diff --git a/nbase/nbase.h b/nbase/nbase.h
index c8d0683f0..4b0c9ed0b 100644
--- a/nbase/nbase.h
+++ b/nbase/nbase.h
@@ -395,8 +395,8 @@ int Vsnprintf(char *, size_t, const char *, va_list)
 int Snprintf(char *, size_t, const char *, ...)
      __attribute__ ((format (printf, 3, 4)));
 
-char *alloc_vsprintf(const char *fmt, va_list va)
-     __attribute__ ((format (printf, 1, 0)));
+int alloc_vsprintf(char **strp, const char *fmt, va_list va)
+     __attribute__ ((format (printf, 2, 0)));
 
 /* Trivial function that returns nonzero if all characters in str of
    length strlength are printable (as defined by isprint()) */
diff --git a/nbase/nbase_str.c b/nbase/nbase_str.c
index b201b5789..35ad4afc8 100644
--- a/nbase/nbase_str.c
+++ b/nbase/nbase_str.c
@@ -166,20 +166,17 @@ int Snprintf(char *s, size_t n, const char *fmt, ...)
 }
 
 /* vsprintf into a dynamically allocated buffer, similar to asprintf in
-   Glibc. Return the buffer or NULL on error. */
-char *alloc_vsprintf(const char *fmt, va_list va) {
+   Glibc. Return the length of the buffer or -1 on error. */
+int alloc_vsprintf(char **strp, const char *fmt, va_list va) {
   va_list va_tmp;
-  char *s, *p;
+  char *s;
   int size = 32;
   int n;
 
   s = NULL;
   size = 32;
   for (;;) {
-    p = (char *) safe_realloc(s, size);
-    if (p == NULL)
-      return NULL;
-    s = p;
+    s = (char *) safe_realloc(s, size);
 
 #ifdef WIN32
     va_tmp = va;
@@ -195,8 +192,9 @@ char *alloc_vsprintf(const char *fmt, va_list va) {
     else
       break;
   }
+  *strp = s;
 
-  return s;
+  return n;
 }
 
 /* Trivial function that returns nonzero if all characters in str of length strlength are
diff --git a/output.cc b/output.cc
index 979b5b0d8..a31000776 100644
--- a/output.cc
+++ b/output.cc
@@ -939,10 +939,9 @@ void log_vwrite(int logt, const char *fmt, va_list ap) {
   case LOG_MACHINE:
   case LOG_SKID:
   case LOG_XML:
-    writebuf = alloc_vsprintf(fmt, ap);
+    len = alloc_vsprintf(&writebuf, fmt, ap);
     if (writebuf == NULL)
       fatal("%s: alloc_vsprintf failed.", __func__);
-    len = strlen(writebuf);
     l = logt;
     fileidx = 0;
     while ((l & 1) == 0) {
diff --git a/xml.cc b/xml.cc
index 1c5aca67c..2cb95698f 100644
--- a/xml.cc
+++ b/xml.cc
@@ -236,7 +236,7 @@ int xml_write_raw(const char *fmt, ...) {
   char *s;
 
   va_start(va, fmt);
-  s = alloc_vsprintf(fmt, va);
+  alloc_vsprintf(&s, fmt, va);
   va_end(va);
   if (s == NULL)
     return -1;
@@ -264,7 +264,7 @@ int xml_write_escaped(const char *fmt, ...) {
 int xml_write_escaped_v(const char *fmt, va_list va) {
   char *s, *esc_s;
 
-  s = alloc_vsprintf(fmt, va);
+  alloc_vsprintf(&s, fmt, va);
   if (s == NULL)
     return -1;
   esc_s = escape(s);
@@ -386,7 +386,7 @@ int xml_attribute(const char *name, const char *fmt, ...) {
   assert(xml.tag_open);
 
   va_start(va, fmt);
-  val = alloc_vsprintf(fmt, va);
+  alloc_vsprintf(&val, fmt, va);
   va_end(va);
   if (val == NULL)
     return -1;