Manpage of MSGGET
MSGGETSection: Linux Programmer's Manual (2)
NAMEmsgget - get a System V message queue identifier
#include <sys/types.h>#include <sys/ipc.h>#include <sys/msg.h>int msgget(key_t key, int msgflg);
DESCRIPTIONThe msgget() system call returns the System V message queue identifier associated with the value of the keyargument. A new message queue is created if keyhas the value IPC_PRIVATEor keyisn't IPC_PRIVATE, no message queue with the given key keyexists, and IPC_CREATis specified in msgflg.
If msgflgspecifies both IPC_CREATand IPC_EXCLand a message queue already exists for key, then msgget() fails with errnoset to EEXIST. (This is analogous to the effect of the combination O_CREAT | O_EXCLfor open(2).)
Upon creation, the least significant bits of the argument msgflgdefine the permissions of the message queue. These permission bits have the same format and semantics as the permissions specified for the modeargument 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.cuidand msg_perm.uidare set to the effective user ID of the calling process.
- msg_perm.cgidand msg_perm.gidare set to the effective group ID of the calling process.
- The least significant 9 bits of msg_perm.modeare set to the least significant 9 bits of msgflg.
- msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtimeare set to 0.
- msg_ctimeis set to the current time.
- msg_qbytesis set to the system limit MSGMNB.
RETURN VALUEIf successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errnoindicating the error.
ERRORSOn failure, errnois set to one of the following values:
- 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_OWNERcapability in the user namespace that governs its IPC namespace.
- IPC_CREATand IPC_EXCLwere specified in msgflg, but a message queue already exists for key.
- No message queue exists for keyand msgflgdid not specify IPC_CREAT.
- A message queue has to be created but the system does not have enough memory for the new data structure.
- A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, SVr4.
NOTESThe 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_PRIVATEisn't a flag field but a key_ttype. If this special value is used for key, the system call ignores everything but the least significant 9 bits of msgflgand creates a new message queue (on success).
The following is a system limit on message queue resources affecting a msgget() call:
- 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.
Linux notesUntil version 2.3.20, Linux would return EIDRMfor a msgget() on a message queue scheduled for deletion.
BUGSThe name choice IPC_PRIVATEwas perhaps unfortunate, IPC_NEWwould more clearly show its function.
SEE ALSOmsgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)
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