid_ds as follows: .IP .B shm_perm.cuid and .B shm_perm.uid are set to the effective user\-ID of the calling process. .IP .B shm_perm.cgid and .B shm_perm.gid are set to the effective group\-ID of the calling process. .IP The lowest order 9 bits of .B shm_perm.mode are set to the lowest order 9 bit of .IR shmflg . .IP .B shm_segsz is set to the value of .IR size. .IP .BR shm_lpid , .BR shm_nattch , .B shm_atime and .B shm_dtime are set to .BR 0. .IP .B shm_ctime is set to the current time. .PP 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. .PP .SH "SYSTEM CALLS" .TP 0.8i .B fork() After a .B fork() the child inherits the attached shared memory segments. .TP .B exec() After an .B exec() all attached shared memory segments are detached (not destroyed). .TP .B exit() Upon .B exit() all attached shared memory segments are detached (not destroyed). .PP .SH "RETURN VALUE" A valid segment identifier, .IR shmid , is returned on success, \-1 on error. .SH ERRORS On failure, .B errno is set to one of the following: .TP 12 .B EINVAL is returned if \fBSHMMIN\fP > \fIsize\fP or \fIsize\fP > \fBSHMMAX\fP, or \fIsize\fP is greater than size of segment. .TP .B EEXIST is returned if .B IPC_CREAT | IPC_EXCL was specified and the segment exists. .TP .B EIDRM is returned if the segment is marked as destroyed, or was removed. .TP .B ENOSPC is returned if all possible shared memory id's have been taken .BR "" ( SHMMNI ) or if allocating a segment of the requested .I size would cause the system to exceed the system-wide limit on shared memory .BR "" ( SHMALL ). .TP .B ENOENT is returned if no segment exists for the given \fIkey\fP, and .B IPC_CREAT was not specified. .TP .B EACCES is returned if the user does not have permission to access the shared memory segment. .TP .B ENOMEM is returned if no memory could be allocated for segment overhead. .SH NOTES .B IPC_PRIVATE isn't a flag field but a .B key_t type. If this special value is used for .IR key , the system call ignores everything but the lowest order 9 bits of .I shmflg and creates a new shared memory segment (on success). .PP The followings are limits on shared memory segment resources affecting a .B shmget call: .TP 11 .B SHMALL System wide maximum of shared memory pages: policy dependent. .TP .B SHMMAX Maximum size in bytes for a shared memory segment: implementation dependent (currently 4M). .TP .B SHMMIN Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though .B PAGE_SIZE is the effective minimum size). .TP .B SHMMNI System wide maximum number of shared memory segments: implementation dependent (currently 4096). .PP The implementation has no specific limits for the per process maximum number of shared memory segments .RB ( SHMSEG ). .SH BUGS Use of .B IPC_PRIVATE doesn't inhibit to other processes the access to the allocated shared memory segment. .PP As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a shared memory segment. Asserting both .B IPC_CREAT and .B IPC_EXCL in .I shmflg only ensures (on success) that a new shared memory segment will be created, it doesn't imply exclusive access to the segment. .SH "CONFORMING TO" SVr4, SVID. SVr4 documents an additional error condition EEXIST. Neither SVr4 nor SVID documents an EIDRM condition. .SH "SEE ALSO" .BR ftok (3), .BR ipc (5), .BR shmctl (2), .BR shmat (2), .BR shmdt (2).