---
name: "sys_msg.h"
section: "0p"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/sys_msg.h"
---

# sys\_msg.h(0p)

XSI message queue structures

## PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

## NAME

sys/msg.h — XSI message queue structures

## SYNOPSIS

```
#include <sys/msg.h>
```

## DESCRIPTION

The *\<sys/msg.h\>* header shall define the following data types through **typedef**:

- **msgqnum\_t**

  Used for the number of messages in the message queue.

- **msglen\_t**

  Used for the number of bytes allowed in a message queue.

These types shall be unsigned integer types that are able to store values at least as large as a type **unsigned short**.

The *\<sys/msg.h\>* header shall define the following symbolic constant as a message operation flag:

- MSG\_NOERROR

  No error if big message.

The *\<sys/msg.h\>* header shall define the **msqid\_ds** structure, which shall include the following members:

```
struct ipc_perm msg_perm   Operation permission structure.
msgqnum_t       msg_qnum   Number of messages currently on queue.
msglen_t        msg_qbytes Maximum number of bytes allowed on queue.
pid_t           msg_lspid  Process ID of last msgsnd().
pid_t           msg_lrpid  Process ID of last msgrcv().
time_t          msg_stime  Time of last msgsnd().
time_t          msg_rtime  Time of last msgrcv().
time_t          msg_ctime  Time of last change.
```

The *\<sys/msg.h\>* header shall define the **pid\_t**, **size\_t**, **ssize\_t**, and **time\_t** types as described in *\<sys/types.h\>*.

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

```
int       msgctl(int, int, struct msqid_ds *);
int       msgget(key_t, int);
ssize_t   msgrcv(int, void *, size_t, long, int);
int       msgsnd(int, const void *, size_t, int);
```

In addition, the *\<sys/msg.h\>* header shall include the *\<sys/ipc.h\>* header.

*The following sections are informative.*

## APPLICATION USAGE

None.

## RATIONALE

None.

## FUTURE DIRECTIONS

None.

## SEE ALSO

***\<sys\_ipc.h\>***, ***\<sys\_types.h\>***

The System Interfaces volume of POSIX.1‐2017, *msgctl()*, *msgget()*, *msgrcv()*, *msgsnd()*

## COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .