msgget(2) [netbsd man page]
MSGGET(2) BSD System Calls Manual MSGGET(2) NAME
msgget -- get message queue identifier LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/msg.h> int msgget(key_t key, int msgflg); DESCRIPTION
The msgget() system call returns the message queue identifier associated with key. A message queue identifier is a unique integer greater than zero. A message queue is created if either key is equal to IPC_PRIVATE, or key does not have a message queue identifier associated with it and the IPC_CREAT bit is set in msgflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in msgflg, and key has a message queue identifier associated with it already, the operation will fail. If a new message queue is created, the data structure associated with it (the msqid_ds structure, see msgctl(2)) is initialized as follows: o msg_perm.cuid and msg_perm.uid are set to the effective uid of the calling process. o msg_perm.gid and msg_perm.cgid are set to the effective gid of the calling process. o msg_perm.mode is set to the lower 9 bits of msgflg. o msg_qnum, msg_lspid, msg_lrpid, msg_rtime, and msg_stime are set to 0. o msg_qbytes is set to the system wide maximum value for the number of bytes in a queue (MSGMNB). o msg_ctime is set to the current time. RETURN VALUES
Upon successful completion a positive message queue identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error. ERRORS
[EACCES] A message queue is already associated with key and the caller has no permission to access it. [EEXIST] Both IPC_CREAT and IPC_EXCL are set in msgflg, and a message queue is already associated with key. [ENOSPC] A new message queue could not be created because the system limit for the number of message queues has been reached. [ENOENT] IPC_CREAT is not set in msgflg and no message queue associated with key was found. SEE ALSO
msgctl(2), msgrcv(2), msgsnd(2), ftok(3) STANDARDS
The msgget system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5''). HISTORY
Message queues appeared in the first release of AT&T System V UNIX. BSD
May 13, 2004 BSD
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MSGGET(2) Linux Programmer's Manual MSGGET(2) NAME
msgget - get a message queue identifier SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgget(key_t key, int msgflg); DESCRIPTION
The msgget() system call returns the message queue identifier associated with the value of the key argument. A new message queue is cre- ated 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. RETURN VALUE
If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error. ERRORS
On failure, errno is set to one of the following values: 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. EEXIST A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL. 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. CONFORMING TO
SVr4, POSIX.1-2001. NOTES
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 maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/ker- nel/msgmni). Linux Notes Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion. BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function. SEE ALSO
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2004-05-27 MSGGET(2)