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Manual page for SEND(2)

send, sendto, sendmsg - send a message from a socket


#include <sys/types.h>
#include <sys/socket.h>

int send(s, msg, len, flags)
int s;
char *msg;
int len, flags;

int sendto(s, msg, len, flags, to, tolen)
int s;
char *msg;
int len, flags;
struct sockaddr *to;
int tolen;

int sendmsg(s, msg, flags)
int s;
struct msghdr *msg;
int flags;


s is a socket created with socket.2 send(), sendto(), and sendmsg() are used to transmit a message to another socket. send() may be used only when the socket is in a connected state, while sendto() and sendmsg() may be used at any time.

The address of the target is given by to with tolen specifying its size. The length of the message is given by len. If the message is too long to pass atomically through the underlying protocol, then the error EMSGSIZE is returned, and the message is not transmitted.

No indication of failure to deliver is implicit in a send(). Return values of -1 indicate some locally detected errors.

If no buffer space is available at the socket to hold the message to be transmitted, then send() normally blocks, unless the socket has been placed in non-blocking I/O mode. The select.2 call may be used to determine when it is possible to send more data.

If the process calling send(), sendmsg() or sendto() receives a signal before any data are buffered to be sent, the system call is restarted unless the calling process explicitly set the signal to interrupt these calls using sigvec() or sigaction() (see the discussions of SV_INTERRUPT on sigvec.2 and SA_INTERRUPT on sigaction.3v

The flags parameter is formed by ORing one or more of the following:

Send ``out-of-band'' data on sockets that support this notion. The underlying protocol must also support ``out-of-band'' data. Currently, only SOCK_STREAM sockets created in the AF_INET address family support out-of-band data.
The SO_DONTROUTE option is turned on for the duration of the operation. This is usually used only by diagnostic or routing programs.

See recv.2 for a description of the msghdr structure.


On success, these functions return the number of bytes sent. On failure, they return -1 and set errno to indicate the error.


s is an invalid descriptor.
The data was specified to be sent to a non-existent or protected part of the process address space.
The calling process received a signal before any data could be buffered to be sent, and the signal was set to interrupt the system call.
len is not the size of a valid address for the specified address family.
The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.
The output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion.
s is a descriptor for a file, not a socket.
The socket is marked non-blocking and the requested operation would block.


connect.2 fcntl.2v getsockopt.2 recv.2 select.2 socket.2 write.2v

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Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).

Last modified 21/April/97