This documentation is for version v1.3.0 of NNG, but the latest released version is v1.8.0. see the documentation for v1.8.0 for the most up-to-date information.


#include <nanomsg/nn.h>

int nn_sendmsg(int sock, const struct nn_msghdr *hdr, int flags);


The nn_sendmsg() function sends the message described by hdr using the socket sock.

This function is provided for API compatibility with legacy libnanomsg. Consider using the relevant modern API instead.

The flags field may contain the special flag NN_DONTWAIT. In this case, if the socket is unable to accept more data for sending, the operation shall not block, but instead will fail with the error EAGAIN.

The hdr points to a structure of type struct nn_msghdr, which has the following definition:

struct nn_iovec {
void * iov_base;
size_t iov_len;

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

The msg_iov is an array of gather items, permitting the message to be spread into different memory blocks. There are msg_iovlen elements in this array, each of which has the base address (iov_base) and length (iov_len) indicated.

For buffers allocated for zero copy (such as by nn_allocmsg()), the value of iov_base should be the address of the pointer to the buffer, rather than the address of the buffer itself. In this case, the value of iov_len should be NN_MSG, as the length is inferred from the allocated message. If the msg_iovlen field is NN_MSG, then this function will free the associated buffer after it is done with it, if it returns successfully. (If the function returns with an error, then the caller retains ownership of the associated buffer and may retry the operation or free the buffer at its choice.)

The values of msg_control and msg_controllen describe a buffer of ancillary data to send the message. This is currently only useful to provide the message headers used with raw mode sockets. In all other circumstances these fields should be zero. Details about this structure are covered in nn_cmsg(3compat).

The send operation is performed asynchronously, and may not have completed before this function returns control to the caller.


This function returns the number of bytes sent on success, and -1 on error.



The operation would block.


The socket sock is not open.


The socket cannot send in this state.


The hdr is invalid.


This protocol cannot send.


Operation timed out.