Add utstring_truncate, analogous to man (2) truncate

doc/Makefile

@@ -1,6 +1,9 @@
 HTML=$(patsubst %.txt,%.html,$(wildcard *.txt))
 
 all: $(HTML)
+
+clean:
+	rm $(HTML)
 
 # when each target of a multi-target rule has its own prereq 
 # we use a static pattern rule. 

doc/utstring.txt

@@ -164,6 +164,7 @@ Operations
 | utstring_body(s) | get `char*` to body of s (buffer is always null-terminated)
 | utstring_find(s,pos,str,len) | forward search from pos for a substring
 | utstring_findR(s,pos,str,len) | reverse search from pos a substring
+| utstring_truncate(s,len) | change utstring length, extending with `NULL`s if needed
 |===============================================================================
 
 New/free vs. init/done

src/utstring.h

@@ -150,6 +150,17 @@ _UNUSED_ static void utstring_printf(UT_string *s, const char *fmt, ...) {
    va_end(ap);
 }
 
+#define utstring_truncate(s,amt)                           \
+do {                                                       \
+  size_t ext = (size_t) (amt) + 1 - (s)->n;                \
+  if (ext < (size_t) (amt)) {                              \
+    utstring_reserve((s), ext);                            \
+    memset((s)->d + (s)->n - ext, 0, ext);                 \
+  }                                                        \
+  (s)->i = amt;                                            \
+  (s)->d[amt] = '\0';                                      \
+} while(0)
+
 /*******************************************************************************
  * begin substring search functions                                            *
  ******************************************************************************/

tests/Makefile

@@ -13,7 +13,7 @@ PROGS = test1 test2 test3 test4 test5 test6 test7 test8 test9   \
         test58 test59 test60 test61 test62 test63 test64 test65 \
         test66 test67 test68 test69 test70 test71 test72 test73 \
         test74 test75 test76 test77 test78 test79 test80 test81 \
-        test82 test83 test84
+        test82 test83 test84 test85
 CFLAGS = -I$(HASHDIR) 
 #CFLAGS += -DHASH_BLOOM=16
 #CFLAGS += -O2

tests/test85.c

@@ -0,0 +1,24 @@
+#include <unistd.h>    /* write */
+#include "utstring.h"
+
+int main() {
+    UT_string *s;
+    char V_TestStr[] = "The quick brown dog, &c.";
+    utstring_new(s);
+
+    utstring_bincpy(s, V_TestStr, sizeof(V_TestStr)-1);
+    write(STDOUT_FILENO, utstring_body(s), utstring_len(s));
+    write(STDOUT_FILENO, "\n", 1);
+
+    utstring_truncate(s, 10);
+    write(STDOUT_FILENO, utstring_body(s), utstring_len(s));
+    write(STDOUT_FILENO, "\n", 1);
+
+    utstring_truncate(s, 10000);
+    write(STDOUT_FILENO, utstring_body(s), utstring_len(s));
+    write(STDOUT_FILENO, "\n", 1);
+
+    utstring_free(s);
+
+    return 0;
+}