diff --git a/otherbackends/raw.c b/otherbackends/raw.c
index 9c803594..2f099ce3 100644
--- a/otherbackends/raw.c
+++ b/otherbackends/raw.c
@@ -207,16 +207,14 @@ static void raw_reconfig(Backend *be, Conf *conf)
 /*
  * Called to send data down the raw connection.
  */
-static size_t raw_send(Backend *be, const char *buf, size_t len)
+static void raw_send(Backend *be, const char *buf, size_t len)
 {
     Raw *raw = container_of(be, Raw, backend);
 
     if (raw->s == NULL)
-        return 0;
+        return;
 
     raw->bufsize = sk_write(raw->s, buf, len);
-
-    return raw->bufsize;
 }
 
 /*
diff --git a/otherbackends/rlogin.c b/otherbackends/rlogin.c
index 30ad0526..d0da322a 100644
--- a/otherbackends/rlogin.c
+++ b/otherbackends/rlogin.c
@@ -269,13 +269,13 @@ static void rlogin_reconfig(Backend *be, Conf *conf)
 /*
  * Called to send data down the rlogin connection.
  */
-static size_t rlogin_send(Backend *be, const char *buf, size_t len)
+static void rlogin_send(Backend *be, const char *buf, size_t len)
 {
     Rlogin *rlogin = container_of(be, Rlogin, backend);
     bufchain bc;
 
     if (rlogin->s == NULL)
-        return 0;
+        return;
 
     bufchain_init(&bc);
     bufchain_add(&bc, buf, len);
@@ -305,8 +305,6 @@ static size_t rlogin_send(Backend *be, const char *buf, size_t len)
     }
 
     bufchain_clear(&bc);
-
-    return rlogin->bufsize;
 }
 
 /*
diff --git a/otherbackends/supdup.c b/otherbackends/supdup.c
index e7eff6ee..7111c9a6 100644
--- a/otherbackends/supdup.c
+++ b/otherbackends/supdup.c
@@ -797,14 +797,14 @@ static void supdup_reconfig(Backend *be, Conf *conf)
 /*
 * Called to send data down the Supdup connection.
 */
-static size_t supdup_send(Backend *be, const char *buf, size_t len)
+static void supdup_send(Backend *be, const char *buf, size_t len)
 {
     Supdup *supdup = container_of(be, Supdup, backend);
     char c;
     int i;
 
     if (supdup->s == NULL)
-        return 0;
+        return;
 
     for (i = 0; i < len; i++) {
         if (buf[i] == 034)
@@ -814,7 +814,6 @@ static size_t supdup_send(Backend *be, const char *buf, size_t len)
             supdup->bufsize = sk_write(supdup->s, &c, 1);
         }
     }
-    return supdup->bufsize;
 }
 
 /*
diff --git a/otherbackends/telnet.c b/otherbackends/telnet.c
index 4b784036..15afd82e 100644
--- a/otherbackends/telnet.c
+++ b/otherbackends/telnet.c
@@ -813,7 +813,7 @@ static void telnet_reconfig(Backend *be, Conf *conf)
 /*
  * Called to send data down the Telnet connection.
  */
-static size_t telnet_send(Backend *be, const char *buf, size_t len)
+static void telnet_send(Backend *be, const char *buf, size_t len)
 {
     Telnet *telnet = container_of(be, Telnet, backend);
     unsigned char *p, *end;
@@ -824,7 +824,7 @@ static size_t telnet_send(Backend *be, const char *buf, size_t len)
 #endif
 
     if (telnet->s == NULL)
-        return 0;
+        return;
 
     p = (unsigned char *)buf;
     end = (unsigned char *)(buf + len);
@@ -841,8 +841,6 @@ static size_t telnet_send(Backend *be, const char *buf, size_t len)
             p++;
         }
     }
-
-    return telnet->bufsize;
 }
 
 /*
diff --git a/otherbackends/testback.c b/otherbackends/testback.c
index 8a158439..4ff6fa16 100644
--- a/otherbackends/testback.c
+++ b/otherbackends/testback.c
@@ -39,9 +39,10 @@ static char *loop_init(const BackendVtable *, Seat *, Backend **, LogContext *,
 static void null_free(Backend *);
 static void loop_free(Backend *);
 static void null_reconfig(Backend *, Conf *);
-static size_t null_send(Backend *, const char *, size_t);
-static size_t loop_send(Backend *, const char *, size_t);
+static void null_send(Backend *, const char *, size_t);
+static void loop_send(Backend *, const char *, size_t);
 static size_t null_sendbuffer(Backend *);
+static size_t loop_sendbuffer(Backend *);
 static void null_size(Backend *, int, int);
 static void null_special(Backend *, SessionSpecialCode, int);
 static const SessionSpecial *null_get_specials(Backend *);
@@ -80,7 +81,7 @@ const BackendVtable loop_backend = {
     .free = loop_free,
     .reconfig = null_reconfig,
     .send = loop_send,
-    .sendbuffer = null_sendbuffer,
+    .sendbuffer = loop_sendbuffer,
     .size = null_size,
     .special = null_special,
     .get_specials = null_get_specials,
@@ -100,6 +101,7 @@ const BackendVtable loop_backend = {
 struct loop_state {
     Seat *seat;
     Backend backend;
+    size_t sendbuffer;
 };
 
 static char *null_init(const BackendVtable *vt, Seat *seat,
@@ -143,15 +145,14 @@ static void null_reconfig(Backend *be, Conf *conf) {
 
 }
 
-static size_t null_send(Backend *be, const char *buf, size_t len) {
+static void null_send(Backend *be, const char *buf, size_t len) {
 
-    return 0;
 }
 
-static size_t loop_send(Backend *be, const char *buf, size_t len) {
+static void loop_send(Backend *be, const char *buf, size_t len) {
     struct loop_state *st = container_of(be, struct loop_state, backend);
 
-    return seat_output(st->seat, 0, buf, len);
+    st->sendbuffer = seat_output(st->seat, 0, buf, len);
 }
 
 static size_t null_sendbuffer(Backend *be) {
@@ -159,6 +160,12 @@ static size_t null_sendbuffer(Backend *be) {
     return 0;
 }
 
+static size_t loop_sendbuffer(Backend *be) {
+    struct loop_state *st = container_of(be, struct loop_state, backend);
+
+    return st->sendbuffer;
+}
+
 static void null_size(Backend *be, int width, int height) {
 
 }
diff --git a/pscp.c b/pscp.c
index a200484f..d759fb4e 100644
--- a/pscp.c
+++ b/pscp.c
@@ -851,7 +851,8 @@ int scp_send_filedata(char *data, int len)
         scp_sftp_fileoffset += len;
         return 0;
     } else {
-        int bufsize = backend_send(backend, data, len);
+        backend_send(backend, data, len);
+        int bufsize = backend_sendbuffer(backend);
 
         /*
          * If the network transfer is backing up - that is, the
diff --git a/putty.h b/putty.h
index 382c010d..4492d515 100644
--- a/putty.h
+++ b/putty.h
@@ -636,9 +636,8 @@ struct BackendVtable {
     void (*free) (Backend *be);
     /* Pass in a replacement configuration. */
     void (*reconfig) (Backend *be, Conf *conf);
-    /* send() returns the current amount of buffered data. */
-    size_t (*send) (Backend *be, const char *buf, size_t len);
-    /* sendbuffer() does the same thing but without attempting a send */
+    void (*send) (Backend *be, const char *buf, size_t len);
+    /* sendbuffer() returns the current amount of buffered data */
     size_t (*sendbuffer) (Backend *be);
     void (*size) (Backend *be, int width, int height);
     void (*special) (Backend *be, SessionSpecialCode code, int arg);
@@ -687,8 +686,8 @@ static inline void backend_free(Backend *be)
 { be->vt->free(be); }
 static inline void backend_reconfig(Backend *be, Conf *conf)
 { be->vt->reconfig(be, conf); }
-static inline size_t backend_send(Backend *be, const char *buf, size_t len)
-{ return be->vt->send(be, buf, len); }
+static inline void backend_send(Backend *be, const char *buf, size_t len)
+{ be->vt->send(be, buf, len); }
 static inline size_t backend_sendbuffer(Backend *be)
 { return be->vt->sendbuffer(be); }
 static inline void backend_size(Backend *be, int width, int height)
diff --git a/ssh/sesschan.c b/ssh/sesschan.c
index 7a2062a8..e1496023 100644
--- a/ssh/sesschan.c
+++ b/ssh/sesschan.c
@@ -270,7 +270,8 @@ static size_t sesschan_send(Channel *chan, bool is_stderr,
     if (!sess->backend || sess->ignoring_input)
         return 0;
 
-    return backend_send(sess->backend, data, length);
+    backend_send(sess->backend, data, length);
+    return backend_sendbuffer(sess->backend);
 }
 
 static void sesschan_send_eof(Channel *chan)
diff --git a/ssh/ssh.c b/ssh/ssh.c
index 096c8b01..4ad10169 100644
--- a/ssh/ssh.c
+++ b/ssh/ssh.c
@@ -1003,18 +1003,16 @@ static void ssh_reconfig(Backend *be, Conf *conf)
 /*
  * Called to send data down the SSH connection.
  */
-static size_t ssh_send(Backend *be, const char *buf, size_t len)
+static void ssh_send(Backend *be, const char *buf, size_t len)
 {
     Ssh *ssh = container_of(be, Ssh, backend);
 
     if (ssh == NULL || ssh->s == NULL)
-        return 0;
+        return;
 
     bufchain_add(&ssh->user_input, buf, len);
     if (ssh->base_layer)
         ssh_ppl_got_user_input(ssh->base_layer);
-
-    return backend_sendbuffer(&ssh->backend);
 }
 
 /*
diff --git a/sshproxy.c b/sshproxy.c
index 955012a8..9c6708ab 100644
--- a/sshproxy.c
+++ b/sshproxy.c
@@ -106,7 +106,8 @@ static size_t sshproxy_write(Socket *s, const void *data, size_t len)
     SshProxy *sp = container_of(s, SshProxy, sock);
     if (!sp->backend)
         return 0;
-    return backend_send(sp->backend, data, len);
+    backend_send(sp->backend, data, len);
+    return backend_sendbuffer(sp->backend);
 }
 
 static size_t sshproxy_write_oob(Socket *s, const void *data, size_t len)
diff --git a/unix/pty.c b/unix/pty.c
index 8321f982..e74ebd4b 100644
--- a/unix/pty.c
+++ b/unix/pty.c
@@ -1367,17 +1367,15 @@ static void pty_try_write(Pty *pty)
 /*
  * Called to send data down the pty.
  */
-static size_t pty_send(Backend *be, const char *buf, size_t len)
+static void pty_send(Backend *be, const char *buf, size_t len)
 {
     Pty *pty = container_of(be, Pty, backend);
 
     if (pty->master_i < 0 || pty->pending_eof)
-        return 0;                      /* ignore all writes if fd closed */
+        return;                   /* ignore all writes if fd closed */
 
     bufchain_add(&pty->output_data, buf, len);
     pty_try_write(pty);
-
-    return bufchain_size(&pty->output_data);
 }
 
 static void pty_close(Pty *pty)
diff --git a/unix/serial.c b/unix/serial.c
index d17e4cdd..905c1b2d 100644
--- a/unix/serial.c
+++ b/unix/serial.c
@@ -461,17 +461,15 @@ static void serial_try_write(Serial *serial)
 /*
  * Called to send data down the serial connection.
  */
-static size_t serial_send(Backend *be, const char *buf, size_t len)
+static void serial_send(Backend *be, const char *buf, size_t len)
 {
     Serial *serial = container_of(be, Serial, backend);
 
     if (serial->fd < 0)
-        return 0;
+        return;
 
     bufchain_add(&serial->output_data, buf, len);
     serial_try_write(serial);
-
-    return bufchain_size(&serial->output_data);
 }
 
 /*
diff --git a/windows/conpty.c b/windows/conpty.c
index 843d6725..f2882316 100644
--- a/windows/conpty.c
+++ b/windows/conpty.c
@@ -287,15 +287,14 @@ static void conpty_reconfig(Backend *be, Conf *conf)
 {
 }
 
-static size_t conpty_send(Backend *be, const char *buf, size_t len)
+static void conpty_send(Backend *be, const char *buf, size_t len)
 {
     ConPTY *conpty = container_of(be, ConPTY, backend);
 
     if (conpty->out == NULL)
-        return 0;
+        return;
 
     conpty->bufsize = handle_write(conpty->out, buf, len);
-    return conpty->bufsize;
 }
 
 static size_t conpty_sendbuffer(Backend *be)
diff --git a/windows/plink.c b/windows/plink.c
index 2c68fb63..5bb001be 100644
--- a/windows/plink.c
+++ b/windows/plink.c
@@ -204,7 +204,8 @@ size_t stdin_gotdata(struct handle *h, const void *data, size_t len, int err)
     noise_ultralight(NOISE_SOURCE_IOLEN, len);
     if (backend_connected(backend)) {
         if (len > 0) {
-            return backend_send(backend, data, len);
+            backend_send(backend, data, len);
+            return backend_sendbuffer(backend);
         } else {
             backend_special(backend, SS_EOF, 0);
             return 0;
diff --git a/windows/serial.c b/windows/serial.c
index 3d5ea8e5..470c979f 100644
--- a/windows/serial.c
+++ b/windows/serial.c
@@ -306,15 +306,14 @@ static void serial_reconfig(Backend *be, Conf *conf)
 /*
  * Called to send data down the serial connection.
  */
-static size_t serial_send(Backend *be, const char *buf, size_t len)
+static void serial_send(Backend *be, const char *buf, size_t len)
 {
     Serial *serial = container_of(be, Serial, backend);
 
     if (serial->out == NULL)
-        return 0;
+        return;
 
     serial->bufsize = handle_write(serial->out, buf, len);
-    return serial->bufsize;
 }
 
 /*