---
name: "msgget"
section: "2"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/msgget/2"
---

# msgget(2)

get a System V message queue identifier

## NAME

msgget - get a System V 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 System V message queue identifier associated with the value of the *key* argument. It may be used either to obtain the identifier of a previously created message queue (when *msgflg* is zero and *key* does not have the value **IPC\_PRIVATE**), or to create a new set.

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)](/open/2.md).)

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)](/open/2.md). (The execute permissions are not used.)

If a new message queue is created, then its associated data structure *msqid\_ds* (see [**msgctl**(2)](/msgctl/2.md)) 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

On success, **msgget**() returns the message queue identifier (a nonnegative integer). On failure, -1 is returned, and *errno* is set to indicate the error.

## ERRORS

- **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 in the user namespace that governs its IPC namespace.

- **EEXIST**

  **IPC\_CREAT** and **IPC\_EXCL** were specified in *msgflg*, but a message queue already exists for *key*.

- **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.

## STANDARDS

POSIX.1-2024 XSI.

## HISTORY

SVr4, SUSv1, POSIX.1-2001 XSI.

### Linux

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

## 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 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*.

## BUGS

The name choice **IPC\_PRIVATE** was perhaps unfortunate, **IPC\_NEW** would more clearly show its function.

## SEE ALSO

[**msgctl**(2)](/msgctl/2.md), [**msgrcv**(2)](/msgrcv/2.md), [**msgsnd**(2)](/msgsnd/2.md), [**ftok**(3)](/ftok/3.md), [**capabilities**(7)](/capabilities/7.md), [**mq\_overview**(7)](/mq_overview/7.md), [**sysvipc**(7)](/sysvipc/7.md)