recv(2)
NAME
recv, recvfrom, recvmsg − receive a message from a socket
SYNTAX
#include <sys/types.h>
#include <sys/socket.h>
int cc = recv(s, buf, len, flags)
char *buf;
int s, len, flags;
int cc = recvfrom(s, buf, len, flags, from, fromlen)
char *buf;
int s, len, flags, *fromlen;
struct sockaddr *from;
int cc = recvmsg(s, msg, flags)
struct msghdr msg[];
int s, flags;
DESCRIPTION
The recv, recvfrom, and recvmsg system calls are used to receive messages from a socket. The recv call may be used only on a connected socket. For further information, see connect(2). The recvfrom and recvmsg calls may be used to receive data on a socket whether it is in a connected state or not.
If from is non-zero, the source address of the message is filled in. The specified fromlen is a value-result parameter. It is 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 cc. If a message is too long to fit in the supplied buffer, the excess bytes may be discarded depending on the type of socket the message is received from. For further information, see socket(2).
If no messages are available at the socket, the receive call waits for a message to arrive. If the socket is nonblocking, however, returns a −1 and the global variable errno is set to EWOULDBLOCK. For further information, see ioctl(2).
The select call may be used to determine when more data arrives.
The specified flags to a send call is formed by a bitwise or of one or more of the following values:
#define MSG_PEEK 0x1 /* peek at incoming message */
#define MSG_OOB 0x2 /* process out-of-band data */
The recvmsg call uses a msghdr structure to minimize the number of directly supplied parameters. This structure includes:
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 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(2). Access rights to be sent along with the message are specified in msg_accrights, which has length msg_accrightslen.
RETURN VALUE
If successful, these calls return the number of bytes received. If unsuccessful, they return a −1, and the global variable errno indicates the error code.
DIAGNOSTICS
The recv, recvfrom, and recvmsg calls will fail if:
[EBADF] The specified s is an invalid descriptor.
[ENOTSOCK] The specified s is not a socket.
[EWOULDBLOCK] The socket is marked non-blocking, 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 non-existent or protected part of the process address space.