diff --git a/src/tcp_subr.c b/src/tcp_subr.c
index a699117b37f4bfd3344e642df6f3144d31ae5a06..a72c86b47131a28ac0dd713423159c4f0a8092e2 100644
--- a/src/tcp_subr.c
+++ b/src/tcp_subr.c
@@ -643,8 +643,7 @@ int tcp_emu(struct socket *so, struct mbuf *m)
                 NTOHS(n1);
                 NTOHS(n2);
                 m_inc(m, snprintf(NULL, 0, "%d,%d\r\n", n1, n2) + 1);
-                m->m_len = snprintf(m->m_data, M_ROOM(m), "%d,%d\r\n", n1, n2);
-                assert(m->m_len < M_ROOM(m));
+                m->m_len = slirp_fmt(m->m_data, M_ROOM(m), "%d,%d\r\n", n1, n2);
             } else {
                 *eol = '\r';
             }
@@ -684,9 +683,9 @@ int tcp_emu(struct socket *so, struct mbuf *m)
             n4 = (laddr & 0xff);
 
             m->m_len = bptr - m->m_data; /* Adjust length */
-            m->m_len += snprintf(bptr, M_FREEROOM(m),
-                                 "ORT %d,%d,%d,%d,%d,%d\r\n%s", n1, n2, n3, n4,
-                                 n5, n6, x == 7 ? buff : "");
+            m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
+                                  "ORT %d,%d,%d,%d,%d,%d\r\n%s",
+                                  n1, n2, n3, n4, n5, n6, x == 7 ? buff : "");
             return 1;
         } else if ((bptr = (char *)strstr(m->m_data, "27 Entering")) != NULL) {
             /*
@@ -719,10 +718,9 @@ int tcp_emu(struct socket *so, struct mbuf *m)
             n4 = (laddr & 0xff);
 
             m->m_len = bptr - m->m_data; /* Adjust length */
-            m->m_len += snprintf(bptr, M_FREEROOM(m),
-                         "27 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n%s",
-                         n1, n2, n3, n4, n5, n6, x == 7 ? buff : "");
-
+            m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
+                                  "27 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n%s",
+                                  n1, n2, n3, n4, n5, n6, x == 7 ? buff : "");
             return 1;
         }
 
@@ -745,8 +743,8 @@ int tcp_emu(struct socket *so, struct mbuf *m)
         if (m->m_data[m->m_len - 1] == '\0' && lport != 0 &&
             (so = tcp_listen(slirp, INADDR_ANY, 0, so->so_laddr.s_addr,
                              htons(lport), SS_FACCEPTONCE)) != NULL)
-            m->m_len = snprintf(m->m_data, M_ROOM(m),
-                                "%d", ntohs(so->so_fport)) + 1;
+            m->m_len = slirp_fmt0(m->m_data, M_ROOM(m),
+                                  "%d", ntohs(so->so_fport));
         return 1;
 
     case EMU_IRC:
@@ -765,10 +763,10 @@ int tcp_emu(struct socket *so, struct mbuf *m)
                 return 1;
             }
             m->m_len = bptr - m->m_data; /* Adjust length */
-            m->m_len += snprintf(bptr, M_FREEROOM(m),
-                                 "DCC CHAT chat %lu %u%c\n",
-                                 (unsigned long)ntohl(so->so_faddr.s_addr),
-                                 ntohs(so->so_fport), 1);
+            m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
+                                  "DCC CHAT chat %lu %u%c\n",
+                                  (unsigned long)ntohl(so->so_faddr.s_addr),
+                                  ntohs(so->so_fport), 1);
         } else if (sscanf(bptr, "DCC SEND %256s %u %u %u", buff, &laddr, &lport,
                           &n1) == 4) {
             if ((so = tcp_listen(slirp, INADDR_ANY, 0, htonl(laddr),
@@ -776,10 +774,10 @@ int tcp_emu(struct socket *so, struct mbuf *m)
                 return 1;
             }
             m->m_len = bptr - m->m_data; /* Adjust length */
-            m->m_len += snprintf(bptr, M_FREEROOM(m),
-                         "DCC SEND %s %lu %u %u%c\n", buff,
-                         (unsigned long)ntohl(so->so_faddr.s_addr),
-                         ntohs(so->so_fport), n1, 1);
+            m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
+                                  "DCC SEND %s %lu %u %u%c\n", buff,
+                                  (unsigned long)ntohl(so->so_faddr.s_addr),
+                                  ntohs(so->so_fport), n1, 1);
         } else if (sscanf(bptr, "DCC MOVE %256s %u %u %u", buff, &laddr, &lport,
                           &n1) == 4) {
             if ((so = tcp_listen(slirp, INADDR_ANY, 0, htonl(laddr),
@@ -787,10 +785,10 @@ int tcp_emu(struct socket *so, struct mbuf *m)
                 return 1;
             }
             m->m_len = bptr - m->m_data; /* Adjust length */
-            m->m_len += snprintf(bptr, M_FREEROOM(m),
-                         "DCC MOVE %s %lu %u %u%c\n", buff,
-                         (unsigned long)ntohl(so->so_faddr.s_addr),
-                         ntohs(so->so_fport), n1, 1);
+            m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
+                                  "DCC MOVE %s %lu %u %u%c\n", buff,
+                                  (unsigned long)ntohl(so->so_faddr.s_addr),
+                                  ntohs(so->so_fport), n1, 1);
         }
         return 1;