Introduce string_appendf/string_vappendf

string_appendf is like string_printf, but instead of allocating a new
string, it appends to an existing string.  This allows reusing a
std::string's memory buffer across several calls, for example.

gdb/ChangeLog:
2017-10-30  Pedro Alves  <palves@redhat.com>

	* common/common-utils.c (string_appendf, string_vappendf): New
	functions.
	* common/common-utils.h (string_appendf, string_vappendf): New
	declarations.
	* unittests/common-utils-selftests.c (string_appendf_func)
	(test_appendf_func, string_vappendf_wrapper, string_appendf_tests)
	(string_vappendf_tests): New functions.
	(_initialize_common_utils_selftests): Register "string_appendf" and
	"string_vappendf tests".

2 files changed, 43 insertions, 0 deletions

diff --git a/gdb/common/common-utils.c b/gdb/common/common-utils.c
index d8c546a..7139302 100644
--- a/gdb/common/common-utils.c
+++ b/gdb/common/common-utils.c
@@ -195,6 +195,40 @@ string_vprintf (const char* fmt, va_list args)
   return str;
 }
 
+
+/* See documentation in common-utils.h.  */
+
+void
+string_appendf (std::string &str, const char *fmt, ...)
+{
+  va_list vp;
+
+  va_start (vp, fmt);
+  string_vappendf (str, fmt, vp);
+  va_end (vp);
+}
+
+
+/* See documentation in common-utils.h.  */
+
+void
+string_vappendf (std::string &str, const char *fmt, va_list args)
+{
+  va_list vp;
+  int grow_size;
+
+  va_copy (vp, args);
+  grow_size = vsnprintf (NULL, 0, fmt, vp);
+  va_end (vp);
+
+  size_t curr_size = str.size ();
+  str.resize (curr_size + grow_size);
+
+  /* C++11 and later guarantee std::string uses contiguous memory and
+     always includes the terminating '\0'.  */
+  vsprintf (&str[curr_size], fmt, args);
+}
+
 char *
 savestring (const char *ptr, size_t len)
 {
diff --git a/gdb/common/common-utils.h b/gdb/common/common-utils.h
index 19724f9..a32863c 100644
--- a/gdb/common/common-utils.h
+++ b/gdb/common/common-utils.h
@@ -67,6 +67,15 @@ std::string string_printf (const char* fmt, ...)
 std::string string_vprintf (const char* fmt, va_list args)
   ATTRIBUTE_PRINTF (1, 0);
 
+/* Like string_printf, but appends to DEST instead of returning a new
+   std::string.  */
+void string_appendf (std::string &dest, const char* fmt, ...)
+  ATTRIBUTE_PRINTF (2, 3);
+
+/* Like string_appendf, but takes a va_list.  */
+void string_vappendf (std::string &dest, const char* fmt, va_list args)
+  ATTRIBUTE_PRINTF (2, 0);
+
 /* Make a copy of the string at PTR with LEN characters
    (and add a null character at the end in the copy).
    Uses malloc to get the space.  Returns the address of the copy.  */