RECV(2) | System Calls Manual | RECV(2)

# NAME

`recv`, `recvfrom`, `recvmsg`, `recvmmsg` — receive a message from a socket

# SYNOPSIS

`#include <sys/socket.h>`

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 *from, socklen_t *fromlen);

ssize_t
`recvmsg`(int s, struct msghdr *msg, int flags);

int
`recvmmsg`(int s, struct mmsghdr *mmsg, unsigned int vlen, int flags, struct timespec *timeout);

# DESCRIPTION

`recv`(), `recvfrom`(), `recvmsg`(), and `recvmmsg`() are used to receive messages from a socket, s. `recv`() is normally used only on a connected socket (see connect(2)). `recvfrom`(), `recvmsg`(), and `recvmmsg`() may be used to receive data on a socket whether or not it is connection-oriented.

`recv`() is identical to `recvfrom`() with a null from parameter.

If from is non-null and the socket is not connection-oriented, the source address of the message is filled in. fromlen is a value-result parameter, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there.

If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1 is returned and the external variable errno set to `EAGAIN`. The receive calls 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).

The select(2) or poll(2) system calls may be used to determine when more data arrive.

The flags argument is the bitwise OR of zero or more of the following values:

`MSG_OOB`
  process out-of-band data
`MSG_PEEK`
  peek at incoming message
`MSG_WAITALL`
  wait for full request or error
`MSG_DONTWAIT`
  don't block
`MSG_CMSG_CLOEXEC`
  set the close-on-exec flag on received file descriptors
`MSG_CMSG_CLOFORK`
  set the close-on-fork flag on received file descriptors

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 strict ANSI or C99 compilation mode. The `MSG_CMSG_CLOEXEC` and `MSG_CMSG_CLOFORK` flags request that any file descriptors received as ancillary data with `recvmsg`() and `recvmmsg`() (see below) have their close-on-exec flag or close-on-fork flag, respectively, set.

The `recvmsg`() call uses a msghdr structure to minimize the number of directly supplied parameters. 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 */
    	unsigned 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. msg_iov and msg_iovlen describe scatter gather locations, as discussed in read(2). msg_control, 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[]; */
    };

See CMSG_DATA(3) for how these messages are constructed and decomposed.

Open file descriptors are now passed as ancillary data for `AF_UNIX` domain and socketpair(2) sockets, with cmsg_level set to `SOL_SOCKET` and cmsg_type set to `SCM_RIGHTS`.

The msg_flags field is set on return according to the message received. It will contain zero or more of the following values:

`MSG_OOB`
  Returned to indicate that expedited or out-of-band data was 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_BCAST`
  Indicates that the packet was received as broadcast.
`MSG_MCAST`
  Indicates that the packet was received as multicast.

The `recvmmsg`() call uses an array of the mmsghdr structure of length vlen to group multiple msghdr structures into a single system call. vlen is capped at maximum `1024` messages that are received in a single call. The flags field allows setting `MSG_WAITFORONE` to wait for one msghdr, and set `MSG_DONTWAIT` for all subsequent messages. A provided timeout limits the time spent in the function but it does not limit the time spent in lower parts of the kernel.

The mmsghdr structure has the following form, as defined in `<sys/socket.h>`:

    struct mmsghdr {
    	struct msghdr msg_hdr;
    	unsigned int msg_len;
    };

Here msg_len indicated the number of bytes received for each msg_hdr member.

# RETURN VALUES

The `recv`(), `recvfrom`(), and `recvmsg`() calls return the number of bytes received, or -1 if an error occurred. The `recvmmsg`() call returns the number of messages received, or -1 if an error occurred before the first message has been received.

# ERRORS

`recv`(), `recvfrom`(), `recvmsg`(), and `recvmmsg`() fail if:

[`EBADF`]
  The argument s is an invalid descriptor.
[`ENOTCONN`]
  The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
[`ENOTSOCK`]
  The argument s does not refer to a socket.
[`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.
[`EHOSTUNREACH`]
  A socket operation was attempted to an unreachable host.
[`EHOSTDOWN`]
  A socket operation failed because the destination host was down.
[`ENETDOWN`]
  A socket operation encountered a dead network.
[`ECONNREFUSED`]
  The socket is associated with a connection-oriented protocol and the connection was forcefully rejected (see connect(2)).

In addition, `recv`() and `recvfrom`() may return the following error:

[`EINVAL`]
  len was larger than `SSIZE_MAX`.

And `recvmsg`() and `recvmmsg`() may return one of the following errors:

[`EINVAL`]
  The sum of the iov_len values in the msg_iov array overflowed an ssize_t.
[`EMSGSIZE`]
  The msg_iovlen member of msg was less than 0 or larger than `IOV_MAX`.
[`EMSGSIZE`]
  The receiving program did not have sufficient free file descriptor slots. The descriptors are closed and any pending data can be returned by another call to `recvmsg`().

# SEE ALSO

connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2), socketpair(2), CMSG_DATA(3), sockatmark(3)

# STANDARDS

The `recv`(), `recvfrom`(), and `recvmsg`() functions conform to. The `MSG_DONTWAIT`, `MSG_BCAST`, and `MSG_MCAST` flags are extensions to that specification.

# HISTORY

The `recv`() function call appeared in 4.1cBSD. The `recvmmsg`() syscall first appeared in Linux 2.6.33 and was added to OpenBSD 7.2.

# CAVEATS

Calling `recvmsg`() with a control message having no or an empty scatter/gather array exposes variations in implementations. To avoid these, always use an iovec with at least a one-byte buffer and set msg_iov and an msg_iovlen to use this vector.

August 11, 2025 | Debian