2 files changed, 80 insertions, 4 deletions
diff --git a/src/glsl/ralloc.c b/src/glsl/ralloc.c
index fb48a91c564..2967598aecb 100644
--- a/src/glsl/ralloc.c
+++ b/src/glsl/ralloc.c
@@ -439,7 +439,27 @@ ralloc_asprintf_append(char **str, const char *fmt, ...)
 bool
 ralloc_vasprintf_append(char **str, const char *fmt, va_list args)
 {
-   size_t existing_length, new_length;
+   assert(str != NULL);
+   size_t existing_length = *str ? strlen(*str) : 0;
+   return ralloc_vasprintf_rewrite_tail(str, existing_length, fmt, args);
+}
+
+bool
+ralloc_asprintf_rewrite_tail(char **str, size_t start, const char *fmt, ...)
+{
+   bool success;
+   va_list args;
+   va_start(args, fmt);
+   success = ralloc_vasprintf_rewrite_tail(str, start, fmt, args);
+   va_end(args);
+   return success;
+}
+
+bool
+ralloc_vasprintf_rewrite_tail(char **str, size_t start, const char *fmt,
+			      va_list args)
+{
+   size_t new_length;
    char *ptr;
 
    assert(str != NULL);
@@ -450,14 +470,13 @@ ralloc_vasprintf_append(char **str, const char *fmt, va_list args)
       return true;
    }
 
-   existing_length = strlen(*str);
    new_length = printf_length(fmt, args);
 
-   ptr = resize(*str, existing_length + new_length + 1);
+   ptr = resize(*str, start + new_length + 1);
    if (unlikely(ptr == NULL))
       return false;
 
-   vsnprintf(ptr + existing_length, new_length + 1, fmt, args);
+   vsnprintf(ptr + start, new_length + 1, fmt, args);
    *str = ptr;
    return true;
 }
diff --git a/src/glsl/ralloc.h b/src/glsl/ralloc.h
index d5338152f10..1324f3466bb 100644
--- a/src/glsl/ralloc.h
+++ b/src/glsl/ralloc.h
@@ -314,9 +314,60 @@ char *ralloc_asprintf (const void *ctx, const char *fmt, ...);
 char *ralloc_vasprintf(const void *ctx, const char *fmt, va_list args);
 
 /**
+ * Rewrite the tail of an existing string, starting at a given index.
+ *
+ * Overwrites the contents of *str starting at \p start with newly formatted
+ * text, including a new null-terminator.  Allocates more memory as necessary.
+ *
+ * This can be used to append formatted text when the length of the existing
+ * string is already known, saving a strlen() call.
+ *
+ * \sa ralloc_asprintf_append
+ *
+ * \param str   The string to be updated.
+ * \param start The index to start appending new data at.
+ * \param fmt   A printf-style formatting string
+ *
+ * \p str will be updated to the new pointer unless allocation fails.
+ *
+ * \return True unless allocation failed.
+ */
+bool ralloc_asprintf_rewrite_tail(char **str, size_t start,
+				  const char *fmt, ...);
+
+/**
+ * Rewrite the tail of an existing string, starting at a given index.
+ *
+ * Overwrites the contents of *str starting at \p start with newly formatted
+ * text, including a new null-terminator.  Allocates more memory as necessary.
+ *
+ * This can be used to append formatted text when the length of the existing
+ * string is already known, saving a strlen() call.
+ *
+ * \sa ralloc_vasprintf_append
+ *
+ * \param str   The string to be updated.
+ * \param start The index to start appending new data at.
+ * \param fmt   A printf-style formatting string
+ * \param args  A va_list containing the data to be formatted
+ *
+ * \p str will be updated to the new pointer unless allocation fails.
+ *
+ * \return True unless allocation failed.
+ */
+bool ralloc_vasprintf_rewrite_tail(char **str, size_t start, const char *fmt,
+				   va_list args);
+
+/**
  * Append formatted text to the supplied string.
  *
+ * This is equivalent to
+ * \code
+ * ralloc_asprintf_rewrite_tail(str, strlen(*str), fmt, ...)
+ * \endcode
+ *
  * \sa ralloc_asprintf
+ * \sa ralloc_asprintf_rewrite_tail
  * \sa ralloc_strcat
  *
  * \p str will be updated to the new pointer unless allocation fails.
@@ -328,7 +379,13 @@ bool ralloc_asprintf_append (char **str, const char *fmt, ...);
 /**
  * Append formatted text to the supplied string, given a va_list.
  *
+ * This is equivalent to
+ * \code
+ * ralloc_vasprintf_rewrite_tail(str, strlen(*str), fmt, args)
+ * \endcode
+ *
  * \sa ralloc_vasprintf
+ * \sa ralloc_vasprintf_rewrite_tail
  * \sa ralloc_strcat
  *
  * \p str will be updated to the new pointer unless allocation fails.