.\" Copyright (C) 2011 by Andi Kleen .\" and Copyright (c) 2011 by Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" Syscall added in following commit .\" commit a2e2725541fad72416326798c2d7fa4dafb7d337 .\" Author: Arnaldo Carvalho de Melo .\" Date: Mon Oct 12 23:40:10 2009 -0700 .\" .TH recvmmsg 2 2022-12-04 "Linux man-pages 6.03" .SH NAME recvmmsg \- receive multiple messages on a socket .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .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 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 .I 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 glibc 2.12. .SH STANDARDS .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 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. .SH EXAMPLES 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 \& .\" SRC BEGIN (recvmmsg.c) .EX #define _GNU_SOURCE #include #include #include #include #include #include #include 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\en", 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); } .EE .\" SRC END .SH SEE ALSO .BR clock_gettime (2), .BR recvmsg (2), .BR sendmmsg (2), .BR sendmsg (2), .BR socket (2), .BR socket (7)