recv, recvfrom, recvmsg - receive a message from a socket
cc [ flag... ] file... -lsocket -lnsl [ library... ]
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);
The recv(), recvfrom(), and recvmsg() functions are used to
receive messages from another socket. The s socket is created with
If from is a non-NULL pointer, the source address of
the message is filled in. The value-result parameter fromlen is
initialized to the size of the buffer associated with from and
modified on return to indicate the actual size of the address stored in the
buffer. The length of the message is returned. If a message is too long to
fit in the supplied buffer, excess bytes may be discarded depending on the
type of socket from which the message is received. See
If no messages are available at the socket, the receive call waits
for a message to arrive. If the socket is non-blocking, -1 is
returned with the external variable errno set to EWOULDBLOCK.
For processes on the same host, recvmsg() can be used to
receive a file descriptor from another process, but it cannot receive
ancillary data. See libxnet(3LIB).
If a zero-length buffer is specified for a message, an EOF
condition results that is indistinguishable from the successful transfer of
a file descriptor. For that reason, one or more bytes of data should be
provided when recvmsg() passes a file descriptor.
The poll(2), select(3C), and port_get(3C)
functions can be used to determine when more data arrives.
The flags parameter is formed by an OR operation on
one or more of the following:
Read any out-of-band data present on the socket
rather than the regular in-band data.
Peek at the data present on the socket. The data is
returned, but not consumed to allow a subsequent receive operation to see the
Messages are blocked until the full amount of data
requested is returned. The recv() function can return a smaller amount
of data if a signal is caught, the connection is terminated, MSG_PEEK
is specified, or if an error is pending for the socket.
Pending messages received on the connection are returned.
If data is unavailable, the function does not block. This behavior is the
equivalent to specifying O_NONBLOCK on the file descriptor of a socket,
except that write requests are unaffected.
The recvmsg() function call uses a msghdr structure
defined in <sys/socket.h> to minimize the number of directly
Upon successful completion, these functions return the number of bytes received.
Otherwise, they return -1 and set errno to indicate the error.
In addition to the errors documented below, an asynchronous error generated by
the underlying socket protocol may be returned. For the full list of errors,
please see the corresponding socket protocol manual page. For example, for a
list of TCP errors, please see tcp(4P).
The recv(), recvfrom(), and recvmsg()
functions return errors under the following conditions:
The s file descriptor is invalid.
The s argument refers to a connection oriented
socket and the connection was forcibly closed by the peer and is no longer
valid. I/O can no longer be performed to filedes.
The MSG_OOB flag is set and no out-of-band data is
The operation is interrupted by the delivery of a signal
before any data is available to be received.
An I/O error occurs while reading from or writing to the
Insufficient user memory is available to complete
Insufficient STREAMS resources are available for
the operation to complete.
s is not a socket.
A stale NFS file handle exists.
The socket is marked non-blocking and the requested
operation would block.
The requested connection was refused by the peer. For
connected IPv4 and IPv6 datagram sockets, this indicates that the system
received an ICMP Destination Port Unreachable message from the
The recv() and recvfrom() functions fail under the
The len argument overflows a ssize_t.
The recvmsg() function returns errors under the following
member of the msghdr
structure pointed to by msg
is less than or equal to 0
greater than [IOV_MAX}
. See Intro(2)
for a definition of
One of the iov_len values in the msg_iov
array member of the msghdr structure pointed to by msg is
negative, or the sum of the iov_len values in the msg_iov array
overflows a ssize_t.
See attributes(7) for descriptions of the following attributes:
fcntl(2), ioctl(2), poll(2), read(2),
connect(3SOCKET), getsockopt(3SOCKET), libxnet(3LIB),
port_get(3C), select(3C), socket.h(3HEAD),
send(3SOCKET), sockaddr(3SOCKET), socket(3SOCKET),