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 <nng/nng.h>

int nng_send(nng_socket s, void *data, size_t size, int flags);


The nng_send() function sends a message containing the data of length size using the socket s.

The semantics of what sending a message means vary from protocol to protocol, so examination of the protocol documentation is encouraged. (For example, with an pub socket the data is broadcast, so that any peers who have a suitable subscription will be able to receive it using nng_recv() or a similar function.) Furthermore, some protocols may not support sending data (such as sub) or may require other conditions. (For example, rep sockets cannot normally send data, which are responses to requests, until they have first received a request.)

The flags may contain either of (or neither of) the following values:


The function returns immediately, regardless of whether the socket is able to accept the data or not. If the socket is unable to accept the data (such as if backpressure exists because the peers are consuming messages too slowly, or no peer is present), then the function will return with NNG_EAGAIN. If this flag is not specified, then the function will block if such a condition exists.


The data was allocated using nng_alloc(), or was obtained from a call to nng_recv() with the NNG_FLAG_ALLOC flag. If this function returns success, then the data is "owned" by the function, and it will assume responsibility for calling nng_free() when it is no longer needed. In the absence of this flag, the data is copied by the implementation before the function returns to the caller.

The NNG_FLAG_ALLOC flag can be used to reduce data copies, thereby increasing performance.
Regardless of the presence or absence of NNG_FLAG_NONBLOCK, there may be queues between the sender and the receiver. Furthermore, there is no guarantee that the message has actually been delivered. Finally, with some protocols, the semantic is implicitly NNG_FLAG_NONBLOCK, such as with pub sockets, which are best-effort delivery only.
When using NNG_FLAG_ALLOC, it is important that the value of size match the actual allocated size of the data. Using an incorrect size results in unspecified behavior, which may include heap corruption, program crashes, or trans-dimensional mutation of the program’s author.


This function returns 0 on success, and non-zero otherwise.



The operation would block, but NNG_FLAG_NONBLOCK was specified.


The socket s is not open.


An invalid set of flags was specified.


The value of size is too large.


Insufficient memory is available.


The protocol for socket s does not support sending.


The socket s cannot send data in this state.


The operation timed out.