diff --git a/CHANGES b/CHANGES
index 96c3f31..d0fe646 100644
--- a/CHANGES
+++ b/CHANGES
@@ -29,6 +29,7 @@
   if SOCK_NONBLOCK is defined, use it instead of socket+fcntl
   ... but if errno==EINVAL still fall back to socket+fcntl (Robert Henney)
   SECURITY: fix botched integer overflow handling logic in stralloc_ready (Giorgio)
+  add critbit
 
 0.29:
   save 8 bytes in taia.h for 64-bit systems
diff --git a/GNUmakefile b/GNUmakefile
index 73cd05a..9a06b04 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -11,7 +11,7 @@ MAN3DIR=${prefix}/man/man3
 
 LIBS=byte.a fmt.a scan.a str.a uint.a open.a stralloc.a unix.a socket.a \
 buffer.a mmap.a taia.a tai.a dns.a case.a mult.a array.a io.a \
-textcode.a cdb.a
+textcode.a cdb.a critbit.a
 
 all: ent $(LIBS) libowfat.a libsocket t
 
@@ -58,7 +58,7 @@ endif
 # to build without diet libc support, use $ make DIET=
 # see http://www.fefe.de/dietlibc/ for details about the diet libc
 
-VPATH=str:byte:fmt:scan:uint:open:stralloc:unix:socket:buffer:mmap:textcode:taia:tai:dns:case:array:mult:io:cdb
+VPATH=str:byte:fmt:scan:uint:open:stralloc:unix:socket:buffer:mmap:textcode:taia:tai:dns:case:array:mult:io:cdb:critbit
 
 BYTE_OBJS=$(patsubst byte/%.c,%.o,$(wildcard byte/*.c))
 FMT_OBJS=$(patsubst fmt/%.c,%.o,$(wildcard fmt/*.c))
@@ -80,6 +80,7 @@ ARRAY_OBJS=$(patsubst array/%.c,%.o,$(wildcard array/*.c))
 MULT_OBJS=$(patsubst mult/%.c,%.o,$(wildcard mult/*.c))
 IO_OBJS=$(patsubst io/%.c,%.o,$(wildcard io/*.c))
 CDB_OBJS=$(patsubst cdb/%.c,%.o,$(wildcard cdb/*.c))
+CRITBIT_OBJS=$(patsubst critbit/%.c,%.o,$(wildcard critbit/*.c))
 
 $(BYTE_OBJS): byte.h
 $(FMT_OBJS): fmt.h
@@ -99,6 +100,7 @@ $(ARRAY_OBJS): uint64.h array.h
 $(MULT_OBJS): uint64.h uint32.h uint16.h safemult.h
 $(IO_OBJS): uint64.h array.h io.h io_internal.h taia.h tai.h haveepoll.h havekqueue.h havesigio.h havebsdsf.h havedevpoll.h havesendfile.h
 $(CDB_OBJS): cdb.h uint32.h
+$(CRITBIT_OBJS): critbit.h
 
 mult64.o: haveuint128.h
 
@@ -133,12 +135,13 @@ array.a: $(ARRAY_OBJS)
 mult.a: $(MULT_OBJS)
 io.a: $(IO_OBJS)
 cdb.a: $(CDB_OBJS)
+critbit.a: $(CRITBIT_OBJS)
 
 ALL_OBJS=$(DNS_OBJS) $(BYTE_OBJS) $(FMT_OBJS) $(SCAN_OBJS) \
 $(STR_OBJS) $(UINT_OBJS) $(OPEN_OBJS) $(STRALLOC_OBJS) $(UNIX_OBJS) \
 $(SOCKET_OBJS) $(BUFFER_OBJS) $(MMAP_OBJS) $(TEXTCODE_OBJS) \
 $(TAIA_OBJS) $(TAI_OBJS) $(CASE_OBJS) $(ARRAY_OBJS) $(MULT_OBJS) \
-$(IO_OBJS) $(CDB_OBJS)
+$(IO_OBJS) $(CDB_OBJS) $(CRITBIT_OBJS)
 
 libowfat.a: $(ALL_OBJS)
 	$(CROSS)ar cru $@ $(ALL_OBJS)
diff --git a/critbit.h b/critbit.h
new file mode 100644
index 0000000..11d096b
--- /dev/null
+++ b/critbit.h
@@ -0,0 +1,23 @@
+#ifndef CRITBIT_H_
+#define CRITBIT_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+  void *root;
+} critbit0_tree;
+
+int critbit0_contains(critbit0_tree *t, const char *u);
+int critbit0_insert(critbit0_tree *t, const char *u);
+int critbit0_delete(critbit0_tree *t, const char *u);
+void critbit0_clear(critbit0_tree *t);
+int critbit0_allprefixed(critbit0_tree *t, const char *prefix,
+                         int (*handle) (const char *, void *), void *arg);
+
+#ifdef __cplusplus
+};
+#endif
+
+#endif
diff --git a/critbit/critbit.c b/critbit/critbit.c
new file mode 100644
index 0000000..19442e3
--- /dev/null
+++ b/critbit/critbit.c
@@ -0,0 +1,227 @@
+#include <stddef.h>	/* size_t, uintptr_t */
+#include <stdint.h>	/* for uint8_t, uint32_t */
+
+#include <string.h>
+#include <stdlib.h>
+
+#include "critbit.h"
+
+typedef struct {
+  void* child[2];
+  uint32_t byte;
+  uint8_t otherbits;
+} critbit0_node;
+
+#if 0
+typedef struct{
+  void* root;
+} critbit0_tree;
+#endif
+
+int critbit0_contains(critbit0_tree* t,const char* u) {
+  const uint8_t* ubytes= (void*)u;
+  const size_t ulen= strlen(u);
+  uint8_t* p= t->root;
+
+  if (!p) return 0;
+
+  while ((uintptr_t)p & 1) {
+    critbit0_node* q = (void*)(p-1);
+
+    uint8_t c = 0;
+    if (q->byte<ulen)
+      c = ubytes[q->byte];
+
+    const int direction = (1+(q->otherbits|c))>>8;
+
+    p = q->child[direction];
+  }
+
+  return 0==strcmp(u,(const char*)p);
+}
+
+int critbit0_insert(critbit0_tree* t,const char* u) {
+  const uint8_t* const ubytes = (void*)u;
+  const size_t ulen = strlen(u);
+  uint8_t* p = t->root;
+
+  if (!p) {
+    char* x = malloc(ulen+1);
+    if (!x) return 0;
+    memcpy(x,u,ulen+1);
+    t->root= x;
+    return 2;
+  }
+
+  while (1&(intptr_t)p) {
+    critbit0_node* q = (void*)(p-1);
+
+    uint8_t c = 0;
+    if (q->byte<ulen)
+      c = ubytes[q->byte];
+    const int direction = (1+(q->otherbits|c))>>8;
+
+    p = q->child[direction];
+  }
+
+  uint32_t newbyte;
+  uint32_t newotherbits;
+
+  for (newbyte = 0; newbyte < ulen; ++newbyte) {
+    if (p[newbyte] != ubytes[newbyte]) {
+      newotherbits = p[newbyte]^ubytes[newbyte];
+      goto different_byte_found;
+    }
+  }
+
+  if (p[newbyte]!=0) {
+    newotherbits = p[newbyte];
+    goto different_byte_found;
+  }
+  return 1;
+
+different_byte_found:
+
+  newotherbits |= newotherbits>>1;
+  newotherbits |= newotherbits>>2;
+  newotherbits |= newotherbits>>4;
+  newotherbits = (newotherbits&~(newotherbits>>1))^255;
+  uint8_t c = p[newbyte];
+  int newdirection = (1+(newotherbits|c))>>8;
+
+  critbit0_node* newnode;
+  if (!(newnode=malloc(sizeof(critbit0_node))))
+    return 0;
+
+  char* x;
+  if (!(x = malloc(ulen+1))) {
+    free(newnode);
+    return 0;
+  }
+  memcpy(x,ubytes,ulen+1);
+
+  newnode->byte= newbyte;
+  newnode->otherbits= newotherbits;
+  newnode->child[1-newdirection]= x;
+
+  void** wherep= &t->root;
+  for(;;) {
+    uint8_t* p = *wherep;
+    if (!((intptr_t)p&1))
+      break;
+    critbit0_node* q = (void*)(p-1);
+    if (q->byte > newbyte)break;
+    if (q->byte==newbyte && q->otherbits>newotherbits)break;
+    uint8_t c = 0;
+    if (q->byte<ulen)
+      c = ubytes[q->byte];
+    const int direction = (1+(q->otherbits|c))>>8;
+    wherep = q->child+direction;
+  }
+
+  newnode->child[newdirection]= *wherep;
+  *wherep= (void*)(1+(char*)newnode);
+
+  return 2;
+}
+
+int critbit0_delete(critbit0_tree* t,const char* u) {
+  const uint8_t* ubytes = (void*)u;
+  const size_t ulen = strlen(u);
+  uint8_t* p = t->root;
+  void** wherep = &t->root;
+  void** whereq = 0;
+  critbit0_node* q = 0;
+  int direction = 0;
+
+  if (!p) return 0;
+
+  while ((intptr_t)p&1) {
+    whereq = wherep;
+    q = (void*)(p-1);
+    uint8_t c = 0;
+    if (q->byte<ulen)
+      c = ubytes[q->byte];
+    direction = (1+(q->otherbits|c))>>8;
+    wherep = q->child+direction;
+    p = *wherep;
+  }
+
+  if (0!=strcmp(u,(const char*)p))
+    return 0;
+  free(p);
+
+  if (!whereq) {
+    t->root = 0;
+    return 1;
+  }
+
+  *whereq = q->child[1-direction];
+  free(q);
+
+  return 1;
+}
+
+static void traverse(void* top) {
+  uint8_t* p = top;
+
+  if ((intptr_t)p&1) {
+    critbit0_node* q = (void*)(p-1);
+    traverse(q->child[0]);
+    traverse(q->child[1]);
+    free(q);
+  } else {
+    free(p);
+  }
+}
+
+void critbit0_clear(critbit0_tree* t) {
+  if (t->root)
+    traverse(t->root);
+  t->root = NULL;
+}
+
+static int allprefixed_traverse(uint8_t* top,int(*handle)(const char*,void*),void* arg) {
+  if ((uintptr_t)top&1) {
+    critbit0_node* q = (void*)(top-1);
+    int direction;
+    for (direction=0; direction<2; ++direction)
+      switch (allprefixed_traverse(q->child[direction],handle,arg)) {
+      case 1: break;
+      case 0: return 0;
+      default: return-1;
+      }
+    return 1;
+  }
+
+  return handle((const char*)top,arg);
+}
+
+int critbit0_allprefixed(critbit0_tree* t,const char* prefix,int(*handle)(const char*,void*),void* arg) {
+  const uint8_t* ubytes = (void*)prefix;
+  const size_t ulen = strlen(prefix);
+  uint8_t* p = t->root;
+  uint8_t* top = p;
+
+  if (!p) return 1;
+
+  while ((uintptr_t)p&1) {
+    critbit0_node* q = (void*)(p-1);
+    uint8_t c = 0;
+    if (q->byte<ulen)
+      c=ubytes[q->byte];
+    const int direction = (1+(q->otherbits|c))>>8;
+    p = q->child[direction];
+    if (q->byte<ulen)
+      top = p;
+  }
+
+  size_t i;
+  for (i=0; i<ulen; ++i) {
+    if (p[i]!=ubytes[i])
+      return 1;
+  }
+
+  return allprefixed_traverse(top,handle,arg);
+}
+
diff --git a/critbit/critbit0_allprefixed.3 b/critbit/critbit0_allprefixed.3
new file mode 100644
index 0000000..e095de8
--- /dev/null
+++ b/critbit/critbit0_allprefixed.3
@@ -0,0 +1,33 @@
+.TH critbit0_allprefixed 3
+.SH NAME
+critbit0_allprefixed \- search a critbit tree by prefix
+.SH SYNTAX
+.B #include <critbit.h>
+
+critbit0_tree cb = { 0 };
+
+int \fBcritbit0_allprefixed\fP(critbit0_tree* \fIcb\fR,const char* \fIprefix\fR,
+    int (*handle)(const char*,void*),void* arg);
+.SH DESCRIPTION
+critbit0_allprefixed calls the \fIhandle\fR function on all members of
+\fIcb\fR that start with \fIprefix\fR. It passes \fIarg\fR as second
+argument to \fIhandle\fR.
+
+If \fIhandle\fR returns 0, the search stops and critbit0_allprefixed
+returns 0.
+
+If \fIhandle\fR returns 1, the search continues and critbit0_allprefixed
+returns 1 if all keys with the given prefix were handled.
+
+If \fIhandle\fR returns something else, the search stops and
+critbit0_allprefixed returns -1.
+.SH "RETURN VALUE"
+1 if all keys were found and handled (or if there were no keys with that
+prefix).
+
+0 if at least one key was found, \fIhandle\fR was called and returned 0.
+
+-1 if at least one key was found, \fIhandle\fR was called and returned
+something other than 0 or 1.
+.SH "SEE ALSO"
+critbit0_contains(3)
diff --git a/critbit/critbit0_clear.3 b/critbit/critbit0_clear.3
new file mode 100644
index 0000000..486bbb7
--- /dev/null
+++ b/critbit/critbit0_clear.3
@@ -0,0 +1,16 @@
+.TH critbit0_clear 3
+.SH NAME
+critbit0_clear \- free all memory associated with a critbit tree
+.SH SYNTAX
+.B #include <critbit.h>
+
+critbit0_tree cb = { 0 };
+
+void \fBcritbit0_clear\fP(critbit0_tree* \fIcb\fR);
+.SH DESCRIPTION
+critbit0_clear deletes all keys in \fIcb\fR and frees all memory
+associated with it.
+.SH "RETURN VALUE"
+none.
+.SH "SEE ALSO"
+critbit0_delete(3)
diff --git a/critbit/critbit0_contains.3 b/critbit/critbit0_contains.3
new file mode 100644
index 0000000..a02185b
--- /dev/null
+++ b/critbit/critbit0_contains.3
@@ -0,0 +1,17 @@
+.TH critbit0_contains 3
+.SH NAME
+critbit0_contains \- check whether a string is in the critbit tree
+.SH SYNTAX
+.B #include <critbit.h>
+
+critbit0_tree cb = { 0 };
+
+int \fBcritbit0_contains\fP(critbit0_tree* \fIcb\fR,const char* \fIstr\fR);
+.SH DESCRIPTION
+critbit0_contains looks up the given string in the critbit0 tree.
+If \fIstr\fR is in \fIcb\fR, critbit0_contains returns 1. Otherwise it
+returns 0.
+.SH "RETURN VALUE"
+1 if the key was found, 0 otherwise.
+.SH "SEE ALSO"
+critbit0_insert(3)
diff --git a/critbit/critbit0_delete.3 b/critbit/critbit0_delete.3
new file mode 100644
index 0000000..48508a5
--- /dev/null
+++ b/critbit/critbit0_delete.3
@@ -0,0 +1,18 @@
+.TH critbit0_delete 3
+.SH NAME
+critbit0_delete \- delete a string from a critbit tree
+.SH SYNTAX
+.B #include <critbit.h>
+
+critbit0_tree cb = { 0 };
+
+int \fBcritbit0_delete\fP(critbit0_tree* \fIcb\fR,const char* \fIstr\fR);
+.SH DESCRIPTION
+critbit0_delete attempts to delete a string from a critbit0 tree.
+If \fIstr\fR is in \fIcb\fR, critbit0_delete removes it and returns 1.
+If \fIstr\fR is not in \fIcb\fR, critbit0_delete leaves it alone and
+returns 0.
+.SH "RETURN VALUE"
+1 if it was in cb and has now been removed, 0 if it was not in cb.
+.SH "SEE ALSO"
+critbit0_insert(3)
diff --git a/critbit/critbit0_insert.3 b/critbit/critbit0_insert.3
new file mode 100644
index 0000000..13b409a
--- /dev/null
+++ b/critbit/critbit0_insert.3
@@ -0,0 +1,21 @@
+.TH critbit0_insert 3
+.SH NAME
+critbit0_insert \- insert a string into a critbit tree
+.SH SYNTAX
+.B #include <critbit.h>
+
+critbit0_tree cb = { 0 };
+
+int \fBcritbit0_insert\fP(critbit0_tree* \fIcb\fR,const char* \fIstr\fR);
+.SH DESCRIPTION
+critbit0_insert attempts to insert a string into a critbit0 tree.
+If \fIstr\fR is already in \fIcb\fR, critbit0_insert returns 1.
+If \fIstr\fR is not in \fIcb\fR, it is inserted and critbit0_insert
+returns 2.
+If there is a memory allocation failure on the way, critbit0_insert
+leaves \fIcb\fR alone and returns 0.
+.SH "RETURN VALUE"
+2 if the key was inserted, 1 if it was already in cb, 0 on memory
+allocation failure.
+.SH "SEE ALSO"
+critbit0_contains(3)
diff --git a/t.c b/t.c
index ca2192e..57d7ede 100644
--- a/t.c
+++ b/t.c
@@ -23,6 +23,8 @@
 #include "iob.h"
 #include "safemult.h"
 #include "iarray.h"
+#include "critbit.h"
+#include <assert.h>
 
 #include "CAS.h"
 
@@ -47,7 +49,47 @@ static int64 writecb(int64 fd,const void* buf,uint64 n) {
   return -1;
 }
 
+static int ret0(const char* s,void* foo) {
+  (void)foo;
+  assert(strcmp(s,"fnord")==0);
+  return 0;
+}
+
+static int ret1(const char* s,void* foo) {
+  static int i;
+  (void)foo;
+  switch (i) {
+  case 0: assert(strcmp(s,"fnord")==0); break;
+  case 1: assert(strcmp(s,"fnord2")==0); break;
+  default: return -1;
+  }
+  ++i;
+  return 1;
+}
+
 int main(int argc,char* argv[]) {
+  static critbit0_tree t;
+  assert(critbit0_insert(&t,"fnord")==2);
+  assert(critbit0_insert(&t,"fnord2")==2);
+  assert(critbit0_insert(&t,"fnord2")==1);
+  assert(critbit0_contains(&t,"foo")==0);
+  assert(critbit0_contains(&t,"fnord")==1);
+  assert(critbit0_allprefixed(&t,"fnord",ret1,NULL)==1);
+  assert(critbit0_allprefixed(&t,"fnord",ret0,NULL)==0);
+  assert(critbit0_delete(&t,"fnord2")==1);
+  assert(critbit0_delete(&t,"foo")==0);
+#if 0
+  int s = socket_tcp6();
+#endif
+#if 0
+  iarray i;
+  iarray_init(&i,sizeof(size_t));
+  printf("%p\n",iarray_get(&i,0));
+  printf("%p\n",iarray_allocate(&i,0));
+  printf("%p\n",iarray_allocate(&i,0));
+  printf("%p\n",iarray_get(&i,0));
+#endif
+#if 0
   char buf[1024];
   size_t l;
   unsigned char c;
@@ -70,6 +112,7 @@ int main(int argc,char* argv[]) {
 		  f   0    9   d    8   4    9   e
 */
 
+#endif
 #if 0
   static size_t x;
   x=23;