recv(2) System Calls Manual recv(2)
Name
recv, recvfrom, recvmsg - receive a message from a socket
Syntax
#include <sys/types.h>
#include <sys/socket.h>
cc = recv(s, buf, len, flags)
int cc, s;
char *buf;
int len, flags;
cc = recvfrom(s, buf, len, flags, from, fromlen)
int cc, s;
char *buf;
int len, flags;
struct sockaddr *from;
int *fromlen;
cc = recvmsg(s, msg, flags)
int cc, s;
struct msghdr msg[];
int flags;
Description
The and system calls are used to receive messages from a socket.
The call can be used only on a connected socket. The and calls can be used to receive data on a socket, whether or not it is in a connected
state. For further information, see
If from is nonzero, the source address of the message is filled in. The 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 in If a message is too long to fit in the supplied buffer, excess bytes can be discarded, depending on the type of socket the
message is received from. For further information, see
If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking. If the
socket is nonblocking, a of -1 is returned, and the external variable errno is set to EWOULDBLOCK. For further information, see
The call can be used to determine when more data arrives.
The flags argument to a send call is formed by ORing one or more of the values following values:
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_PEEK 0x2 /* peek at incoming message */
The 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 {
caddr_t msg_name; /* optional address */
int msg_namelen; /* size of address */
struct iov *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_accrights; /* access rights sent/received */
int msg_accrightslen;
};
Here, msg_name and msg_namelen specify the destination address if the socket is unconnected; msg_name can 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 Access rights to be sent
along with the message are specified in msg_accrights , which has length msg_accrightslen .
Return Values
These calls return the number of bytes received, or -1 if an error occurred.
Diagnostics
The call fails under the following conditions:
[EBADF] The argument s is an invalid descriptor.
[EINVAL] The argument length of the message is less than 0.
[EMSGSIZE] The message sent on the socket was larger than the internal message buffer.
[ENOTCONN] A call was made to from an unconnected stream socket.
[ENOTSOCK] The argument s is not a socket.
[EWOULDBLOCK] The socket is marked nonblocking and the receive operation would block.
[EINTR] The receive was interrupted by delivery of a signal before any data was available for the receive.
[EFAULT] The data was specified to be received into a nonexistent or protected part of the process address space. The argument
fromlen points outside the process address space.
See Also
read(2), send(2), socket(2)
recv(2)