## nn_sendmsg

Send message (compatible API).

### Synopsis

```c
#include <nanomsg/nn.h>

int nn_sendmsg(int sock, const struct nn_msghdr *hdr, int flags);
```

### Description

The `nn_sendmsg` function sends the message described by _hdr_ using the socket _sock_.

The _flags_ field may contain the special flag `NN_DONTWAIT`.
In this case, if the socket is unable to accept more data for sending, the operation shall not block, but instead will fail with the error `EAGAIN`.

The _hdr_ points to a structure of type `struct nn_msghdr`, which has the following definition:

```c
struct nn_iovec {
    void * iov_base;
    size_t iov_len;
};

struct nn_msghdr {
    struct nn_iovec *msg_iov;
    int              msg_iovlen;
    void *           msg_control;
    size_t           msg_controllen;
};
```

The `msg_iov` is an array of gather items, permitting the message to be spread into different memory blocks.
There are `msg_iovlen` elements in this array, each of which has the base address (`iov_base`) and length (`iov_len`) indicated.

For buffers allocated for zero copy (such as by xref:nn_allocmsg.adoc[`nn_allocmsg`]), the value of `iov_base` should be the address of the pointer to the buffer, rather than the address of the buffer itself.
In this case, the value of `iov_len` should be `NN_MSG`, as the length is inferred from the allocated message.
If the `msg_iovlen` field is `NN_MSG`, then this function will free the associated buffer after it is done with it, if it returns successfully.
(If the function returns with an error, then the caller retains ownership of the associated buffer and may retry the operation or free the buffer at its choice.)

The values of `msg_control` and `msg_controllen` describe a buffer of ancillary data to send the message.
This is only useful to provide the message headers used with raw mode sockets.
In all other circumstances these fields should be zero.
Details about this structure are covered in xref:nn_cmsg.adoc[`nn_cmsg`].

NOTE: The send operation is performed asynchronously, and may not have completed before this function returns control to the caller.

### Return Values

This function returns the number of bytes sent on success, and -1 on error.

### Errors

[horizontal]
`EAGAIN`:: The operation would block.
`EBADF`:: The socket _sock_ is not open.
`EFSM`:: The socket cannot send in this state.
`EINVAL`:: The _hdr_ is invalid.
`ENOTSUP`:: This protocol cannot send.
`ETIMEDOUT`:: Operation timed out.

### See Also

xref:nn_cmsg.adoc[nn_cmsg],
xref:nn_errno.adoc[nn_errno],
xref:nn_recv.adoc[nn_recv],
xref:nn_send.adoc[nn_send],
xref:nn_socket.adoc[nn_socket]
