Introduce socket_nbd_write_hello() and a macro to display errno results nicely

src/readwrite.c

@@ -56,6 +56,25 @@ fail:
 	return 0;
 }
 
+int socket_nbd_write_hello(int fd, off64_t out_size)
+{
+	struct nbd_init init;
+	struct nbd_init_raw init_raw;
+
+	memcpy(&init.passwd, INIT_PASSWD, 8);
+	init.magic  = INIT_MAGIC;
+	init.size   = out_size;
+
+	memset( &init_raw, 0, sizeof( init_raw ) ); // ensure reserved is 0s
+	nbd_h2r_init(&init, &init_raw);
+
+	if ( 0 > writeloop( fd, &init_raw, sizeof( init_raw ) ) ) {
+		warn( SHOW_ERRNO( "failed to write hello to socket" ) );
+		return 0;
+	}
+	return 1;
+}
+
 void fill_request(struct nbd_request *request, int type, off64_t from, int len)
 {
 	request->magic  = htobe32(REQUEST_MAGIC);

src/readwrite.h

@@ -7,6 +7,7 @@
 
 int socket_connect(struct sockaddr* to, struct sockaddr* from);
 int socket_nbd_read_hello(int fd, off64_t * size);
+int socket_nbd_write_hello(int fd, off64_t size);
 void socket_nbd_read(int fd, off64_t from, int len, int out_fd, void* out_buf, int timeout_secs);
 void socket_nbd_write(int fd, off64_t from, int len, int out_fd, void* out_buf, int timeout_secs);
 int socket_nbd_disconnect( int fd );

src/util.h

@@ -148,6 +148,7 @@ void mylog(int line_level, const char* format, ...);
 
 #define NULLCHECK(value) FATAL_IF_NULL(value, "BUG: " #value " is null")
 
+#define SHOW_ERRNO( msg ) msg ": %s (%i)", ( errno == 0 ? "EOF" : strerror(errno) ), errno
 
 #endif