Add new functions for pretty-printing an OpenFlow message to a string.

This makes it possible to send pretty-printed versions of OpenFlow
messages to vlog.

diff --git a/include/dynamic-string.h b/include/dynamic-string.h
index c23dfb7e732829db5fbedcdd8ff3d2702874e30b..e43d006aa6f8013686f04bb091f7712df2fd9741 100644 (file)
--- a/include/dynamic-string.h
+++ b/include/dynamic-string.h
@@ -35,7 +35,9 @@
 #define DYNAMIC_STRING_H 1
 
 #include <stdarg.h>
+#include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 #include "compiler.h"
 
 struct ds {
@@ -48,9 +50,13 @@ struct ds {
 
 void ds_init(struct ds *);
 void ds_reserve(struct ds *, size_t min_length);
+void ds_put_char(struct ds *, char);
+void ds_put_cstr(struct ds *, const char *);
 void ds_put_format(struct ds *, const char *, ...) PRINTF_FORMAT(2, 3);
 void ds_put_format_valist(struct ds *, const char *, va_list)
     PRINTF_FORMAT(2, 0);
+void ds_put_hex_dump(struct ds *ds, const void *buf_, size_t size,
+                     uintptr_t ofs, bool ascii);
 char *ds_cstr(struct ds *);
 void ds_destroy(struct ds *);
 

diff --git a/include/ofp-print.h b/include/ofp-print.h
index 36981decdd7b4ba82315ff0ba01fe993a7284842..167809ad3178c6ddc3e7b4ebbeae7069e5420a08 100644 (file)
--- a/include/ofp-print.h
+++ b/include/ofp-print.h
@@ -47,11 +47,11 @@ extern "C" {
 
 void ofp_print(FILE *, const void *, size_t, int verbosity);
 void ofp_print_table(FILE *stream, const struct ofp_table* ot);
-void ofp_print_flow_mod(FILE *stream, const void *data, size_t len, int verbosity);
-void ofp_print_flow_expired(FILE *stream, const void *data, size_t len, int verbosity);
-void ofp_print_data_hello(FILE *stream, const void *data, size_t len, int verbosity);
 void ofp_print_packet(FILE *stream, const void *data, size_t len, size_t total_len);
-void ofp_print_port_status(FILE *stream, const void *oh, size_t len, int verbosity);
+
+char *ofp_to_string(const void *, size_t, int verbosity);
+char *ofp_table_to_string(const struct ofp_table* ot);
+char *ofp_packet_to_string(const void *data, size_t len, size_t total_len);
 
 #ifdef  __cplusplus
 }

diff --git a/lib/dpif.c b/lib/dpif.c
index b36df5f48e615d526237732e5e5ce1449dadb4dc..791c4bec0ef8d6f157714c9d0506922124726a40 100644 (file)
--- a/lib/dpif.c
+++ b/lib/dpif.c
@@ -300,7 +300,7 @@ dpif_show(struct dpif *dp)
     }
 
     len = nl_attr_get_size(attrs[DP_GENL_A_DP_INFO]);
-    ofp_print_data_hello(stdout, odh, len, 1);
+    ofp_print(stdout, odh, len, 1);
 
     return retval;
 }
@@ -431,8 +431,7 @@ dpif_dump_flows(struct dpif *dp, int table, struct ofp_match *match)
                 return EPROTO;
             }
 
-            ofp_print_flow_mod(stdout, &ofm->ofm, 
-                    sizeof(struct ofp_flow_mod), 1);
+            ofp_print(stdout, &ofm->ofm, sizeof(struct ofp_flow_mod), 1);
             putc('\n', stdout);
         }
 

diff --git a/lib/dynamic-string.c b/lib/dynamic-string.c
index 01d63fe5a3f36f9972475783dca6bd01ea330c89..68dd341912a4e1eba6ac550ee5e1052d5dd41f59 100644 (file)
--- a/lib/dynamic-string.c
+++ b/lib/dynamic-string.c
@@ -34,6 +34,7 @@
 #include "dynamic-string.h"
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 #include "util.h"
 
 void
@@ -54,6 +55,23 @@ ds_reserve(struct ds *ds, size_t min_length)
     }
 }
 
+void
+ds_put_char(struct ds *ds, char c)
+{
+    ds_reserve(ds, ds->length + 1);
+    ds->string[ds->length++] = c;
+    ds->string[ds->length] = '\0';
+}
+
+void
+ds_put_cstr(struct ds *ds, const char *s)
+{
+    size_t s_len = strlen(s);
+    ds_reserve(ds, ds->length + s_len);
+    memcpy(&ds->string[ds->length], s, s_len + 1);
+    ds->length += s_len;
+}
+
 void
 ds_put_format(struct ds *ds, const char *format, ...)
 {
@@ -108,3 +126,56 @@ ds_destroy(struct ds *ds)
 {
     free(ds->string);
 }
+
+/* Writes the 'size' bytes in 'buf' to 'string' as hex bytes arranged 16 per
+ * line.  Numeric offsets are also included, starting at 'ofs' for the first
+ * byte in 'buf'.  If 'ascii' is true then the corresponding ASCII characters
+ * are also rendered alongside. */
+void
+ds_put_hex_dump(struct ds *ds, const void *buf_, size_t size,
+                uintptr_t ofs, bool ascii)
+{
+  const uint8_t *buf = buf_;
+  const size_t per_line = 16; /* Maximum bytes per line. */
+
+  while (size > 0)
+    {
+      size_t start, end, n;
+      size_t i;
+
+      /* Number of bytes on this line. */
+      start = ofs % per_line;
+      end = per_line;
+      if (end - start > size)
+        end = start + size;
+      n = end - start;
+
+      /* Print line. */
+      ds_put_format(ds, "%08jx  ", (uintmax_t) ROUND_DOWN(ofs, per_line));
+      for (i = 0; i < start; i++)
+        ds_put_format(ds, "   ");
+      for (; i < end; i++)
+        ds_put_format(ds, "%02hhx%c",
+                buf[i - start], i == per_line / 2 - 1? '-' : ' ');
+      if (ascii)
+        {
+          for (; i < per_line; i++)
+            ds_put_format(ds, "   ");
+          ds_put_format(ds, "|");
+          for (i = 0; i < start; i++)
+            ds_put_format(ds, " ");
+          for (; i < end; i++) {
+              int c = buf[i - start];
+              ds_put_char(ds, c >= 32 && c < 127 ? c : '.');
+          }
+          for (; i < per_line; i++)
+            ds_put_format(ds, " ");
+          ds_put_format(ds, "|");
+        }
+      ds_put_format(ds, "\n");
+
+      ofs += n;
+      buf += n;
+      size -= n;
+    }
+}

diff --git a/lib/ofp-print.c b/lib/ofp-print.c
index 8ee1af58704253df51d04f259c50c139458f4bd2..98190a4f6539db1598322ee716f8191b5e78239d 100644 (file)
--- a/lib/ofp-print.c
+++ b/lib/ofp-print.c
@@ -43,17 +43,21 @@
 #include <ctype.h>
 
 #include "compiler.h"
+#include "dynamic-string.h"
 #include "util.h"
 #include "openflow.h"
 #include "packets.h"
 
-/* Dumps the contents of the Ethernet frame in the 'len' bytes starting at
- * 'data' to 'stream' using tcpdump.  'total_len' specifies the full length of
- * the Ethernet frame (of which 'len' bytes were captured).
+/* Returns a string that represents the contents of the Ethernet frame in the
+ * 'len' bytes starting at 'data' to 'stream' as output by tcpdump.
+ * 'total_len' specifies the full length of the Ethernet frame (of which 'len'
+ * bytes were captured).
+ *
+ * The caller must free the returned string.
  *
  * This starts and kills a tcpdump subprocess so it's quite expensive. */
-void ofp_print_packet(FILE *stream, const void *data, size_t len,
-                     size_t total_len)
+char *
+ofp_packet_to_string(const void *data, size_t len, size_t total_len)
 {
     struct pcap_hdr {
         uint32_t magic_number;   /* magic number */
@@ -75,17 +79,18 @@ void ofp_print_packet(FILE *stream, const void *data, size_t len,
     struct pcap_hdr ph;
     struct pcaprec_hdr prh;
 
+    struct ds ds = DS_EMPTY_INITIALIZER;
+
     char command[128];
+    FILE *pcap;
     FILE *tcpdump;
     int status;
+    int c;
 
-    fflush(stream);
-    snprintf(command, sizeof command, "tcpdump -n -r - %d>&1 2>/dev/null",
-             fileno(stream));
-    tcpdump = popen(command, "w");
-    if (!tcpdump) {
-        error(errno, "exec(\"%s\")", command);
-        return;
+    pcap = tmpfile();
+    if (!pcap) {
+        error(errno, "tmpfile");
+        return xstrdup("<error>");
     }
 
     /* The pcap reader is responsible for figuring out endianness based on the
@@ -103,13 +108,28 @@ void ofp_print_packet(FILE *stream, const void *data, size_t len,
     prh.incl_len = len;
     prh.orig_len = total_len;
 
-    fwrite(&ph, 1, sizeof ph, tcpdump);
-    fwrite(&prh, 1, sizeof prh, tcpdump);
-    fwrite(data, 1, len, tcpdump);
+    fwrite(&ph, 1, sizeof ph, pcap);
+    fwrite(&prh, 1, sizeof prh, pcap);
+    fwrite(data, 1, len, pcap);
+
+    fflush(pcap);
+    if (ferror(pcap)) {
+        error(errno, "error writing temporary file");
+    }
+    rewind(pcap);
+
+    snprintf(command, sizeof command, "tcpdump -n -r /dev/fd/%d 2>/dev/null",
+             fileno(pcap));
+    tcpdump = popen(command, "r");
+    fclose(pcap);
+    if (!tcpdump) {
+        error(errno, "exec(\"%s\")", command);
+        return xstrdup("<error>");
+    }
 
-    fflush(tcpdump);
-    if (ferror(tcpdump))
-        error(errno, "error writing \"%s\" subprocess", command);
+    while ((c = getc(tcpdump)) != EOF) {
+        ds_put_char(&ds, c);
+    }
 
     status = pclose(tcpdump);
     if (WIFEXITED(status)) {
@@ -118,116 +138,124 @@ void ofp_print_packet(FILE *stream, const void *data, size_t len,
     } else if (WIFSIGNALED(status)) {
         error(0, "tcpdump exited with signal %d", WTERMSIG(status)); 
     }
+    return ds_cstr(&ds);
 }
 
 /* Pretty-print the OFPT_PACKET_IN packet of 'len' bytes at 'oh' to 'stream'
  * at the given 'verbosity' level. */
-static void ofp_packet_in(FILE *stream, const void *oh, size_t len,
-                            int verbosity)
+static void
+ofp_packet_in(struct ds *string, const void *oh, size_t len, int verbosity)
 {
     const struct ofp_packet_in *op = oh;
     size_t data_len;
 
-    fprintf(stream, " total_len=%"PRIu16" in_port=%"PRIu8,
+    ds_put_format(string, " total_len=%"PRIu16" in_port=%"PRIu8,
             ntohs(op->total_len), ntohs(op->in_port));
 
     if (op->reason == OFPR_ACTION)
-        fputs(" (via action)", stream);
+        ds_put_cstr(string, " (via action)");
     else if (op->reason != OFPR_NO_MATCH)
-        fprintf(stream, " (***reason %"PRIu8"***)", op->reason);
+        ds_put_format(string, " (***reason %"PRIu8"***)", op->reason);
 
     data_len = len - offsetof(struct ofp_packet_in, data);
-    fprintf(stream, " data_len=%zu", data_len);
+    ds_put_format(string, " data_len=%zu", data_len);
     if (htonl(op->buffer_id) == UINT32_MAX) {
-        fprintf(stream, " (unbuffered)");
+        ds_put_format(string, " (unbuffered)");
         if (ntohs(op->total_len) != data_len)
-            fprintf(stream, " (***total_len != data_len***)");
+            ds_put_format(string, " (***total_len != data_len***)");
     } else {
-        fprintf(stream, " buffer=%08"PRIx32, ntohl(op->buffer_id));
+        ds_put_format(string, " buffer=%08"PRIx32, ntohl(op->buffer_id));
         if (ntohs(op->total_len) < data_len)
-            fprintf(stream, " (***total_len < data_len***)");
+            ds_put_format(string, " (***total_len < data_len***)");
     }
-    putc('\n', stream);
+    ds_put_char(string, '\n');
 
-    if (verbosity > 0)
-        ofp_print_packet(stream, op->data, data_len, ntohs(op->total_len));
+    if (verbosity > 0) {
+        char *packet = ofp_packet_to_string(op->data, data_len,
+                                            ntohs(op->total_len)); 
+        ds_put_cstr(string, packet);
+        free(packet);
+    }
 }
 
-static void ofp_print_port_name(FILE *stream, uint16_t port) 
+static void ofp_print_port_name(struct ds *string, uint16_t port) 
 {
     if (port == UINT16_MAX) {
-        fputs("none", stream);
+        ds_put_cstr(string, "none");
     } else if (port == OFPP_FLOOD) {
-        fputs("flood", stream);
+        ds_put_cstr(string, "flood");
     } else if (port == OFPP_CONTROLLER) {
-        fputs("controller", stream);
+        ds_put_cstr(string, "controller");
     } else {
-        fprintf(stream, "%"PRIu16, port);
+        ds_put_format(string, "%"PRIu16, port);
     }
 }
 
-static void ofp_print_action(FILE *stream, const struct ofp_action *a) 
+static void
+ofp_print_action(struct ds *string, const struct ofp_action *a) 
 {
     switch (ntohs(a->type)) {
     case OFPAT_OUTPUT:
-        fputs("output(", stream);
-        ofp_print_port_name(stream, ntohs(a->arg.output.port));
+        ds_put_cstr(string, "output(");
+        ofp_print_port_name(string, ntohs(a->arg.output.port));
         if (a->arg.output.port == htons(OFPP_CONTROLLER)) {
-            fprintf(stream, ", max %"PRIu16" bytes", ntohs(a->arg.output.max_len));
+            ds_put_format(string, ", max %"PRIu16" bytes", ntohs(a->arg.output.max_len));
         }
-        fputs(")", stream);
+        ds_put_cstr(string, ")");
         break;
 
     default:
-        fprintf(stream, "(decoder %"PRIu16" not implemented)", ntohs(a->type));
+        ds_put_format(string, "(decoder %"PRIu16" not implemented)", ntohs(a->type));
         break;
     }
 }
 
-static void ofp_print_actions(FILE *stream,
-                                const struct ofp_action actions[],
-                                size_t n_bytes) 
+static void ofp_print_actions(struct ds *string,
+                              const struct ofp_action actions[],
+                              size_t n_bytes) 
 {
     size_t i;
 
-    fputs(" actions[", stream);
+    ds_put_cstr(string, " actions[");
     for (i = 0; i < n_bytes / sizeof *actions; i++) {
         if (i) {
-            fputs("; ", stream);
+            ds_put_cstr(string, "; ");
         }
-        ofp_print_action(stream, &actions[i]);
+        ofp_print_action(string, &actions[i]);
     }
     if (n_bytes % sizeof *actions) {
         if (i) {
-            fputs("; ", stream);
+            ds_put_cstr(string, "; ");
         }
-        fputs("; ***trailing garbage***", stream);
+        ds_put_cstr(string, "; ***trailing garbage***");
     }
-    fputs("]", stream);
+    ds_put_cstr(string, "]");
 }
 
-/* Pretty-print the OFPT_PACKET_OUT packet of 'len' bytes at 'oh' to 'stream'
+/* Pretty-print the OFPT_PACKET_OUT packet of 'len' bytes at 'oh' to 'string'
  * at the given 'verbosity' level. */
-static void ofp_packet_out(FILE *stream, const void *oh, size_t len,
-                            int verbosity) 
+static void ofp_packet_out(struct ds *string, const void *oh, size_t len,
+                           int verbosity) 
 {
     const struct ofp_packet_out *opo = oh;
 
-    fputs(" in_port=", stream);
-    ofp_print_port_name(stream, ntohs(opo->in_port));
+    ds_put_cstr(string, " in_port=");
+    ofp_print_port_name(string, ntohs(opo->in_port));
 
     if (ntohl(opo->buffer_id) == UINT32_MAX) {
-        fputs(" out_port=", stream);
-        ofp_print_port_name(stream, ntohs(opo->out_port));
+        ds_put_cstr(string, " out_port=");
+        ofp_print_port_name(string, ntohs(opo->out_port));
         if (verbosity > 0 && len > sizeof *opo) {
-            ofp_print_packet(stream, opo->u.data, len - sizeof *opo,
-                               len - sizeof *opo);
+            char *packet = ofp_packet_to_string(opo->u.data, len - sizeof *opo,
+                                                len - sizeof *opo);
+            ds_put_cstr(string, packet);
+            free(packet);
         }
     } else {
-        fprintf(stream, " buffer=%08"PRIx32, ntohl(opo->buffer_id));
-        ofp_print_actions(stream, opo->u.actions, len - sizeof *opo);
+        ds_put_format(string, " buffer=%08"PRIx32, ntohl(opo->buffer_id));
+        ofp_print_actions(string, opo->u.actions, len - sizeof *opo);
     }
-    putc('\n', stream);
+    ds_put_char(string, '\n');
 }
 
 /* qsort comparison function. */
@@ -242,8 +270,8 @@ compare_ports(const void *a_, const void *b_)
     return ap < bp ? -1 : ap > bp;
 }
 
-static
-void ofp_print_phy_port(FILE *stream, const struct ofp_phy_port *port)
+static void
+ofp_print_phy_port(struct ds *string, const struct ofp_phy_port *port)
 {
     uint8_t name[OFP_MAX_PORT_NAME_LEN];
     int j;
@@ -256,7 +284,7 @@ void ofp_print_phy_port(FILE *stream, const struct ofp_phy_port *port)
     }
     name[j] = '\0';
 
-    fprintf(stream, " %2d(%s): addr:"ETH_ADDR_FMT", speed:%d, flags:%#x, "
+    ds_put_format(string, " %2d(%s): addr:"ETH_ADDR_FMT", speed:%d, flags:%#x, "
             "feat:%#x\n", ntohs(port->port_no), name, 
             ETH_ADDR_ARGS(port->hw_addr), ntohl(port->speed),
             ntohl(port->flags), ntohl(port->features));
@@ -264,7 +292,7 @@ void ofp_print_phy_port(FILE *stream, const struct ofp_phy_port *port)
 
 /* Pretty-print the OFPT_DATA_HELLO packet of 'len' bytes at 'oh' to 'stream'
  * at the given 'verbosity' level. */
-void ofp_print_data_hello(FILE *stream, const void *oh, size_t len, 
+void ofp_print_data_hello(struct ds *string, const void *oh, size_t len, 
         int verbosity)
 {
     const struct ofp_data_hello *odh = oh;
@@ -273,14 +301,14 @@ void ofp_print_data_hello(FILE *stream, const void *oh, size_t len,
     int i;
 
 
-    fprintf(stream, "dp id:%"PRIx64"\n", ntohll(odh->datapath_id));
-    fprintf(stream, "tables: exact:%d, mac:%d, compressed:%d, general:%d\n",
+    ds_put_format(string, "dp id:%"PRIx64"\n", ntohll(odh->datapath_id));
+    ds_put_format(string, "tables: exact:%d, mac:%d, compressed:%d, general:%d\n",
            ntohl(odh->n_exact), ntohl(odh->n_mac_only),
            ntohl(odh->n_compression), ntohl(odh->n_general));
-    fprintf(stream, "buffers: size:%d, number:%d, miss_len:%d\n",
+    ds_put_format(string, "buffers: size:%d, number:%d, miss_len:%d\n",
            ntohl(odh->buffer_mb), ntohl(odh->n_buffers),
            ntohs(odh->miss_send_len));
-    fprintf(stream, "features: capabilities:%#x, actions:%#x\n",
+    ds_put_format(string, "features: capabilities:%#x, actions:%#x\n",
            ntohl(odh->capabilities), ntohl(odh->actions));
 
     if (ntohs(odh->header.length) >= sizeof *odh) {
@@ -291,30 +319,30 @@ void ofp_print_data_hello(FILE *stream, const void *oh, size_t len,
     memcpy(port_list, odh->ports, (len - sizeof *odh));
     qsort(port_list, n_ports, sizeof port_list[0], compare_ports);
     for (i = 0; i < n_ports; i++) {
-        ofp_print_phy_port(stream, &port_list[i]);
+        ofp_print_phy_port(string, &port_list[i]);
     }
 }
 
-static void print_wild(FILE *stream, const char *leader, int is_wild,
+static void print_wild(struct ds *string, const char *leader, int is_wild,
             const char *format, ...) __attribute__((format(printf, 4, 5)));
 
-static void print_wild(FILE *stream, const char *leader, int is_wild,
+static void print_wild(struct ds *string, const char *leader, int is_wild,
                        const char *format, ...) 
 {
-    fputs(leader, stream);
+    ds_put_cstr(string, leader);
     if (!is_wild) {
         va_list args;
 
         va_start(args, format);
-        vfprintf(stream, format, args);
+        ds_put_format_valist(string, format, args);
         va_end(args);
     } else {
-        putc('?', stream);
+        ds_put_char(string, '?');
     }
 }
 
 /* Pretty-print the ofp_match structure */
-static void ofp_print_match(FILE *f, const struct ofp_match *om)
+static void ofp_print_match(struct ds *f, const struct ofp_match *om)
 {
     uint16_t w = ntohs(om->wildcards);
 
@@ -330,58 +358,59 @@ static void ofp_print_match(FILE *f, const struct ofp_match *om)
     print_wild(f, "] proto", w & OFPFW_NW_PROTO, "%u", om->nw_proto);
     print_wild(f, " tport[", w & OFPFW_TP_SRC, "%d", ntohs(om->tp_src));
     print_wild(f, "->", w & OFPFW_TP_DST, "%d", ntohs(om->tp_dst));
-    fputs("]\n", f);
+    ds_put_cstr(f, "]\n");
 }
 
-/* Pretty-print the OFPT_FLOW_MOD packet of 'len' bytes at 'oh' to 'stream'
+/* Pretty-print the OFPT_FLOW_MOD packet of 'len' bytes at 'oh' to 'string'
  * at the given 'verbosity' level. */
-void ofp_print_flow_mod(FILE *stream, const void *oh, size_t len, 
-        int verbosity)
+static void
+ofp_print_flow_mod(struct ds *string, const void *oh, size_t len, 
+                   int verbosity)
 {
     const struct ofp_flow_mod *ofm = oh;
 
-    ofp_print_match(stream, &ofm->match);
-    fprintf(stream, " cmd:%d idle:%d buf:%#x grp:%d\n", ntohs(ofm->command),
+    ofp_print_match(string, &ofm->match);
+    ds_put_format(string, " cmd:%d idle:%d buf:%#x grp:%d\n", ntohs(ofm->command),
          ntohs(ofm->max_idle), ntohl(ofm->buffer_id), ntohl(ofm->group_id));
 }
 
-/* Pretty-print the OFPT_FLOW_EXPIRED packet of 'len' bytes at 'oh' to 'stream'
+/* Pretty-print the OFPT_FLOW_EXPIRED packet of 'len' bytes at 'oh' to 'string'
  * at the given 'verbosity' level. */
-void ofp_print_flow_expired(FILE *stream, const void *oh, size_t len, 
+void ofp_print_flow_expired(struct ds *string, const void *oh, size_t len, 
         int verbosity)
 {
     const struct ofp_flow_expired *ofe = oh;
 
-    ofp_print_match(stream, &ofe->match);
-    fprintf(stream, 
+    ofp_print_match(string, &ofe->match);
+    ds_put_format(string, 
          " secs%d pkts%lld bytes%lld\n", ntohl(ofe->duration),
          ntohll(ofe->packet_count), ntohll(ofe->byte_count));
 }
 
-/* Pretty-print the OFPT_PORT_STATUS packet of 'len' bytes at 'oh' to 'stream'
+/* Pretty-print the OFPT_PORT_STATUS packet of 'len' bytes at 'oh' to 'string'
  * at the given 'verbosity' level. */
-void ofp_print_port_status(FILE *stream, const void *oh, size_t len, 
+void ofp_print_port_status(struct ds *string, const void *oh, size_t len, 
         int verbosity)
 {
     const struct ofp_port_status *ops = oh;
 
     if (ops->reason == OFPPR_ADD) {
-        fprintf(stream, "add:");
+        ds_put_format(string, "add:");
     } else if (ops->reason == OFPPR_DELETE) {
-        fprintf(stream, "del:");
+        ds_put_format(string, "del:");
     } else if (ops->reason == OFPPR_MOD) {
-        fprintf(stream, "mod:");
+        ds_put_format(string, "mod:");
     } else {
-        fprintf(stream, "err:");
+        ds_put_format(string, "err:");
     }
 
-    ofp_print_phy_port(stream, &ops->desc);
+    ofp_print_phy_port(string, &ops->desc);
 }
 
 struct openflow_packet {
     const char *name;
     size_t min_size;
-    void (*printer)(FILE *, const void *, size_t len, int verbosity);
+    void (*printer)(struct ds *, const void *, size_t len, int verbosity);
 };
 
 static const struct openflow_packet packets[] = {
@@ -427,58 +456,97 @@ static const struct openflow_packet packets[] = {
     },
 };
 
-/* Pretty-print the OpenFlow packet of 'len' bytes at 'oh' to 'stream' at the
- * given 'verbosity' level.  0 is a minimal amount of verbosity and higher
- * numbers increase verbosity. */
-void ofp_print(FILE *stream, const void *oh_, size_t len, int verbosity)
+/* Composes and returns a string representing the OpenFlow packet of 'len'
+ * bytes at 'oh' at the given 'verbosity' level.  0 is a minimal amount of
+ * verbosity and higher numbers increase verbosity.  The caller is responsible
+ * for freeing the string. */
+char *
+ofp_to_string(const void *oh_, size_t len, int verbosity)
 {
+    struct ds string = DS_EMPTY_INITIALIZER;
     const struct ofp_header *oh = oh_;
     const struct openflow_packet *pkt;
 
     if (len < sizeof(struct ofp_header)) {
-        fprintf(stream, "OpenFlow packet too short:\n");
-        hex_dump(stream, oh, len, 0, true);
-        return;
+        ds_put_cstr(&string, "OpenFlow packet too short:\n");
+        ds_put_hex_dump(&string, oh, len, 0, true);
+        return ds_cstr(&string);
     } else if (oh->version != 1) {
-        fprintf(stream, "Bad OpenFlow version %"PRIu8":\n", oh->version);
-        hex_dump(stream, oh, len, 0, true);
-        return;
+        ds_put_format(&string, "Bad OpenFlow version %"PRIu8":\n", oh->version);
+        ds_put_hex_dump(&string, oh, len, 0, true);
+        return ds_cstr(&string);
     } else if (oh->type >= ARRAY_SIZE(packets) || !packets[oh->type].name) {
-        fprintf(stream, "Unknown OpenFlow packet type %"PRIu8":\n",
+        ds_put_format(&string, "Unknown OpenFlow packet type %"PRIu8":\n",
                 oh->type);
-        hex_dump(stream, oh, len, 0, true);
-        return;
+        ds_put_hex_dump(&string, oh, len, 0, true);
+        return ds_cstr(&string);
     }
 
     pkt = &packets[oh->type];
-    fprintf(stream, "%s (xid=%"PRIx32"):", pkt->name, oh->xid);
+    ds_put_format(&string, "%s (xid=%"PRIx32"):", pkt->name, oh->xid);
 
     if (ntohs(oh->length) > len)
-        fprintf(stream, " (***truncated to %zu bytes from %"PRIu16"***)",
+        ds_put_format(&string, " (***truncated to %zu bytes from %"PRIu16"***)",
                 len, ntohs(oh->length));
     else if (ntohs(oh->length) < len) {
-        fprintf(stream, " (***only uses %"PRIu16" bytes out of %zu***)\n",
+        ds_put_format(&string, " (***only uses %"PRIu16" bytes out of %zu***)\n",
                 ntohs(oh->length), len);
         len = ntohs(oh->length);
     }
 
     if (len < pkt->min_size) {
-        fprintf(stream, " (***length=%zu < min_size=%zu***)\n",
+        ds_put_format(&string, " (***length=%zu < min_size=%zu***)\n",
                 len, pkt->min_size);
     } else if (!pkt->printer) {
-        fprintf(stream, " length=%zu (decoder not implemented)\n",
+        ds_put_format(&string, " length=%zu (decoder not implemented)\n",
                 ntohs(oh->length));
     } else {
-        pkt->printer(stream, oh, len, verbosity);
+        pkt->printer(&string, oh, len, verbosity);
+    }
+    if (verbosity >= 3) {
+        ds_put_hex_dump(&string, oh, len, 0, true);
     }
-    if (verbosity >= 3)
-        hex_dump(stream, oh, len, 0, true);
+    return ds_cstr(&string);
+}
+
+char *
+ofp_table_to_string(const struct ofp_table* ot)
+{
+    return xasprintf("id: %d name: %-8s n_flows: %6d max_flows: %6d",
+                     ntohs(ot->table_id), ot->name, ntohl(ot->n_flows),
+                     ntohl(ot->max_flows));
+}
+\f
+static void
+print_and_free(FILE *stream, char *string) 
+{
+    fputs(string, stream);
+    free(string);
+}
+
+/* Pretty-print the OpenFlow packet of 'len' bytes at 'oh' to 'stream' at the
+ * given 'verbosity' level.  0 is a minimal amount of verbosity and higher
+ * numbers increase verbosity. */
+void
+ofp_print(FILE *stream, const void *oh, size_t len, int verbosity)
+{
+    print_and_free(stream, ofp_to_string(oh, len, verbosity));
 }
 
 /* Pretty print a openflow table */
-void ofp_print_table(FILE *stream, const struct ofp_table* ot)
+void
+ofp_print_table(FILE *stream, const struct ofp_table *ot)
+{
+    print_and_free(stream, ofp_table_to_string(ot));
+}
+
+/* Dumps the contents of the Ethernet frame in the 'len' bytes starting at
+ * 'data' to 'stream' using tcpdump.  'total_len' specifies the full length of
+ * the Ethernet frame (of which 'len' bytes were captured).
+ *
+ * This starts and kills a tcpdump subprocess so it's quite expensive. */
+void
+ofp_print_packet(FILE *stream, const void *data, size_t len, size_t total_len)
 {
-    fprintf(stream, "id: %d name: %-8s n_flows: %6d max_flows: %6d",
-            ntohs(ot->table_id), ot->name, ntohl(ot->n_flows),
-            ntohl(ot->max_flows));
+    print_and_free(stream, ofp_packet_to_string(data, len, total_len));
 }