recv.2

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 2 代码 · 共 171 行

.\" SCCSID: @(#)recv.2	8.1	9/11/90
.TH recv 2
.SH Name
recv, recvfrom, recvmsg \- receive a message from a socket
.SH Syntax
.nf
.ft B
#include <sys/types.h>
#include <sys/socket.h>
.PP
.ft B
cc = recv(s, buf, len, flags)
int cc, s;
char *buf;
int len, flags;
.PP
.ft B
cc = recvfrom(s, buf, len, flags, from, fromlen)
int cc, s;
char *buf;
int len, flags;
struct sockaddr *from;
int *fromlen;
.PP
.ft B
cc = recvmsg(s, msg, flags)
int cc, s;
struct msghdr msg[];
int flags;
.ft R
.SH Description
.NXR "recv system call"
.NXR "recvfrom system call"
.NXR "recvmsg system call"
.NXA "recv system call" "send system call"
.NXR "socket" "receiving message from"
The
.PN recv ,
.PN recvfrom ,
and
.PN recvmsg
system calls are used to receive messages from a socket.
.PP
The 
.PN recv
call can be used only on a connected socket. The  
.PN recvfrom
and 
.PN recvmsg
calls can be used to receive data on a socket, whether or not
it is in a connected state.
For further information, see 
.MS connect 2 .
.PP
If
.I from
is nonzero, the source address of the message is filled in.
The
.I fromlen
is a value-result parameter, initialized to the size of
the buffer associated with
.IR from ,
and modified on return to indicate the actual size of the
address stored there.
The length of the message is returned in
.PN cc .
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
.MS socket 2 .
.PP
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
.PN cc
of \-1 is returned, and the external variable errno is
set to EWOULDBLOCK.
For further information, see 
.MS ioctl 2 .
.PP
The 
.MS select 2
call can be used to determine when more data arrives.
.PP
The
.I flags
argument to a send call is formed by 
ORing
one or more of the values following values:
.EX 0
#define	MSG_OOB	0x1	/* process out-of-band data */
#define	MSG_PEEK	0x2	/* peek at incoming message */
.EE
The
.PN recvmsg
call uses a 
.I msghdr
structure to minimize the number of directly supplied parameters.
This structure has the following form, as defined in <sys/socket.h>:
.NXR(e) "recv system call" "msghdr structure"
.EX 0
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;
};
.EE
Here,
.I msg_name
and
.I msg_namelen
specify the destination address if the socket is unconnected;
.I msg_name
can be given as a null pointer if no names are desired or required.
The 
.I msg_iov
and
.I msg_iovlen
describe the scatter gather locations, as described in
.MS read 2 .
Access rights to be sent along with the message are specified
in 
.I msg_accrights ,
which has length
.I msg_accrightslen .
.SH Return Values
These calls return the number of bytes received, or \-1
if an error occurred.
.SH Diagnostics
.NXR "recv system call" "diagnostics"
The 
.PN recv
call fails under the following conditions:
.TP 15
[EBADF]
The argument \fIs\fP is an invalid descriptor.
.TP 15
[EINVAL]
The argument length of the message is less than 0.
.TP 15
[EMSGSIZE]
The message sent on the socket was larger than the
internal message buffer.
.TP 15
[ENOTCONN]
A call was made to 
.PN recv
from an unconnected stream socket.
.TP 15
[ENOTSOCK]
The argument \fIs\fP is not a socket.
.TP 15
[EWOULDBLOCK]
The socket is marked nonblocking and the receive operation
would block.
.TP 15
[EINTR]
The receive was interrupted by delivery of a signal before
any data was available for the receive.
.TP 15
[EFAULT]
The data was specified to be received into a nonexistent
or protected part of the process address space. The argument \fIfromlen\fP
points outside the process address space.
.SH See Also
read(2), send(2), socket(2)