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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO |
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msgget(2) System Calls Manual msgget(2)
msgget - get a System V message queue identifier
Standard C library (libc, -lc)
#include <sys/msg.h>
int msgget(key_t key, int msgflg);
The msgget() system call returns the System V message queue
identifier associated with the value of the key argument. It may
be used either to obtain the identifier of a previously created
message queue (when msgflg is zero and key does not have the
value IPC_PRIVATE), or to create a new set.
A new message queue is created if key has the value IPC_PRIVATE
or key isn't IPC_PRIVATE, no message queue with the given key key
exists, and IPC_CREAT is specified in msgflg.
If msgflg specifies both IPC_CREAT and IPC_EXCL and a message
queue already exists for key, then msgget() fails with errno set
to EEXIST. (This is analogous to the effect of the combination
O_CREAT | O_EXCL for open(2).)
Upon creation, the least significant bits of the argument msgflg
define the permissions of the message queue. These permission
bits have the same format and semantics as the permissions
specified for the mode argument of open(2). (The execute
permissions are not used.)
If a new message queue is created, then its associated data
structure msqid_ds (see msgctl(2)) is initialized as follows:
• msg_perm.cuid and msg_perm.uid are set to the effective user
ID of the calling process.
• msg_perm.cgid and msg_perm.gid are set to the effective group
ID of the calling process.
• The least significant 9 bits of msg_perm.mode are set to the
least significant 9 bits of msgflg.
• msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are
set to 0.
• msg_ctime is set to the current time.
• msg_qbytes is set to the system limit MSGMNB.
If the message queue already exists the permissions are verified,
and a check is made to see if it is marked for destruction.
On success, msgget() returns the message queue identifier (a
nonnegative integer). On failure, -1 is returned, and errno is
set to indicate the error.
EACCES A message queue exists for key, but the calling process
does not have permission to access the queue, and does not
have the CAP_IPC_OWNER capability in the user namespace
that governs its IPC namespace.
EEXIST IPC_CREAT and IPC_EXCL were specified in msgflg, but a
message queue already exists for key.
ENOENT No message queue exists for key and msgflg did not specify
IPC_CREAT.
ENOMEM A message queue has to be created but the system does not
have enough memory for the new data structure.
ENOSPC A message queue has to be created but the system limit for
the maximum number of message queues (MSGMNI) would be
exceeded.
POSIX.1-2008.
POSIX.1-2001, SVr4.
Linux
Until Linux 2.3.20, Linux would return EIDRM for a msgget() on a
message queue scheduled for deletion.
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
least significant 9 bits of msgflg and creates a new message
queue (on success).
The following is a system limit on message queue resources
affecting a msgget() call:
MSGMNI System-wide limit on the number of message queues. Before
Linux 3.19, the default value for this limit was
calculated using a formula based on available system
memory. Since Linux 3.19, the default value is 32,000.
On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmni.
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7),
mq_overview(7), sysvipc(7)
Linux man-pages (unreleased) (date) msgget(2)
Pages that refer to this page: ipcrm(1), ipc(2), msgctl(2), msgop(2), syscalls(2), umask(2), ftok(3), mq_overview(7), sysvipc(7)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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