.\" Copyright (c) 1983, 1990, 1991 The Regents of the University of California.

.\" All rights reserved.

.\"

.\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)

.\" Redistribution and use in source and binary forms, with or without

.\" modification, are permitted provided that the following conditions

.\" are met:

.\" 1. Redistributions of source code must retain the above copyright

.\"    notice, this list of conditions and the following disclaimer.

.\" 2. Redistributions in binary form must reproduce the above copyright

.\"    notice, this list of conditions and the following disclaimer in the

.\"    documentation and/or other materials provided with the distribution.

.\" 3. All advertising materials mentioning features or use of this software

.\"    must display the following acknowledgement:

.\"	This product includes software developed by the University of

.\"	California, Berkeley and its contributors.

.\" 4. Neither the name of the University nor the names of its contributors

.\"    may be used to endorse or promote products derived from this software

.\"    without specific prior written permission.

.\"

.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND

.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE

.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

.\" SUCH DAMAGE.

.\" %%%LICENSE_END

.\"

.\"     $Id: recv.2,v 1.3 1999/05/13 11:33:38 freitag Exp $

.\"

.\" Modified Sat Jul 24 00:22:20 1993 by Rik Faith <faith@cs.unc.edu>

.\" Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esr@thyrsus.com>

.\" Modified 1998,1999 by Andi Kleen

.\" 2001-06-19 corrected SO_EE_OFFENDER, bug report by James Hawtin

.\"

.TH RECV 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

recv, recvfrom, recvmsg \- receive a message from a socket

.SH SYNOPSIS

.\" .B #include <sys/uio.h>

.\" .br

.nf

.B #include <sys/types.h>

.br

.B #include <sys/socket.h>

.PP

.BI "ssize_t recv(int " sockfd ", void *" buf ", size_t " len ", int " flags );

.PP

.BI "ssize_t recvfrom(int " sockfd ", void *" buf ", size_t " len ", int " flags ,

.BI "                 struct sockaddr *" src_addr ", socklen_t *" addrlen );

.PP

.BI "ssize_t recvmsg(int " sockfd ", struct msghdr *" msg ", int " flags );

.fi

.SH DESCRIPTION

The

.BR recv (),

.BR recvfrom (),

and

.BR recvmsg ()

calls are used to receive messages from a socket.

They may be used

to receive data on both connectionless and connection-oriented sockets.

This page first describes common features of all three system calls,

and then describes the differences between the calls.

.PP

The only difference between

.BR recv ()

and

.BR read (2)

is the presence of

.IR flags .

With a zero

.I flags

argument,

.BR recv ()

is generally equivalent to

.BR read (2)

(but see NOTES).

Also, the following call

.PP

    recv(sockfd, buf, len, flags);

.PP

is equivalent to

.PP

    recvfrom(sockfd, buf, len, flags, NULL, NULL);

.PP

All three calls 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.

.PP

If no messages are available at the socket, the receive calls wait for a

message to arrive, unless the socket is nonblocking (see

.BR fcntl (2)),

in which case the value \-1 is returned and the external variable

.I errno

is set to

.BR 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.

.PP

An application can use

.BR select (2),

.BR poll (2),

or

.BR epoll (7)

to determine when more data arrives on a socket.

.SS The flags argument

The

.I flags

argument is formed by ORing one or more of the following values:

.TP

.BR MSG_CMSG_CLOEXEC " (" recvmsg "() only; since Linux 2.6.23)"

Set the close-on-exec flag for the file descriptor received

via a UNIX domain file descriptor using the

.B SCM_RIGHTS

operation (described in

.BR unix (7)).

This flag is useful for the same reasons as the

.B O_CLOEXEC

flag of

.BR open (2).

.TP

.BR MSG_DONTWAIT " (since Linux 2.2)"

Enables nonblocking operation; if the operation would block,

the call fails with the error

.BR EAGAIN " or " EWOULDBLOCK .

This provides similar behavior to setting the

.B O_NONBLOCK

flag (via the

.BR fcntl (2)

.B F_SETFL

operation), but differs in that

.B MSG_DONTWAIT

is a per-call option, whereas

.B O_NONBLOCK

is a setting on the open file description (see

.BR open (2)),

which will affect all threads in the calling process

and as well as other processes that hold file descriptors

referring to the same open file description.

.TP

.BR MSG_ERRQUEUE " (since Linux 2.2)"

This flag

specifies that queued errors should be received from the socket error queue.

The error is passed in

an ancillary message with a type dependent on the protocol (for IPv4

.BR IP_RECVERR ).

The user should supply a buffer of sufficient size.

See

.BR cmsg (3)

and

.BR ip (7)

for more information.

The payload of the original packet that caused the error

is passed as normal data via

.IR msg_iovec .

The original destination address of the datagram that caused the error

is supplied via

.IR msg_name .

.IP

The error is supplied in a

.I sock_extended_err

structure:

.IP

.in +4n

.EX

#define SO_EE_ORIGIN_NONE    0

#define SO_EE_ORIGIN_LOCAL   1

#define SO_EE_ORIGIN_ICMP    2

#define SO_EE_ORIGIN_ICMP6   3

struct sock_extended_err

{

    uint32_t ee_errno;   /* error number */

    uint8_t  ee_origin;  /* where the error originated */

    uint8_t  ee_type;    /* type */

    uint8_t  ee_code;    /* code */

    uint8_t  ee_pad;     /* padding */

    uint32_t ee_info;    /* additional information */

    uint32_t ee_data;    /* other data */

    /* More data may follow */

};

struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

.EE

.in

.IP

.I ee_errno

contains the

.I errno

number of the queued error.

.I ee_origin

is the origin code of where the error originated.

The other fields are protocol-specific.

The macro

.B SOCK_EE_OFFENDER

returns a pointer to the address of the network object

where the error originated from given a pointer to the ancillary message.

If this address is not known, the

.I sa_family

member of the

.I sockaddr

contains

.B AF_UNSPEC

and the other fields of the

.I sockaddr

are undefined.

The payload of the packet that caused the error is passed as normal data.

.IP

For local errors, no address is passed (this

can be checked with the

.I cmsg_len

member of the

.IR cmsghdr ).

For error receives,

the

.B MSG_ERRQUEUE

flag is set in the

.IR msghdr .

After an error has been passed, the pending socket error

is regenerated based on the next queued error and will be passed

on the next socket operation.

.TP

.B MSG_OOB

This flag requests receipt of out-of-band data that would not be received

in the normal data stream.

Some protocols place expedited data

at the head of the normal data queue, and thus this flag cannot

be used with such protocols.

.TP

.B MSG_PEEK

This flag causes the receive operation to

return data from the beginning of the

receive queue without removing that data from the queue.

Thus, a

subsequent receive call will return the same data.

.TP

.BR MSG_TRUNC " (since Linux 2.2)"

For raw

.RB ( AF_PACKET ),

Internet datagram (since Linux 2.4.27/2.6.8),

netlink (since Linux 2.6.22), and UNIX datagram

.\" commit 9f6f9af7694ede6314bed281eec74d588ba9474f

(since Linux 3.4) sockets:

return the real length of the packet or datagram,

even when it was longer than the passed buffer.

.IP

For use with Internet stream sockets, see

.BR tcp (7).

.TP

.BR MSG_WAITALL " (since Linux 2.2)"

This flag requests that the operation block until the full request is

satisfied.

However, the call may still return less data than requested if

a signal is caught, an error or disconnect occurs, or the next data to be

received is of a different type than that returned.

This flag has no effect for datagram sockets.

.\"

.SS recvfrom()

.BR recvfrom ()

places the received message into the buffer

.IR buf .

The caller must specify the size of the buffer in

.IR len .

.PP

If

.I src_addr

is not NULL,

and the underlying protocol provides the source address of the message,

that source address is placed in the buffer pointed to by

.IR src_addr .

.\" (Note: for datagram sockets in both the UNIX and Internet domains,

.\" .I src_addr

.\" is filled in.

.\" .I src_addr

.\" is also filled in for stream sockets in the UNIX domain, but is not

.\" filled in for stream sockets in the Internet domain.)

.\" [The above notes on AF_UNIX and AF_INET sockets apply as at

.\" Kernel 2.4.18. (MTK, 22 Jul 02)]

In this case,

.I addrlen

is a value-result argument.

Before the call,

it should be initialized to the size of the buffer associated with

.IR src_addr .

Upon return,

.I addrlen

is updated to contain the actual size of the source address.

The returned address is truncated if the buffer provided is too small;

in this case,

.I addrlen

will return a value greater than was supplied to the call.

.PP

If the caller is not interested in the source address,

.I src_addr

and

.I addrlen

should be specified as NULL.

.\"

.SS recv()

The

.BR recv ()

call is normally used only on a

.I connected

socket (see

.BR connect (2)).

It is equivalent to the call:

.PP

    recvfrom(fd, buf, len, flags, NULL, 0);

.\"

.SS recvmsg()

The

.BR recvmsg ()

call uses a

.I msghdr

structure to minimize the number of directly supplied arguments.

This structure is defined as follows in

.IR <sys/socket.h> :

.PP

.in +4n

.EX

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 */

};

.EE

.in

.PP

The

.I msg_name

field points to a caller-allocated buffer that is used to

return the source address if the socket is unconnected.

The caller should set

.I msg_namelen

to the size of this buffer before this call;

upon return from a successful call,

.I msg_namelen

will contain the length of the returned address.

If the application does not need to know the source address,

.I msg_name

can be specified as NULL.

.PP

The fields

.I msg_iov

and

.I msg_iovlen

describe scatter-gather locations, as discussed in

.BR readv (2).

.PP

The field

.IR msg_control ,

which has length

.IR msg_controllen ,

points to a buffer for other protocol control-related messages or

miscellaneous ancillary data.

When

.BR recvmsg ()

is called,

.I msg_controllen

should contain the length of the available buffer in

.IR msg_control ;

upon return from a successful call it will contain the length

of the control message sequence.

.PP

The messages are of the form:

.PP

.in +4n

.EX

struct cmsghdr {

    size_t cmsg_len;    /* Data byte count, including header

                           (type is socklen_t in POSIX) */

    int    cmsg_level;  /* Originating protocol */

    int    cmsg_type;   /* Protocol-specific type */

/* followed by

    unsigned char cmsg_data[]; */

};

.EE

.in

.PP

Ancillary data should be accessed only by the macros defined in

.BR cmsg (3).

.PP

As an example, Linux uses this ancillary data mechanism to pass extended

errors, IP options, or file descriptors over UNIX domain sockets.

.PP

The

.I msg_flags

field in the

.I msghdr

is set on return of

.BR recvmsg ().

It can contain several flags:

.TP

.B MSG_EOR

indicates end-of-record; the data returned completed a record (generally

used with sockets of type

.BR SOCK_SEQPACKET ).

.TP

.B MSG_TRUNC

indicates that the trailing portion of a datagram was discarded because the

datagram was larger than the buffer supplied.

.TP

.B MSG_CTRUNC

indicates that some control data were discarded due to lack of space in the

buffer for ancillary data.

.TP

.B MSG_OOB

is returned to indicate that expedited or out-of-band data were received.

.TP

.B MSG_ERRQUEUE

indicates that no data was received but an extended error from the socket

error queue.

.SH RETURN VALUE

These calls return the number of bytes received, or \-1

if an error occurred.

In the event of an error,

.I errno

is set to indicate the error.

.PP

When a stream socket peer has performed an orderly shutdown,

the return value will be 0 (the traditional "end-of-file" return).

.PP

Datagram sockets in various domains (e.g., the UNIX and Internet domains)

permit zero-length datagrams.

When such a datagram is received, the return value is 0.

.PP

The value 0 may also be returned if the requested number of bytes

to receive from a stream socket was 0.

.SH ERRORS

These are some standard errors generated by the socket layer.

Additional errors

may be generated and returned from the underlying protocol modules;

see their manual pages.

.TP

.BR EAGAIN " or " EWOULDBLOCK

.\" Actually EAGAIN on Linux

The socket is marked nonblocking and the receive operation

would block, or a receive timeout had been set and the timeout expired

before data was received.

POSIX.1 allows either error to be returned for this case,

and does not require these constants to have the same value,

so a portable application should check for both possibilities.

.TP

.B EBADF

The argument

.I sockfd

is an invalid file descriptor.

.TP

.B ECONNREFUSED

A remote host refused to allow the network connection (typically

because it is not running the requested service).

.TP

.B EFAULT

The receive buffer pointer(s) point outside the process's

address space.

.TP

.B EINTR

The receive was interrupted by delivery of a signal before

any data were available; see

.BR signal (7).

.TP

.B EINVAL

Invalid argument passed.

.\" e.g., msg_namelen < 0 for recvmsg() or addrlen < 0 for recvfrom()

.TP

.B ENOMEM

Could not allocate memory for

.BR recvmsg ().

.TP

.B ENOTCONN

The socket is associated with a connection-oriented protocol

and has not been connected (see

.BR connect (2)

and

.BR accept (2)).

.TP

.B ENOTSOCK

The file descriptor

.I sockfd

does not refer to a socket.

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008,

4.4BSD (these interfaces first appeared in 4.2BSD).

.PP

POSIX.1 describes only the

.BR MSG_OOB ,

.BR MSG_PEEK ,

and

.B MSG_WAITALL

flags.

.SH NOTES

If a zero-length datagram is pending,

.BR read (2)

and

.BR recv ()

with a

.I flags

argument of zero provide different behavior.

In this circumstance,

.BR read (2)

has no effect (the datagram remains pending), while

.BR recv ()

consumes the pending datagram.

.PP

The

.I socklen_t

type was invented by POSIX.

See also

.BR accept (2).

.PP

According to POSIX.1,

.\" POSIX.1-2001, POSIX.1-2008

the

.I msg_controllen

field of the

.I msghdr

structure should be typed as

.IR socklen_t ,

but glibc currently types it as

.IR size_t .

.\" glibc bug raised 12 Mar 2006

.\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448

.\" The problem is an underlying kernel issue: the size of the

.\" __kernel_size_t type used to type this field varies

.\" across architectures, but socklen_t is always 32 bits.

.PP

See

.BR recvmmsg (2)

for information about a Linux-specific system call

that can be used to receive multiple datagrams in a single call.

.SH EXAMPLE

An example of the use of

.BR recvfrom ()

is shown in

.BR getaddrinfo (3).

.SH SEE ALSO

.BR fcntl (2),

.BR getsockopt (2),

.BR read (2),

.BR recvmmsg (2),

.BR select (2),

.BR shutdown (2),

.BR socket (2),

.BR cmsg (3),

.BR sockatmark (3),

.BR ip (7),

.BR ipv6 (7),

.BR socket (7),

.BR tcp (7),

.BR udp (7),

.BR unix (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.