RECV(2) System Calls Manual RECV(2)
recv, recvfrom, recvmsg - receive a message from a socket
int recv(s, buf, len, flags)
int len, flags;
int recvfrom(s, buf, len, flags, from, fromlen)
int len, flags;
struct sockaddr *from;
int recvmsg(s, msg, flags)
struct msghdr *msg;
s is a socket created with socket(2). recv(), recvfrom(), and
recvmsg() are used to receive messages from another socket. recv() may
be used only on a connected socket (see connect(2)), while recvfrom()
and recvmsg() may be used to receive data on a socket whether it is in
a connected state or not.
If from is not a NULL pointer, 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 length of
the message is returned. If a message is too long to fit in the sup-
plied 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 non-blocking (see ioctl(2))
in which case -1 is returned with the external variable errno set to
The select(2) call may be used to determine when more data arrive.
If the process calling recv(), recvfrom() or recvmsg() receives a sig-
nal before any data are available, the system call is restarted unless
the calling process explicitly set the signal to interrupt these calls
using sigvec() or sigaction() (see the discussions of SV_INTERRUPT on
sigvec(2), and SA_INTERRUPT on sigaction(3V)).
The flags parameter is formed by ORing one or more of the following:
MSG_OOB Read any "out-of-band" data present on the socket,
rather than the regular "in-band" data.
MSG_PEEK "Peek" at the data present on the socket; the data are
returned, but not consumed, so that a subsequent receive
operation will see the same data.
The recvmsg() call uses a msghdr structure to minimize the number of
directly supplied parameters. This structure is defined in
<<sys/socket.h>>, and includes the following members:
caddr_t msg_name; /* optional address */
int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_accrights; /* access rights sent/received */
Here msg_name and msg_namelen specify the destination 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 describe the
scatter-gather locations, as described in read(2V). A buffer to
receive any access rights sent along with the message is specified in
msg_accrights, which has length msg_accrightslen.
These calls return the number of bytes received, or -1 if an error
EBADF s is an invalid descriptor.
EFAULT The data were specified to be received into a non-
existent or protected part of the process address
EINTR The calling process received a signal before any
data were available to be received, and the signal
was set to interrupt the system call.
ENOTSOCK s is a descriptor for a file, not a socket.
EWOULDBLOCK The socket is marked non-blocking and the requested
operation would block.
connect(2), fcntl(2V), getsockopt(2), ioctl(2), read(2V), select(2),
21 January 1990 RECV(2)