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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
recvmmsg(2) System Calls Manualrecvmmsg(2)recvmmsg - receive multiple messages on a socket
Standard C library (libc,-lc)
#define _GNU_SOURCE/* See feature_test_macros(7) */#include <sys/socket.h>int recvmmsg(unsigned int n;intsockfd, struct mmsghdrmsgvec[n], unsigned intn,intflags, struct timespec *timeout);
Therecvmmsg() system call is an extension ofrecvmsg(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 overrecvmsg(2) is support for a timeout on the receive operation. Thesockfd argument is the file descriptor of the socket to receive data from. Themsgvec argument is a pointer to an array ofmmsghdr structures. The size of this array is specified inn. Themmsghdr 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 */ }; Themsg_hdr field is amsghdr structure, as described inrecvmsg(2). Themsg_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 singlerecvmsg(2) on the header. Theflags argument contains flags ORed together. The flags are the same as documented forrecvmsg(2), with the following addition:MSG_WAITFORONE(since Linux 2.6.34) Turns onMSG_DONTWAITafter the first message has been received. Thetimeout argument points to astruct timespec (seeclock_gettime(2)) 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.) Iftimeout is NULL, then the operation blocks indefinitely. A blockingrecvmmsg() call blocks untiln messages have been received or until the timeout expires. A nonblocking call reads as many messages as are available (up to the limit specified byn) and returns immediately. On return fromrecvmmsg(), successive elements ofmsgvec are updated to contain information about each received message:msg_len contains the size of the received message; the subfields ofmsg_hdr are updated as described inrecvmsg(2). The return value of the call indicates the number of elements ofmsgvec that have been updated.On success,recvmmsg() returns the number of messages received inmsgvec; on error, -1 is returned, anderrno is set to indicate the error.
Errors are as forrecvmsg(2). In addition, the following error can occur:EINVALtimeout is invalid. See also BUGS.
Linux.
Linux 2.6.33, glibc 2.12.
Thetimeout argument does not work as intended. The timeout is checked only after the receipt of each datagram, so that if up ton-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 torecvmmsg(). 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.
The following program usesrecvmmsg() 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); }clock_gettime(2),recvmsg(2),sendmmsg(2),sendmsg(2),socket(2),socket(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.16.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2026-01-16. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.16 2025-08-20recvmmsg(2)Pages that refer to this page:recv(2), sendmmsg(2), syscalls(2), signal(7)
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