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## nng_send
Send data.
### Synopsis
```c
#include <nng/nng.h>
int nng_send(nng_socket s, void *data, size_t size, int flags);
```
### Description
The `nng_send` function sends a message containing the _data_ of
length _size_ using the xref:nng_socket.adoc[socket] _s_.
NOTE: The semantics of what receiving a message means vary from protocol to protocol, so examination of the protocol documentation is encouraged.
Furthermore, some protocols may not support sending data at all, or may require other conditions be met before they can send.
The _flags_ may contain either of (or neither of) the following values:
((`NNG_FLAG_NONBLOCK`))::
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.
((`NNG_FLAG_ALLOC`))::
The _data_ was allocated using xref:../util/nng_alloc.adoc[`nng_alloc`], or was obtained from a call to xref:nng_recv.adoc[`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 xref:../util/nng_free.adoc[`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.
TIP: The `NNG_FLAG_ALLOC` flag can be used to reduce data copies, thereby increasing performance.
However the xref:nng_sendmsg.adoc[`nng_sendmsg`] function is even better, and should be preferred when possible.
NOTE: 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 best-effort delivery only protocols, the semantic is implicitly `NNG_FLAG_NONBLOCK`.
IMPORTANT: 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.
### Return Values
This function returns 0 on success, and non-zero otherwise.
### Errors
[horizontal]
`NNG_EAGAIN`:: The operation would block, but `NNG_FLAG_NONBLOCK` was specified.
`NNG_ECLOSED`:: The socket _s_ is not open.
`NNG_EINVAL`:: An invalid set of _flags_ was specified.
`NNG_EMSGSIZE`:: The value of _size_ is too large.
`NNG_ENOMEM`:: Insufficient memory is available.
`NNG_ENOTSUP`:: The protocol for socket _s_ does not support sending.
`NNG_ESTATE`:: The socket _s_ cannot send data in this state.
`NNG_ETIMEDOUT`:: The operation timed out.
### See Also
xref:../util/nng_alloc.adoc[nng_alloc],
xref:../util/nng_free.adoc[nng_free],
xref:nng_recv.adoc[nng_recv],
xref:nng_sendmsg.adoc[nng_sendmsg]
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