NN_RECV

Section: nanomsg 1.1.5 (3)
Updated: 2021-01-26
Page Index

NAME

nn_recv - receive a message

SYNOPSIS

int nn_recv (int s, void *buf, size_t len, int flags);

DESCRIPTION

Receive a message from the socket s and store it in the buffer referenced by the buf argument. Any bytes exceeding the length specified by the len argument will be truncated.

Alternatively, nanomsg can allocate the buffer for you. To do so, let the buf parameter be a pointer to a void* variable (pointer to pointer) to the receive buffer and set the len parameter to NN_MSG. If the call is successful the user is responsible for deallocating the message using the nn_freemsg(3) function.

The flags argument is a combination of the flags defined below:

NN_DONTWAIT

: Specifies that the operation should be performed in non-blocking mode. If the message cannot be received straight away, the function will fail with errno set to EAGAIN.

RETURN VALUE

If the function succeeds number of bytes in the message is returned. Otherwise, -1 is returned and errno is set to to one of the values defined below.

ERRORS

EBADF

: The provided socket is invalid.

ENOTSUP

: The operation is not supported by this socket type.

EFSM

: The operation cannot be performed on this socket at the moment because socket is not in the appropriate state. This error may occur with socket types that switch between several states.

EAGAIN

: Non-blocking mode was requested and there's no message to receive at the moment.

EINTR

: The operation was interrupted by delivery of a signal before the message was received.

ETIMEDOUT

: Individual socket types may define their own specific timeouts. If such timeout is hit this error will be returned.

ETERM

: The library is terminating.

EXAMPLES

Receiving a message into a buffer allocated by the user

: This example code will retrieve a message of either 100 bytes or less. If a larger message was sent it will be truncated to 100 bytes.

char buf [100];
nbytes = nn_recv (s, buf, sizeof (buf), 0);

Receiving a message into a buffer allocated by nanomsg

: The following will get a message from the pipe with a buffer allocated by the system. It is large enough to accommodate the entire message. This is a good way to get the entire message without truncating if the size of the message is unknown. It is the user's responsibility to call nn_freemsg(3) after processing the message.

void *buf = NULL;
nbytes = nn_recv (s, &buf, NN_MSG, 0);

if (nbytes < 0) {
    /* handle error */
    ...
}
else {
    /* process message */
    ...
    nn_freemsg (buf);
}

Note that this can be more efficient than manually allocating a buffer since it is a zero-copy operation.