NAME¶

recvmmsg - receive multiple messages on a socket

SYNOPSIS¶

#define _GNU_SOURCE
#include <sys/socket.h>
int recvmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,


             unsigned int flags, struct timespec *timeout);

DESCRIPTION¶

The recvmmsg() system call is an extension of 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 recvmsg(2) 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 vlen.

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 msg_hdr field is a msghdr structure, as described in recvmsg(2). The 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 recvmsg(2) on the header.

The flags argument contains flags ORed together. The flags are the same as documented for recvmsg(2), with the following addition:

MSG_WAITFORONE

Turns on MSG_DONTWAIT after the first message has been received.

The timeout argument points to a struct timespec (see clock_gettime(2)) defining a timeout (seconds plus nanoseconds) for the receive operation. If timeout is NULL then the operation blocks indefinitely.

A blocking recvmmsg() call blocks until 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 vlen) and returns immediately.

On return from recvmmsg(), successive elements of msgvec are updated to contain information about each received message: msg_len contains the size of the received message; the subfields of msg_hdr are updated as described in recvmsg(2). The return value of the call indicates the number of elements of msgvec that have been updated.

RETURN VALUE¶

On success, recvmmsg() returns the number of messages received in msgvec; on error, -1 is returned, and errno is set to indicate the error.

ERRORS¶

Errors are as for recvmsg(2). In addition, the following error can occur:

EINVAL

timeout is invalid.

VERSIONS¶

The recvmmsg() system call was added in Linux 2.6.32. Support in glibc was added in version 2.12.

CONFORMING TO¶

recvmmsg() is Linux-specific.

SEE ALSO¶

clock_gettime(2), recvmsg(2), sendmmsg(2), sendmsg(2), socket(2), socket(7)