After a mention from Ben Kibbey we now let knownhost_writeline() return the

length of the actual data it returns, excluding the trailing zero. I also
updated the man page for libssh2_knownhost_writeline() accordingly.

docs/libssh2_knownhost_writeline.3

@@ -23,7 +23,8 @@ the 'type' output format.
 \fIbuflen\fP is the size of the \fIbuffer\fP. See RETURN VALUE about the size.
 
 \fIoutlen\fP must be a pointer to a size_t variable that will get the output
-length stored
+length of the stored data chunk. The number does not included the trailing
+zero!
 
 \fItype\fP specifies what file type it is, and
 \fILIBSSH2_KNOWNHOST_FILE_OPENSSH\fP is the only currently supported

src/knownhost.c

@@ -728,7 +728,8 @@ knownhost_writeline(LIBSSH2_KNOWNHOSTS *hosts,
             rc = LIBSSH2_ERROR_BUFFER_TOO_SMALL;
     }
 
-    *outlen = nlen;
+    /* we report the full length of the data with the trailing zero excluded */
+    *outlen = nlen-1;
 
     return rc;
 }