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+## 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]