--- name: "sendmmsg" section: "2" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/send/2" --- # sendmmsg(2) send message(s) from a socket Part of [send(2)](/send/2.md). ## NAME **send**, **sendto**, **sendmsg**, **sendmmsg** – send message(s) from a socket ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *ssize\_t*\ **send**(*int s*, *const void \*msg*, *size\_t len*, *int flags*); *ssize\_t*\ **sendto**(*int s*, *const void \*msg*, *size\_t len*, *int flags*, *const struct sockaddr \*to*, *socklen\_t tolen*); *ssize\_t*\ **sendmsg**(*int s*, *const struct msghdr \*msg*, *int flags*); *ssize\_t*\ **sendmmsg**(*int s*, *struct mmsghdr \* restrict msgvec*, *size\_t vlen*, *int flags*); ## DESCRIPTION The **send**() and **sendmmsg**() functions, and **sendto**() and **sendmsg**() system calls are used to transmit one or more messages (with the **sendmmsg**() call) to another socket. The **send**() function may be used only when the socket is in a *connected* state. The functions **sendto**(), **sendmsg**() and **sendmmsg**() may be used at any time if the socket is connectionless-mode. If the socket is connection-mode, the protocol must support implied connect (currently [tcp(4)](/tcp/4.md) is the only protocol with support) or the socket must be in a connected state before use. The address of the target is given by *to* with *tolen* specifying its size, or the equivalent *msg\_name* and *msg\_namelen* in *struct*, *msghdr*. If the socket is in a connected state, the target address passed to **sendto**(), **sendmsg**() or **sendmmsg**() is ignored. The length of the message is given by *len*. If the message is too long to pass atomically through the underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted. The **sendmmsg**() function sends multiple messages at a call. They are given by the *msgvec* vector along with *vlen* specifying the vector size. The number of octets sent per each message is placed in the *msg\_len* field of each processed element of the vector after transmission. No indication of failure to deliver is implicit in a **send**(). Locally detected errors are indicated by a return value of -1. If no messages space is available at the socket to hold the message to be transmitted, then **send**() normally blocks, unless the socket has been placed in non-blocking I/O mode. The [select(2)](/select/2.md) system call may be used to determine when it is possible to send more data. The *flags* argument may include one or more of the following: ``` #define MSG_OOB 0x00001 /* process out-of-band data */ #define MSG_DONTROUTE 0x00004 /* bypass routing, use direct interface */ #define MSG_EOR 0x00008 /* data completes record */ #define MSG_DONTWAIT 0x00080 /* do not block */ #define MSG_EOF 0x00100 /* data completes transaction */ #define MSG_NOSIGNAL 0x20000 /* do not generate SIGPIPE on EOF */ ``` The flag MSG\_OOB is used to send “out-of-band” data on sockets that support this notion (e.g. SOCK\_STREAM); the underlying protocol must also support “out-of-band” data. MSG\_EOR is used to indicate a record mark for protocols which support the concept. The MSG\_DONTWAIT flag request the call to return when it would block otherwise. MSG\_EOF requests that the sender side of a socket be shut down, and that an appropriate indication be sent at the end of the specified data; this flag is only implemented for SOCK\_STREAM sockets in the PF\_INET protocol family. MSG\_DONTROUTE is usually used only by diagnostic or routing programs. MSG\_NOSIGNAL is used to prevent SIGPIPE generation when writing a socket that may be closed. See [recv(2)](/recv/2.md) for a description of the *msghdr* structure and the *mmsghdr* structure. ## RETURN VALUES The **send**(), **sendto**() and **sendmsg**() calls return the number of octets sent. The **sendmmsg**() call returns the number of messages sent. If an error occurred a value of -1 is returned. ## ERRORS The **send**() and **sendmmsg**() functions and **sendto**() and **sendmsg**() system calls fail if: - \[EBADF\] An invalid descriptor was specified. - \[EACCES\] The destination address is a broadcast address, and SO\_BROADCAST has not been set on the socket. - \[ENOTCONN\] The socket is connection-mode but is not connected. - \[ENOTSOCK\] The argument *s* is not a socket. - \[EFAULT\] An invalid user space address was specified for an argument. - \[EMSGSIZE\] The socket requires that message be sent atomically, and the size of the message to be sent made this impossible. - \[EAGAIN\] The socket is marked non-blocking, or MSG\_DONTWAIT is specified, and the requested operation would block. - \[ENOBUFS\] The system was unable to allocate an internal buffer. The operation may succeed when buffers become available. - \[ENOBUFS\] The output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion. - \[EHOSTUNREACH\] The remote host was unreachable. - \[EISCONN\] A destination address was specified and the socket is already connected. - \[ECONNREFUSED\] The socket received an ICMP destination unreachable message from the last message sent. This typically means that the receiver is not listening on the remote port. - \[EHOSTDOWN\] The remote host was down. - \[ENETDOWN\] The remote network was down. - \[EADDRNOTAVAIL\] The process using a SOCK\_RAW socket was jailed and the source address specified in the IP header did not match the IP address bound to the prison. - \[EPERM\] The *msg* contained an SCM\_RIGHTS control message, and the receiving [unix(4)](/unix/4.md) socket is configured to reject new SCM\_RIGHTS. - \[EPIPE\] The socket is unable to send anymore data (SBS\_CANTSENDMORE has been set on the socket). This typically means that the socket is not connected. ## SEE ALSO [connect(2)](/connect/2.md), [fcntl(2)](/fcntl/2.md), [getsockopt(2)](/getsockopt/2.md), [recv(2)](/recv/2.md), [select(2)](/select/2.md), [socket(2)](/socket/2.md), [write(2)](/write/2.md), [CMSG\_DATA(3)](/CMSG_DATA/3.md), [unix(4)](/unix/4.md) ## HISTORY The **send**() function appeared in 4.2BSD. The **sendmmsg**() function appeared in FreeBSD 11.0. ## BUGS Because **sendmsg**() does not necessarily block until the data has been transferred, it is possible to transfer an open file descriptor across an AF\_UNIX domain socket (see [recv(2)](/recv/2.md)), then **close**() it before it has actually been sent, the result being that the receiver gets a closed file descriptor. It is left to the application to implement an acknowledgment mechanism to prevent this from happening.