recv(), recvfrom(), recvmsg()
receive a message from a socket
#include 'sys/type.h' #include 'sys/socket.h' ssize_t recv (int sockfd, void *buf, size_t len , int flags ); ssize_t recvfrom( int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t addrlen); ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

DESCRIPTION :

The recvfrom() and recvmsg() calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.

If src_addr is not NULL, and the underlying protocol provides the source address, this source address is filled in. When src_addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL. The argument addrlen is a value-result argument, which the caller should initialize before the call to the size of the buffer associated with src_addr, and modified on return to indicate the actual size of the source address. The returned address is truncated if the buffer provided is too small; in this case, addrlen will return a value greater than was supplied to the call.

The recv() call is normally used only on a connected socket (see connect(2)) and is identical to recvfrom() with a NULL src_addr argument.

All three routines return the length of the message on successful completion. 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.

If no messages are available at the socket, the receive calls wait 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 is set to EAGAIN or EWOULDBLOCK. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.

The select(2) or poll(2) call may be used to determine when more data arrives.

The recvmsg() call uses a msghdr structure to minimize the number of directly supplied arguments. This structure is defined as follows in :

struct iovec { /* Scatter/gather array items */ void *iov_base; /* Starting address */ size_t iov_len; /* Number of bytes to transfer */ }; struct msghdr { void *msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ size_t msg_iovlen; /* # elements in msg_iov */ void *msg_control; /* ancillary data, see below */ size_t msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ };

Return Value

On success, these calls return the number of characters sent. On error, -1 is returned, and errno is set appropriately.