RECV(2) | System Calls Manual | RECV(2) |
recv
, recvfrom
,
recvmsg
— receive a message
from a socket
#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);
recvfrom
()
and recvmsg
() are used to receive messages from a
socket, s, and may be used to receive data on a socket
whether or not it is connection-oriented.
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.
The
recv
() call is
normally used only on a
connected
socket (see connect(2)) and
is identical to recvfrom
() with a null
from parameter.
On successful completion, all three routines return the number of message bytes read. 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)).
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
MSG_PEEK
MSG_WAITALL
MSG_DONTWAIT
MSG_CMSG_CLOEXEC
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
requests that any file descriptors
received as ancillary data with
recvmsg
()
(see below) have their close-on-exec flag 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
MSG_EOR
SOCK_SEQPACKET
).MSG_TRUNC
MSG_CTRUNC
MSG_BCAST
MSG_MCAST
These calls return the number of bytes received, or -1 if an error occurred.
recv
(),
recvfrom
(), and recvmsg
()
fail if:
EBADF
]ENOTCONN
]ENOTSOCK
]EAGAIN
]EINTR
]EFAULT
]EHOSTUNREACH
]EHOSTDOWN
]ENETDOWN
]ECONNREFUSED
]In addition, recv
() and
recvfrom
() may return the following error:
EINVAL
]SSIZE_MAX
.And recvmsg
() may return one of the
following errors:
EINVAL
]EMSGSIZE
]IOV_MAX
.EMSGSIZE
]recvmsg
().connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2), socketpair(2), CMSG_DATA(3), sockatmark(3)
The recv
(),
recvfrom
(), and recvmsg
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”). The MSG_DONTWAIT
,
MSG_BCAST
, and MSG_MCAST
flags are extensions to that specification.
The recv
() function call appeared in
4.2BSD.
February 16, 2015 | OpenBSD-6.0 |