---
name: "msgsnd"
section: "2freebsd"
platform: "linux"
package: "freebsd-manpages"
version: "12.2-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/msgsnd/2freebsd"
---

# msgsnd(2freebsd)

send a message to a message queue

## NAME

**msgsnd** – send a message to a message queue

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<sys/types.h\>**

**#include \<sys/ipc.h\>**

**#include \<sys/msg.h\>**

*int*\

**msgsnd**(*int msqid*, *const void \*msgp*, *size\_t msgsz*, *int msgflg*);

## DESCRIPTION

The **msgsnd**() function sends a message to the message queue specified in *msqid*. The *msgp* argument points to a structure containing the message. This structure should consist of the following members:

```
    long mtype;    /* message type */
    char mtext[1]; /* body of message */
```

*mtype* is an integer greater than 0 that can be used for selecting messages (see [msgrcv(2)](/msgrcv/2.md)), *mtext* is an array of *msgsz* bytes. The argument *msgsz* can range from 0 to a system-imposed maximum, MSGMAX.

If the number of bytes already on the message queue plus *msgsz* is bigger than the maximum number of bytes on the message queue (*msg\_qbytes*, see [msgctl(2)](/msgctl/2.md)), or the number of messages on all queues system-wide is already equal to the system limit, *msgflg* determines the action of **msgsnd**(). If *msgflg* has IPC\_NOWAIT mask set in it, the call will return immediately. If *msgflg* does not have IPC\_NOWAIT set in it, the call will block until:

- The condition which caused the call to block does no longer exist. The message will be sent.

- The message queue is removed, in which case -1 will be returned, and *errno* is set to EINVAL.

- The caller catches a signal. The call returns with *errno* set to EINTR.

After a successful call, the data structure associated with the message queue is updated in the following way:

- *msg\_cbytes* is incremented by the size of the message.

- *msg\_qnum* is incremented by 1.

- *msg\_lspid* is set to the pid of the calling process.

- *msg\_stime* is set to the current time.

## RETURN VALUES

The **msgsnd()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

## ERRORS

The **msgsnd**() function will fail if:

- \[EINVAL\]

  The *msqid* argument is not a valid message queue identifier.

  The message queue was removed while **msgsnd**() was waiting for a resource to become available in order to deliver the message.

  The *msgsz* argument is greater than *msg\_qbytes*.

  The *mtype* argument is not greater than 0.

- \[EACCES\]

  The calling process does not have write access to the message queue.

- \[EAGAIN\]

  There was no space for this message either on the queue, or in the whole system, and IPC\_NOWAIT was set in *msgflg*.

- \[EFAULT\]

  The *msgp* argument points to an invalid address.

- \[EINTR\]

  The system call was interrupted by the delivery of a signal.

## HISTORY

Message queues appeared in the first release of AT&T Unix System V.

## BUGS

NetBSD and FreeBSD do not define the EIDRM error value, which should be used in the case of a removed message queue.