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nng_recv.3.adoc

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nng_recv(3)

NAME

nng_recv - recv data

SYNOPSIS

#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 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 nng_free() function or passing the message (also with the NNG_FLAG_ALLOC flag) in a call to 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. (For example, with a req socket a message may only be received after a request has been sent, and a sub socket may only receive messages corresponding to topics to which it has subscribed.) Furthermore, some protocols may not support receiving data at all, such as pub.
Tip
 The NNG_FLAG_ALLOC flag can be used to reduce data copies, thereby increasing performance, particularly if the buffer is reused to send a response using the same flag.

RETURN VALUES

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

ERRORS

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.