nn_sendmsg - Man Page

fine-grained alternative to nn_send


#include <nanomsg/nn.h>

int nn_sendmsg (int s, const struct nn_msghdr *msghdr, int flags);


Sends data specified by msghdr parameter to socket s along with any additional control data. msghdr structure should be nullified before being used.

Structure nn_msghdr contains at least following members:

struct nn_iovec *msg_iov;
int msg_iovlen;
void *msg_control;
size_t msg_controllen;

msg_iov points to a scatter array of buffers to send. msg_iovlen specifies the size of the array.

msg_control points to the buffer containing control information to be associated with the message being sent. msg_controllen specifies the length of the buffer. If there’s no control information to send, msg_control should be set to NULL. For detailed discussion of how to set control data check nn_cmsg(3) man page.

Structure nn_iovec defines one element in the scatter array (i.e. a buffer to send to the socket) and contains following members:

void *iov_base;
size_t iov_len;

Alternatively, to send a buffer allocated by nn_allocmsg(3) function set iov_base to point to the pointer to the buffer and iov_len to NN_MSG constant. In this case a successful call to nn_sendmsg will deallocate the buffer. Trying to deallocate it afterwards will result in undefined behaviour. Also, scatter array in nn_msghdr structure can contain only one element in this case.

To which of the peers will the message be sent to is determined by the particular socket type.

The flags argument is a combination of the flags defined below:


Specifies that the operation should be performed in non-blocking mode. If the message cannot be sent straight away, the function will fail with errno set to EAGAIN.

Return Value

If the function succeeds number of bytes in the message is returned. Otherwise, -1 is returned and errno is set to to one of the values defined below.



Either msghdr is NULL, there are multiple scatter buffers but length is set to NN_MSG for one of them, or the sum of iov_len values for the scatter buffers overflows size_t. These are early checks and no pre-allocated message is freed in this case.


msghdr→msg_iovlen is negative. This is an early check and no pre-allocated message is freed in this case.


The supplied pointer for the pre-allocated message buffer or the scatter buffer is NULL, or the length for the scatter buffer is 0.


The provided socket is invalid.


The operation is not supported by this socket type.


The operation cannot be performed on this socket at the moment because socket is not in the appropriate state.  This error may occur with socket types that switch between several states.


Non-blocking mode was requested and the message cannot be sent at the moment.


The operation was interrupted by delivery of a signal before the message was sent.


Individual socket types may define their own specific timeouts. If such timeout is hit this error will be returned.


The library is terminating.


Usage of multiple scatter buffers:

struct nn_msghdr hdr;
struct nn_iovec iov [2];

iov [0].iov_base = "Hello";
iov [0].iov_len = 5;
iov [1].iov_base = "World";
iov [1].iov_len = 5;
memset (&hdr, 0, sizeof (hdr));
hdr.msg_iov = iov;
hdr.msg_iovlen = 2;
nn_sendmsg (s, &hdr, 0);

Usage of a single message:

void *msg;
struct nn_msghdr hdr;
struct nn_iovec iov;

msg = nn_allocmsg(12, 0);
strcpy(msg, "Hello World");
iov.iov_base = &msg;
iov.iov_len = NN_MSG;
memset (&hdr, 0, sizeof (hdr));
hdr.msg_iov = &iov;
hdr.msg_iovlen = 1;
nn_sendmsg (s, &hdr, 0);

See Also

nn_send(3) nn_recvmsg(3) nn_allocmsg(3) nn_freemsg(3) nn_cmsg(3) nanomsg(7)


Martin Sustrik

Referenced By

nanomsg(7), nn_allocmsg(3), nn_cmsg(3), nn_recvmsg(3), nn_send(3).

2022-07-22 nanomsg 1.1.5