Advanced Programming in the UNIX® Environment

ptg10805159

Section 16.5 Data Transfer 611

the destination address is first set by callingconnect,sosendtogives us an alternate

way to send a message.

FreeBSD Linux Mac OS X Solaris

Flag Description POSIX.1 8.0 3.2.0 10.6.8 10

MSG_CONFIRM Provide feedback to the link layer •

to keep address mapping valid.

MSG_DONTROUTE Don’t route packet outside of local ••••

network.

MSG_DONTWAIT Enable nonblocking operation ••••

(equivalent to using

O_NONBLOCK).

MSG_EOF Shut the sender side of the socket ••

down after sending data.

MSG_EOR Mark the end of the recordif •••••

supported by protocol.

MSG_MORE Delay sending the packet to allow •

moredata to be written.

MSG_NOSIGNAL Don’t generateSIGPIPEwhen •••

writing to an unconnected

socket.

MSG_OOB Send out-of-band data if •••••

supported by protocol (see

Section 16.7).

Figure 16.13Flags used withsendsocket calls

We have one morechoice when transmitting data over a socket. We can call

sendmsgwith amsghdrstructure to specify multiple buffers from which to transmit

data, similar to thewritevfunction (Section 14.6).

#include <sys/socket.h>

ssize_t sendmsg(intsockfd,const struct msghdr *msg,intflags);

Returns: number of bytes sent if OK,−1 on error

POSIX.1 defines themsghdrstructure to have at least the following members:

struct msghdr {

void *msg_name; /* optional address */

socklen_t msg_namelen; /* address size in bytes */

struct iovec *msg_iov; /* array of I/O buffers */

int msg_iovlen; /* number of elements in array */

void *msg_control; /* ancillary data */

socklen_t msg_controllen; /* number of ancillary bytes */

int msg_flags; /* flags for received message */

..

.

};

We saw theiovecstructure in Section 14.6. We’ll see the use of ancillary data in

Section 17.4.