--- name: "recvmsg" section: "2freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/recv/2freebsd" --- # recvmsg(2freebsd) receive message(s) from a socket Part of [recv(2freebsd)](/recv/2freebsd.md). ## NAME **recv**, **recvfrom**, **recvmsg**, **recvmmsg** – receive message(s) from a socket ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *ssize\_t*\ **recv**(*int s*, *void \*buf*, *size\_t len*, *int flags*); *ssize\_t*\ **recvfrom**(*int s*, *void \*buf*, *size\_t len*, *int flags*, *struct sockaddr \* restrict from*, *socklen\_t \* restrict fromlen*); *ssize\_t*\ **recvmsg**(*int s*, *struct msghdr \*msg*, *int flags*); *ssize\_t*\ **recvmmsg**(*int s*, *struct mmsghdr \* restrict msgvec*, *size\_t vlen*, *int flags*, *const struct timespec \* restrict timeout*); ## DESCRIPTION The **recvfrom**(), **recvmsg**(), and **recvmmsg**() system calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented. If *from* is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The *fromlen* argument is a value-result argument, initialized to the size of the buffer associated with *from*, and modified on return to indicate the actual size of the address stored there. The **recv**() function is normally used only on a *connected* socket (see [connect(2)](/connect/2.md)) and is identical to **recvfrom**() with a null pointer passed as its *from* argument. The **recvmmsg**() function is used to receive multiple messages at a call. Their number is supplied by *vlen*. The messages are placed in the buffers described by *msgvec* vector, after reception. The size of each received message is placed in the *msg\_len* field of each element of the vector. If *timeout* is NULL the call blocks until the data is available for each supplied message buffer. Otherwise it waits for data for the specified amount of time. If the timeout expired and there is no data received, a value 0 is returned. The [ppoll(2)](/ppoll/2.md) system call is used to implement the timeout mechanism, before first receive is performed. The **recv**(), **recvfrom**() and **recvmsg**() return the length of the message on successful completion, whereas **recvmmsg**() returns the number of received messages. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see [socket(2)](/socket/2.md)). If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is non-blocking (see [fcntl(2)](/fcntl/2.md)) in which case the value -1 is returned and the global variable *errno* is set to EAGAIN. The receive calls except **recvmmsg**() normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options SO\_RCVLOWAT and SO\_RCVTIMEO described in [getsockopt(2)](/getsockopt/2.md). The **recvmmsg**() function implements this behaviour for each message in the vector. The [select(2)](/select/2.md) system call may be used to determine when more data arrives. The *flags* argument to a **recv**() function is formed by *or*'ing one or more of the values: | | | | --- | --- | | MSG\_OOB | process out-of-band data | | MSG\_PEEK | peek at incoming message | | MSG\_WAITALL | wait for full request or error | | MSG\_DONTWAIT | do not block | | MSG\_CMSG\_CLOEXEC | set received fds close-on-exec | | MSG\_WAITFORONE | do not block after receiving the first message | The MSG\_OOB flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. The MSG\_PEEK flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data. The MSG\_WAITALL flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. The MSG\_DONTWAIT flag requests the call to return when it would block otherwise. If no data is available, *errno* is set to EAGAIN. This flag is not available in ANSI X3.159-1989 (“ANSI C89”) or ISO/IEC 9899:1999 (“ISO C99”) compilation mode. The MSG\_WAITFORONE flag sets MSG\_DONTWAIT after the first message has been received. This flag is only relevant for **recvmmsg**(). The **recvmsg**() system call uses a *msghdr* structure to minimize the number of directly supplied arguments. This structure has the following form, as defined in ⟨sys/socket.h⟩: ``` struct msghdr { void *msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ void *msg_control; /* ancillary data, see below */ socklen_t msg_controllen;/* ancillary data buffer len */ int msg_flags; /* flags on received message */ }; ``` Here *msg\_name* and *msg\_namelen* specify the source address if the socket is unconnected; *msg\_name* may be given as a null pointer if no names are desired or required. The *msg\_iov* and *msg\_iovlen* arguments describe scatter gather locations, as discussed in [read(2)](/read/2.md). The *msg\_control* argument, which has length *msg\_controllen*, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form: ``` struct cmsghdr { socklen_t cmsg_len; /* data byte count, including hdr */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by u_char cmsg_data[]; */ }; ``` As an example, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting a **recvmsg**() with no data buffer provided immediately after an **accept**() system call. With AF\_UNIX domain sockets, ancillary data can be used to pass file descriptors and process credentials. See [unix(4)](/unix/4.md) for details. The *msg\_flags* field is set on return according to the message received. MSG\_EOR indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK\_SEQPACKET). MSG\_TRUNC indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied. MSG\_CTRUNC indicates that some control data were discarded due to lack of space in the buffer for ancillary data. MSG\_OOB is returned to indicate that expedited or out-of-band data were received. The **recvmmsg**() system call uses the *mmsghdr* structure, defined as follows in the ⟨sys/socket.h⟩ header: ``` struct mmsghdr { struct msghdr msg_hdr; /* message header */ ssize_t msg_len; /* message length */ }; ``` On data reception the *msg\_len* field is updated to the length of the received message. ## RETURN VALUES These calls except **recvmmsg**() return the number of bytes received. **recvmmsg**() returns the number of messages received. A value of -1 is returned if an error occurred. ## ERRORS The calls fail if: - \[EBADF\] The argument *s* is an invalid descriptor. - \[ECONNRESET\] The remote socket end is forcibly closed. - \[ENOTCONN\] The socket is associated with a connection-oriented protocol and has not been connected (see [connect(2)](/connect/2.md) and [accept(2)](/accept/2.md)). - \[ENOTSOCK\] The argument *s* does not refer to a socket. - \[EMSGSIZE\] The **recvmsg**() system call was used to receive rights (file descriptors) that were in flight on the connection. However, the receiving program did not have enough free file descriptor slots to accept them. In this case the descriptors are closed, any pending data can be returned by another call to **recvmsg**(). - \[EAGAIN\] The socket is marked non-blocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data were received. - \[EINTR\] The receive was interrupted by delivery of a signal before any data were available. - \[EFAULT\] The receive buffer pointer(s) point outside the process's address space. ## SEE ALSO [fcntl(2)](/fcntl/2.md), [getsockopt(2)](/getsockopt/2.md), [read(2)](/read/2.md), [select(2)](/select/2.md), [socket(2)](/socket/2.md), [CMSG\_DATA(3)](/CMSG_DATA/3.md), [unix(4)](/unix/4.md) ## HISTORY The **recv**() function appeared in 4.2BSD. The **recvmmsg**() function appeared in FreeBSD 11.0.