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msgget(2)

NAME

msgget - Returns (and possibly creates) the ID for a message queue

SYNOPSIS

#include <sys/msg.h> int msgget( key_t key, int msgflg); Application developers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/msg.h> if programs are being developed for multiple platforms. The additional #include statements are not required on Tru64 UNIX systems or by ISO or X/Open standards, but may be required on other vendors' systems that conform to these standards.

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows: msgget(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

key Specifies the key that identifies the message queue. The value for the key parameter can be IPC_PRIVATE or a random number other than zero (0). To ensure that the msgget() function returns a new message queue ID, specify the IPC_PRIVATE keyword. msgflg Specifies flags that control the msgget() function. Possible values are as follows: IPC_CREAT Specifies creating a message queue ID. If the specified key does not exist, the msgget function creates a message queue ID and associates it with the specified key. If the specified key does exist, and IPC_EXCL is not set, the msgget function returns the message queue ID associated with key. If the specified key does exist and IPC_EXCL is set, the msgget function fails and returns an error. IPC_EXCL Specifies that you want to ensure that your application is the only one using the specified key parameter. If the specified key already exists, the msgget function fails and returns an error notification.

DESCRIPTION

The msgget function returns the message queue ID for the message queue identified by the key parameter. If the message queue ID does not exist, the msgget() function attempts to create it. If you specify IPC_PRIVATE for the key parameter, the msgget function returns the ID for a private (that is, newly created) message queue. The msgflg parameter supplies creation options for the msgget function. The IPC_CREAT flag specifies that you want the kernel to create a message queue for your application. The kernel returns the message queue ID to your application as the return value of the successful call to the msgget() function. The msgget() function does not create a message queue ID for your application under the following conditions: · A message queue ID that is associated with the specified key already exists. In this case, the msgget() function returns the message queue ID associated with the key parameter (unless you have set the IPC_EXCL flag). · You have set the IPC_EXCL flag and another application is using the specified key. In this case, the msgget() function fails and returns an error to your application. After creating a new message queue ID, the msgget function initializes the msqid_ds structure associated with the ID as follows: · The msg_perm.cuid and msg_perm.uid members are set equal to the effective user ID of the calling process. · The msg_perm.cgid and msg_perm.gid members are set equal to the effective group ID of the calling process. · The low-order nine bits of the msg_perm.mode member are set equal to the low-order nine bits of msgflg. · The msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime members are all set equal to zero. · The msg_ctime member is set equal to the current time. · The msg_qbytes member is set equal to the system limit.

RETURN VALUES

Upon successful completion, the msgget() function returns a message queue identifier. Otherwise, the function returns a value of -1 and sets errno to indicate the error.

ERRORS

The msgget function sets errno to the specified values for the following conditions: [EACCES] A message queue identifier exists for the key parameter, but the calling process does not have the appropriate privileges to perform the operation. The permission is specified by the low- order nine bits of the msgflg parameter. [EEXIST] A message queue identifier exists for the key parameter, and both IPC_CREAT and IPC_EXCL are set. [ENOENT] A message queue identifier does not exist for the key parameter and the IPC_CREAT value is not set. [ENOSPC] A message queue identifier can be created, but the system-imposed limit on the maximum number of allowed message queue identifiers has been exceeded.

RELATED INFORMATION

Functions: msgctl(2), msgrcv(2), msgsnd(2), table(2), ftok(3) Files: msqid_ds(4) Standards: standards(5)

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