diff --git a/io.c b/io.c
index 697d571c4a..a01aca8f61 100644
--- a/io.c
+++ b/io.c
@@ -1885,105 +1885,126 @@ io_write(VALUE io, VALUE str, int nosync)
 #ifdef HAVE_WRITEV
 struct binwritev_arg {
     rb_io_t *fptr;
-    const struct iovec *iov;
+    struct iovec *iov;
     int iovcnt;
+    size_t total;
 };
 
 static VALUE
-call_writev_internal(VALUE arg)
+io_binwritev_internal(VALUE arg)
 {
     struct binwritev_arg *p = (struct binwritev_arg *)arg;
-    return rb_writev_internal(p->fptr, p->iov, p->iovcnt);
+
+    size_t remaining = p->total;
+    size_t offset = 0;
+
+    rb_io_t *fptr = p->fptr;
+    struct iovec *iov = p->iov;
+    int iovcnt = p->iovcnt;
+
+    while (remaining) {
+        long result = rb_writev_internal(fptr, iov, iovcnt);
+
+        if (result >= 0) {
+            offset += result;
+            if (fptr->wbuf.ptr && fptr->wbuf.len) {
+                if (offset < (size_t)fptr->wbuf.len) {
+                    fptr->wbuf.off += result;
+                    fptr->wbuf.len -= result;
+                }
+                else {
+                    offset -= (size_t)fptr->wbuf.len;
+                    fptr->wbuf.off = 0;
+                    fptr->wbuf.len = 0;
+                }
+            }
+
+            if (offset == p->total) {
+                return p->total;
+            }
+
+            while (result >= (ssize_t)iov->iov_len) {
+                /* iovcnt > 0 */
+                result -= iov->iov_len;
+                iov->iov_len = 0;
+                iov++;
+
+                if (!--iovcnt) {
+                    // I don't believe this code path can ever occur.
+                    return offset;
+                }
+            }
+
+            iov->iov_base = (char *)iov->iov_base + result;
+            iov->iov_len -= result;
+
+            errno = EAGAIN;
+        }
+
+        if (rb_io_maybe_wait_writable(errno, fptr->self, Qnil)) {
+            rb_io_check_closed(fptr);
+        } else {
+            return -1;
+        }
+    }
+
+    return offset;
 }
 
 static long
 io_binwritev(struct iovec *iov, int iovcnt, rb_io_t *fptr)
 {
-    int i;
-    long r, total = 0, written_len = 0;
-
-    /* don't write anything if current thread has a pending interrupt. */
+    // Don't write anything if current thread has a pending interrupt:
     rb_thread_check_ints();
 
     if (iovcnt == 0) return 0;
-    for (i = 1; i < iovcnt; i++) total += iov[i].iov_len;
+
+    size_t total = 0;
+    for (int i = 1; i < iovcnt; i++) total += iov[i].iov_len;
 
     io_allocate_write_buffer(fptr, 1);
 
     if (fptr->wbuf.ptr && fptr->wbuf.len) {
-        long offset = fptr->wbuf.off + fptr->wbuf.len;
-        if (offset + total <= fptr->wbuf.capa) {
-            for (i = 1; i < iovcnt; i++) {
+        // The end of the buffered data:
+        size_t offset = fptr->wbuf.off + fptr->wbuf.len;
+
+        if (offset + total <= (size_t)fptr->wbuf.capa) {
+            for (int i = 1; i < iovcnt; i++) {
                 memcpy(fptr->wbuf.ptr+offset, iov[i].iov_base, iov[i].iov_len);
                 offset += iov[i].iov_len;
             }
 
             fptr->wbuf.len += total;
+
             return total;
         }
         else {
             iov[0].iov_base = fptr->wbuf.ptr + fptr->wbuf.off;
-            iov[0].iov_len  = fptr->wbuf.len;
+            iov[0].iov_len = fptr->wbuf.len;
         }
     }
     else {
+        // The first iov is reserved for the internal buffer, and it's empty.
         iov++;
-        if (!--iovcnt) return 0;
+
+        if (!--iovcnt) {
+            // If there are no other io vectors we are done.
+            return 0;
+        }
     }
 
-  retry:
+    struct binwritev_arg arg;
+    arg.fptr = fptr;
+    arg.iov = iov;
+    arg.iovcnt = iovcnt;
+    arg.total = total;
+
     if (fptr->write_lock) {
-        struct binwritev_arg arg;
-        arg.fptr = fptr;
-        arg.iov  = iov;
-        arg.iovcnt = iovcnt;
-        r = rb_mutex_synchronize(fptr->write_lock, call_writev_internal, (VALUE)&arg);
+        return rb_mutex_synchronize(fptr->write_lock, io_binwritev_internal, (VALUE)&arg);
     }
     else {
-        r = rb_writev_internal(fptr, iov, iovcnt);
+        return io_binwritev_internal((VALUE)&arg);
     }
-
-    if (r >= 0) {
-        written_len += r;
-        if (fptr->wbuf.ptr && fptr->wbuf.len) {
-            if (written_len < fptr->wbuf.len) {
-                fptr->wbuf.off += r;
-                fptr->wbuf.len -= r;
-            }
-            else {
-                written_len -= fptr->wbuf.len;
-                fptr->wbuf.off = 0;
-                fptr->wbuf.len = 0;
-            }
-        }
-
-        if (written_len == total) return total;
-
-        while (r >= (ssize_t)iov->iov_len) {
-            /* iovcnt > 0 */
-            r -= iov->iov_len;
-            iov->iov_len = 0;
-            iov++;
-
-            if (!--iovcnt) {
-                // assert(written_len == total);
-
-                return total;
-            }
-        }
-
-        iov->iov_base = (char *)iov->iov_base + r;
-        iov->iov_len -= r;
-
-        errno = EAGAIN;
-    }
-
-    if (rb_io_maybe_wait_writable(errno, fptr->self, Qnil)) {
-        rb_io_check_closed(fptr);
-        goto retry;
-    }
-
-    return -1L;
 }
 
 static long