recvfrom(2) [bsd man page]
RECV(2) System Calls Manual RECV(2) NAME
recv, recvfrom, recvmsg - receive a message from a socket SYNOPSIS
#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
Recv, recvfrom, and recvmsg are used to receive messages from a socket. The recv call is normally 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 non-zero, 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 in cc. 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 ioctl(2)) in which case a cc of -1 is returned with the external variable errno set to EWOULDBLOCK. The select(2) call may be used to determine when more data arrives. The flags argument to a recv call is formed by or'ing one or more of the values, #define MSG_OOB 0x1 /* process out-of-band data */ #define MSG_PEEK 0x2 /* peek at incoming message */ 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 { 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 */ 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). A buffer to receive any access rights sent along with the message is specified in msg_accrights, which has length msg_accrightslen. Access rights are currently limited to file descriptors, which each occupy the size of an int. RETURN VALUE
These calls return the number of bytes received, or -1 if an error occurred. ERRORS
The calls fail if: [EBADF] The argument s is an invalid descriptor. [ENOTSOCK] The argument 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. SEE ALSO
fcntl(2), read(2), send(2), select(2), getsockopt(2), socket(2) 4.2 Berkeley Distribution May 23, 1986 RECV(2)
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recv(3SOCKET) Sockets Library Functions recv(3SOCKET) NAME
recv, recvfrom, recvmsg - receive a message from a socket SYNOPSIS
cc [ flag... ] file... -lsocket -lnsl [ library... ] #include <sys/types.h> #include <sys/socket.h> #include <sys/uio.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, int *fromlen); ssize_t recvmsg(int s, struct msghdr *msg, int flags); DESCRIPTION
The recv(), recvfrom(), and recvmsg() functions are used to receive messages from another socket. The s socket is created with socket(3SOCKET). 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 socket(3SOCKET). 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. See fcntl(2). For processes on the same host, recvmsg() can be used to receive a file descriptor from another process. 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 select(3C) call 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: 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 is returned, but not consumed to allow a subsequent receive operation to see the same data. MSG_WAITALL 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. MSG_DONTWAIT Pending messages received on the connection are returned. If data is unavailable, the function does not block. This behav- ior is the equivalent to specifying O_NONBLOCK on the file descriptor of a socket, except that write requests are unaf- fected. The recvmsg() function 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 */ int msg_accrightslen; The msg_name and msg_namelen parameters specify the destination address when the socket is unconnected The msg_name can be specified as a NULL pointer if no names are desired or required. The msg_iov and msg_iovlen parameters describe the scatter-gather locations, as described in read(2). The msg_accrights parameter specifies the buffer in which access rights sent along with the message are received. The msg_accrightslen specifies the length of the buffer. RETURN VALUES
Upon successful completion, these functions return the number of bytes received. Otherwise, they return -1 and set errno to indicate the error. ERRORS
The recv(), recvfrom(), and recvmsg() functions return errors under the following conditions: EBADF The s file descriptor is invalid. EINVAL The MSG_OOB flag is set and no out-of-band data is available. EINTR The operation is interrupted by the delivery of a signal before any data is available to be received. EIO An I/O error occurs while reading from or writing to the file system. ENOMEM Insufficient user memory is available to complete operation. ENOSR Insufficient STREAMS resources are available for the operation to complete. ENOTSOCK s is not a socket. ESTALE A stale NFS file handle exists. EWOULDBLOCK The socket is marked non-blocking and the requested operation would block. ECONNREFUSED 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 peer. The recv() and recvfrom() functions fail under the following conditions: EINVAL The len argument overflows a ssize_t. The recvmsg() function returns errors under the following conditions: EINVAL The msg_iovlen member of the msghdr structure pointed to by msg is less than or equal to 0, or greater than [IOV_MAX}. See Intro(2) for a definition of [IOV_MAX}. EINVAL 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. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Stable | +-----------------------------+-----------------------------+ |MT-Level |Safe | +-----------------------------+-----------------------------+ SEE ALSO
fcntl(2), ioctl(2), read(2), connect(3SOCKET), getsockopt(3SOCKET), select(3C), send(3SOCKET), socket(3SOCKET), socket.h(3HEAD), attributes(5) SunOS 5.10 05 Feb 2004 recv(3SOCKET)