msgget - 得到一個消息隊列標識符

內容簡介

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

int msgget(key_t key**,** int msgflg**);**

描述

The  msgget () system call returns the message queue identifier associated with the value of the  key  argument. 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 (seemsgctl(2)) is initialised as follows:

標籤

描述

 

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.

如果消息隊列中已經存在的權限進行了驗證，並進行檢查，看它是否被標記銷燬。

返回值

If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with  errno  indicating the error.

錯誤

On failure,  errno  is set to one of the following values:

標籤

描述

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 bothIPC_CREAT and IPC_EXCL.

ENOENT

No message queue exists for key and msgflg did not specifyIPC_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.

注意

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:

標籤

描述

MSGMNI

System wide maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via*/proc/sys/kernel/msgmni*).

BUGS

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

遵循於

SVr4, POSIX.1-2001.

LINUX 注意

Until version 2.3.20 Linux would return EIDRM for a  msgget () on a message queue scheduled for deletion.

另請參閱

• msgctl (2)

• msgrcv (2)

• msgsnd (2)