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 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 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 receive call is formed by or’ing one or more of the values,

MSG_PEEK/∗ peek at incoming message ∗/
MSG_OOB/∗ process out-of-band data ∗/

The recvmsg call uses a msghdr structure to minimize the number of directly supplied parameters, and to receive file descriptors from another process (Unix domain only).  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 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.

RECEIVING OUT-OF-BAND DATA IN INTERNET DOMAIN

When out-of-band data arrives at a socket, the system could signal a group of processes, or a specific process, or no process at all. 

 
- If the socket’s process group id is positive, the system signals all  process in that process group. 
 
- If the socket’s process group id is negative, the system only signals the process whose process id is the absolute value of the socket’s process group id. 
 
- If the socket’s process group id is 0 (the default value), the system does not signal any process.
 

To assign a socket process group id, use the ioctl(2) system call with the command SIOCSPGRP. 

RECEIVING FILE DESCRIPTORS IN CX/UX DOMAIN

In Unix domain, process A can open a set of files, then send these file descriptors to process B.  Process B then can read or write those files which were already opened (and perhaps accessible only) by A. 

The active process stores the file descriptors in the field msg_accrights of the structure msghdr, and use the system call sendmsg(2) to send the descriptors. The passive process then can receive the descriptors via the system call recvmsg(2); the descriptors are returned in the field msg_accrights of msghdr. 

NOTE

The ability to send and receive file descriptors is not available in CX/SX when configured to B1 level security. 

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. 

NOTE

These functions are defined in the 88open Binary and Object Compatibility Standards’ Networking Supplements (BCSNS and OCSNS) for use in BCSNS/OCSNS compliant applications on Series.  OCSNS-defined functions may be accessed by passing OCS options to cc(1) and ld(1). 

SEE ALSO

read(2), send(2), socket(2). See also bind(2) for address formats in different communication domains. 

CX/UX Programmer’s Reference Manual