msgget(2) [redhat man page]
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 function returns the message queue identifier associated to the value of the key argument. A new message queue is created if key has value IPC_PRIVATE or key isn't IPC_PRIVATE, no existing message queue is associated to key, and IPC_CREAT is asserted in msgflg (i.e. msgflg&IPC_CREAT is nonzero). The presence in msgflg of the fields IPC_CREAT and IPC_EXCL plays the same role, with respect to the exis- tence of the message queue, as the presence of O_CREAT and O_EXCL in the mode argument of the open(2) system call: i.e. the msgget function fails if msgflg asserts both IPC_CREAT and IPC_EXCL and a message queue already exists for key. Upon creation, the lower 9 bits of the argument msgflg define the access permissions of the message queue. These permission bits have the same format and semantics as the access permissions parameter in open(2) or creat(2) system calls. (The execute permissions are not used.) Furthermore, while creating, the system call initializes the system message queue data structure msqid_ds 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 lowest order 9 bits of msg_perm.mode are set to the lowest order 9 bit 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 access 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
For a failing return, errno will be set to one among the following values: EACCES A message queue exists for key, but the calling process has no access permissions to the queue. EEXIST A message queue exists for key and msgflg was asserting both IPC_CREAT and IPC_EXCL. EIDRM The message queue is marked for removal. ENOENT No message queue exists for key and msgflg wasn't asserting IPC_CREAT. ENOMEM A message queue has to be created but the system has not 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. 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 lowest order 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. BUGS
Use of IPC_PRIVATE does not actually prohibit other processes from getting access to the allocated message queue. There is currently no intrinsic way for a process to ensure exclusive access to a message queue. Asserting both IPC_CREAT and IPC_EXCL in msgflg only ensures (on success) that a new message queue will be created, it doesn't imply exclusive access to the message queue. CONFORMING TO
SVr4, SVID. SVr4 does not document the EIDRM error code. SEE ALSO
ftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2) Linux 0.99.13 1993-11-01 MSGGET(2)
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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
The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. 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 3.44 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 2012-05-31 MSGGET(2)