recvmmsg(2) - Linux manual page (original) (raw)
recvmmsg(2) System Calls Manual recvmmsg(2)
NAME top
recvmmsg - receive multiple messages on a socketLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#define _GNU_SOURCE** /* See feature_test_macros(7) */
**#include <sys/socket.h>**
**int recvmmsg(int** _sockfd_**, struct mmsghdr** _msgvec_**[.**_n_**], unsigned int** _n_**,**
**int** _flags_**, struct timespec ***_timeout_**);**DESCRIPTION top
The **recvmmsg**() system call is an extension of [recvmsg(2)](../man2/recvmsg.2.html) 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 [recvmsg(2)](../man2/recvmsg.2.html) is support for
a timeout on the receive operation.
The _sockfd_ argument is the file descriptor of the socket to
receive data from.
The _msgvec_ argument is a pointer to an array of _mmsghdr_
structures. The size of this array is specified in _n_.
The _mmsghdr_ structure is defined in _<sys/socket.h>_ as:
struct mmsghdr {
struct msghdr msg_hdr; /* Message header */
unsigned int msg_len; /* Number of received bytes for header */
};
The _msghdr_ field is a _msghdr_ structure, as described in
[recvmsg(2)](../man2/recvmsg.2.html). The _msglen_ 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 [recvmsg(2)](../man2/recvmsg.2.html) on the header.
The _flags_ argument contains flags ORed together. The flags are
the same as documented for [recvmsg(2)](../man2/recvmsg.2.html), with the following
addition:
**MSG_WAITFORONE** (since Linux 2.6.34)
Turns on **MSG_DONTWAIT** after the first message has been
received.
The _timeout_ argument points to a _struct timespec_ (see
[clock_gettime(2)](../man2/clock%5Fgettime.2.html)) defining a timeout (seconds plus nanoseconds)
for the receive operation (_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 _timeout_ is NULL, then the operation blocks
indefinitely.
A blocking **recvmmsg**() call blocks until _n_ 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 _n_)
and returns immediately.
On return from **recvmmsg**(), successive elements of _msgvec_ are
updated to contain information about each received message:
_msglen_ contains the size of the received message; the subfields
of _msghdr_ are updated as described in [recvmsg(2)](../man2/recvmsg.2.html). The return
value of the call indicates the number of elements of _msgvec_ that
have been updated.RETURN VALUE top
On success, **recvmmsg**() returns the number of messages received in
_msgvec_; on error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the
error.ERRORS top
Errors are as for [recvmsg(2)](../man2/recvmsg.2.html). In addition, the following error
can occur:
**EINVAL** _timeout_ is invalid.
See also BUGS.STANDARDS top
Linux.HISTORY top
Linux 2.6.33, glibc 2.12.BUGS top
The _timeout_ argument does not work as intended. The timeout is
checked only after the receipt of each datagram, so that if up to
_n-1_ datagrams are received before the timeout expires, but then no
further datagrams are received, the call will block forever.
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
**recvmmsg**(). 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.EXAMPLES top
The following program uses **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.
The following snippet periodically generates UDP datagrams
containing a random number:
$ **while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234;**
**sleep 0.25; done**
These datagrams are read by the example application, which can
give the following output:
$ **./a.out**
5 messages received
1 11782
2 11345
3 304
4 13514
5 28421Program source
#define _GNU_SOURCE
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <time.h>
int
main(void)
{
#define VLEN 10
#define BUFSIZE 200
#define TIMEOUT 1
int sockfd, retval;
char bufs[VLEN][BUFSIZE+1];
struct iovec iovecs[VLEN];
struct mmsghdr msgs[VLEN];
struct timespec timeout;
struct sockaddr_in addr;
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 (size_t 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 (size_t i = 0; i < retval; i++) {
bufs[i][msgs[i].msg_len] = 0;
printf("%zu %s", i+1, bufs[i]);
}
exit(EXIT_SUCCESS);
}SEE ALSO top
[clock_gettime(2)](../man2/clock%5Fgettime.2.html), [recvmsg(2)](../man2/recvmsg.2.html), [sendmmsg(2)](../man2/sendmmsg.2.html), [sendmsg(2)](../man2/sendmsg.2.html), [socket(2)](../man2/socket.2.html),
[socket(7)](../man7/socket.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 recvmmsg(2)
Pages that refer to this page:recv(2), sendmmsg(2), syscalls(2), signal(7)