## nng_recv

Recv data.

### Synopsis

```c
#include <nng/nng.h>

int nng_recv(nng_socket s, void *data, size_t *sizep, int flags);
```

### Description

The `nng_recv` receives a message.

The _flags_ is a bit mask that may contain any of the following values:

((`NNG_FLAG_NONBLOCK`))::
  The function returns immediately, even if no message is available.
  Without this flag, the function will wait until a message is received by the socket _s_, or any configured timer expires.

((`NNG_FLAG_ALLOC`))::
  If this flag is present, then a ((zero-copy)) mode is used.
  In this case the caller must set the value of _data_ to the location of another pointer (of type `void *`), and the _sizep_ pointer must be set to a location to receive the size of the message body.
  The function will then allocate a message buffer (as if by xref:../util/nng_alloc.adoc[`nng_alloc`]), fill it with the message body, and store it at the address referenced by _data_, and update the size referenced by _sizep_.
  The caller is responsible for disposing of the received buffer either by the xref:../util/nng_free.adoc[`nng_free`] function or passing the message (also with the `NNG_FLAG_ALLOC` flag) in a call to xref:nng_send.adoc[`nng_send`].

If the special flag `NNG_FLAG_ALLOC` (see above) is not specified, then the caller must set _data_ to a buffer to receive the message body content, and must store the size of that buffer at the location pointed to by _sizep_.
When the function returns, if it is successful, the size at _sizep_ will be updated with the actual message body length copied into _data_.

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 receiving data at all, or may require other conditions be met before they can receive.

TIP: The `NNG_FLAG_ALLOC` flag can be used to reduce data copies, thereby increasing performance.
However the xref:nng_recvmsg.adoc[`nng_recvmsg`] function is even better, and should be preferred when possible.

### 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 received message did not fit in the size provided.
`NNG_ENOMEM`:: Insufficient memory is available.
`NNG_ENOTSUP`:: The protocol for socket _s_ does not support receiving.
`NNG_ESTATE`:: The socket _s_ cannot receive 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_recvmsg.adoc[nng_recvmsg],
xref:nng_send.adoc[nng_send]
