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.\" Syscall added in following commit

.\"	commit a2e2725541fad72416326798c2d7fa4dafb7d337

.\"	Author: Arnaldo Carvalho de Melo <acme@redhat.com>

.\"	Date:   Mon Oct 12 23:40:10 2009 -0700

.\"

.TH RECVMMSG 2 2018-02-02 "Linux" "Linux Programmer's Manual"

.SH NAME

recvmmsg \- receive multiple messages on a socket

.SH SYNOPSIS

.nf

.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"

.BI "#include <sys/socket.h>"

.PP

.BI "int recvmmsg(int " sockfd ", struct mmsghdr *" msgvec \

", unsigned int " vlen ","

.BI "             int " flags ", struct timespec *" timeout ");"

.fi

.SH DESCRIPTION

The

.BR recvmmsg ()

system call is an extension of

.BR recvmsg (2)

that allows the caller to receive multiple messages from a socket

using a single system call.

(This has performance benefits for some applications.)

A further extension over

.BR recvmsg (2)

is support for a timeout on the receive operation.

.PP

The

.I sockfd

argument is the file descriptor of the socket to receive data from.

.PP

The

.I msgvec

argument is a pointer to an array of

.I mmsghdr

structures.

The size of this array is specified in

.IR vlen .

.PP

The

.I mmsghdr

structure is defined in

.I <sys/socket.h>

as:

.PP

.in +4n

.EX

struct mmsghdr {

    struct msghdr msg_hdr;  /* Message header */

    unsigned int  msg_len;  /* Number of received bytes for header */

};

.EE

.in

.PP

The

.I msg_hdr

field is a

.I msghdr

structure, as described in

.BR recvmsg (2).

The

.I msg_len

field is the number of bytes returned for the message in the entry.

This field has the same value as the return value of a single

.BR recvmsg (2)

on the header.

.PP

The

.I flags

argument contains flags ORed together.

The flags are the same as documented for

.BR recvmsg (2),

with the following addition:

.TP

.BR MSG_WAITFORONE " (since Linux 2.6.34)"

Turns on

.B MSG_DONTWAIT

after the first message has been received.

.PP

The

.I timeout

argument points to a

.I struct timespec

(see

.BR clock_gettime (2))

defining a timeout (seconds plus nanoseconds) for the receive operation

.RI ( "but see BUGS!" ).

(This interval will be rounded up to the system clock granularity,

and kernel scheduling delays mean that the blocking interval

may overrun by a small amount.)

If

.I timeout

is NULL, then the operation blocks indefinitely.

.PP

A blocking

.BR recvmmsg ()

call blocks until

.I vlen

messages have been received

or until the timeout expires.

A nonblocking call reads as many messages as are available

(up to the limit specified by

.IR vlen )

and returns immediately.

.PP

On return from

.BR recvmmsg (),

successive elements of

.IR msgvec

are updated to contain information about each received message:

.I msg_len

contains the size of the received message;

the subfields of

.I msg_hdr

are updated as described in

.BR recvmsg (2).

The return value of the call indicates the number of elements of

.I msgvec

that have been updated.

.SH RETURN VALUE

On success,

.BR recvmmsg ()

returns the number of messages received in

.IR msgvec ;

on error, \-1 is returned, and

.I errno

is set to indicate the error.

.SH ERRORS

Errors are as for

.BR recvmsg (2).

In addition, the following error can occur:

.TP

.B EINVAL

.I timeout

is invalid.

.PP

See also BUGS.

.SH VERSIONS

The

.BR recvmmsg ()

system call was added in Linux 2.6.33.

Support in glibc was added in version 2.12.

.SH CONFORMING TO

.BR recvmmsg ()

is Linux-specific.

.SH BUGS

The

.I timeout

argument does not work as intended.

.\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=75371

.\" http://thread.gmane.org/gmane.linux.man/5677

The timeout is checked only after the receipt of each datagram,

so that if up to

.I vlen\-1

datagrams are received before the timeout expires,

but then no further datagrams are received, the call will block forever.

.PP

If an error occurs after at least one message has been received,

the call succeeds, and returns the number of messages received.

The error code is expected to be returned on a subsequent call to

.BR recvmmsq ().

In the current implementation, however, the error code can be overwritten

in the meantime by an unrelated network event on a socket,

for example an incoming ICMP packet.

.SH EXAMPLE

.PP

The following program uses

.BR recvmmsg ()

to receive multiple messages on a socket and stores

them in multiple buffers.

The call returns if all buffers are filled or if the

timeout specified has expired.

.PP

The following snippet periodically generates UDP datagrams

containing a random number:

.PP

.in +4n

.EX

.RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; "

.B      "      sleep 0.25; done"

.EE

.in

.PP

These datagrams are read by the example application, which

can give the following output:

.PP

.in +4n

.EX

.RB "$" " ./a.out"

5 messages received

1 11782

2 11345

3 304

4 13514

5 28421

.EE

.in

.SS Program source

\&

.EX

#define _GNU_SOURCE

#include <netinet/ip.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/socket.h>

int

main(void)

{

#define VLEN 10

#define BUFSIZE 200

#define TIMEOUT 1

    int sockfd, retval, i;

    struct sockaddr_in addr;

    struct mmsghdr msgs[VLEN];

    struct iovec iovecs[VLEN];

    char bufs[VLEN][BUFSIZE+1];

    struct timespec timeout;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);

    if (sockfd == \-1) {

        perror("socket()");

        exit(EXIT_FAILURE);

    }

    addr.sin_family = AF_INET;

    addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

    addr.sin_port = htons(1234);

    if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == \-1) {

        perror("bind()");

        exit(EXIT_FAILURE);

    }

    memset(msgs, 0, sizeof(msgs));

    for (i = 0; i < VLEN; i++) {

        iovecs[i].iov_base         = bufs[i];

        iovecs[i].iov_len          = BUFSIZE;

        msgs[i].msg_hdr.msg_iov    = &iovecs[i];

        msgs[i].msg_hdr.msg_iovlen = 1;

    }

    timeout.tv_sec = TIMEOUT;

    timeout.tv_nsec = 0;

    retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);

    if (retval == \-1) {

        perror("recvmmsg()");

        exit(EXIT_FAILURE);

    }

    printf("%d messages received\\n", retval);

    for (i = 0; i < retval; i++) {

        bufs[i][msgs[i].msg_len] = 0;

        printf("%d %s", i+1, bufs[i]);

    }

    exit(EXIT_SUCCESS);

}

.EE

.SH SEE ALSO

.BR clock_gettime (2),

.BR recvmsg (2),

.BR sendmmsg (2),

.BR sendmsg (2),

.BR socket (2),

.BR socket (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/.