diff --git a/libraries/AP_Common/ExpandingString.cpp b/libraries/AP_Common/ExpandingString.cpp
new file mode 100644
index 0000000000..c6a701eec3
--- /dev/null
+++ b/libraries/AP_Common/ExpandingString.cpp
@@ -0,0 +1,102 @@
+/*
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/*
+  expanding string for easy construction of text buffers
+ */
+
+#include "ExpandingString.h"
+
+extern const AP_HAL::HAL& hal;
+
+#define EXPAND_INCREMENT 512
+
+/*
+  expand the string buffer
+ */
+bool ExpandingString::expand(uint32_t min_extra_space_needed)
+{
+    // expand a reasonable amount
+    uint32_t newsize = (5*buflen/4) + EXPAND_INCREMENT;
+    if (newsize - used < min_extra_space_needed) {
+        newsize = used + min_extra_space_needed;
+    }
+    
+    // add one to ensure we are always null terminated
+    void *newbuf = hal.util->std_realloc(buf, newsize+1);
+
+    if (newbuf == nullptr) {
+        allocation_failed = true;
+        return false;
+    }
+
+    buflen = newsize;
+    buf = (char *)newbuf;
+
+    return true;
+}
+
+/*
+  print into the buffer, expanding if needed
+ */
+void ExpandingString::printf(const char *format, ...)
+{
+    if (allocation_failed) {
+        return;
+    }
+    if (buflen == used && !expand(0)) {
+        return;
+    }
+    int n;
+
+    /*
+      print into the buffer, expanding the buffer if needed
+     */
+    while (true) {
+        va_list arg;
+        va_start(arg, format);
+        n = hal.util->vsnprintf(&buf[used], buflen-used, format, arg);
+        va_end(arg);
+        if (n < 0) {
+            return;
+        }
+        if (uint32_t(n) < buflen - used) {
+            break;
+        }
+        if (!expand(n+1)) {            
+            return;
+        }
+    }
+    used += n;
+}
+
+/*
+  print into the buffer, expanding if needed
+ */
+void ExpandingString::append(const char *s, uint32_t len)
+{
+    if (allocation_failed) {
+        return;
+    }
+    if (buflen - used < len && !expand(len)) {
+        return;
+    }
+    memcpy(&buf[used], s, len);
+    used += len;
+}
+
+ExpandingString::~ExpandingString()
+{
+    free(buf);
+}
diff --git a/libraries/AP_Common/ExpandingString.h b/libraries/AP_Common/ExpandingString.h
new file mode 100644
index 0000000000..54a124def0
--- /dev/null
+++ b/libraries/AP_Common/ExpandingString.h
@@ -0,0 +1,54 @@
+/*
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/*
+  expanding string for easy construction of text buffers
+ */
+
+#pragma once
+
+#include <AP_HAL/AP_HAL.h>
+
+class ExpandingString {
+public:
+
+    const char *get_string(void) const {
+        return buf;
+    }
+    uint32_t get_length(void) const {
+        return used;
+    }
+
+    // print into the string
+    void printf(const char *format, ...) FMT_PRINTF(2,3);
+
+    // append data to the string
+    void append(const char *s, uint32_t len);
+
+    // destructor
+    ~ExpandingString();
+
+    bool has_failed_allocation() const {
+        return allocation_failed;
+    }
+
+private:
+    char *buf;
+    uint32_t buflen;
+    uint32_t used;
+    bool allocation_failed;
+
+    // try to expand the buffer
+    bool expand(uint32_t min_needed) WARN_IF_UNUSED;
+};