NN_RECVMSG

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

nn_recvmsg - fine-grained alternative to nn_recv

SYNOPSIS

NN_EXPORT int nn_recvmsg (int s, struct nn_msghdr *msghdr, int flags);

DESCRIPTION

Receives a message from socket s into buffers specified by msghdr parameter along with any additional control data. msghdr parameter should be nullified before being used.

Structure nn_msghdr contains at least following members:

struct nn_iovec *msg_iov;
int msg_iovlen;
void *msg_control;
size_t msg_controllen;

msg_iov points to a gather array of buffers to fill in. msg_iovlen specifies the size of the array.

msg_control points to the buffer to hold control information associated with the received message. msg_controllen specifies the length of the buffer. If the control information should not be retrieved, set msg_control parameter to NULL. For detailed discussion of how to parse the control information check nn_cmsg(3) man page.

Structure nn_iovec defines one element in the gather array (a buffer to be filled in by message data) and contains following members:

void *iov_base;
size_t iov_len;

Alternatively, nanomsg library can allocate the buffer for you. To do so, let the iov_base point to void* variable to receive the buffer and set iov_len to NN_MSG. After successful completion user is responsible for deallocating the message using nn_freemsg(3) function. Gather array in nn_msghdr structure can contain only one element in this case.

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.

EXAMPLE

struct nn_msghdr hdr;
struct nn_iovec iov [2];
char buf0 [4];
char buf1 [2];
iov [0].iov_base = buf0;
iov [0].iov_len = sizeof (buf0);
iov [1].iov_base = buf1;
iov [1].iov_len = sizeof (buf1);
memset (&hdr, 0, sizeof (hdr));
hdr.msg_iov = iov;
hdr.msg_iovlen = 2;
nn_recvmsg (s, &hdr, 0);