Add internal facility for dynamic array handling

This is intended as a type-safe alternative to obstacks and
hand-written realloc constructs.  The implementation avoids
writing function pointers to the heap.

support/capture_subprocess.h

@@ -0,0 +1,42 @@
+/* Capture output from a subprocess.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_CAPTURE_SUBPROCESS_H
+#define SUPPORT_CAPTURE_SUBPROCESS_H
+
+#include <support/xmemstream.h>
+
+struct support_capture_subprocess
+{
+  struct xmemstream out;
+  struct xmemstream err;
+  int status;
+};
+
+/* Invoke CALLBACK (CLOSURE) in a subprocess and capture standard
+   output, standard error, and the exit status.  The out.buffer and
+   err.buffer members in the result are null-terminated strings which
+   can be examined by the caller (out.out and err.out are NULL).  */
+struct support_capture_subprocess support_capture_subprocess
+  (void (*callback) (void *), void *closure);
+
+/* Deallocate the subprocess data captured by
+   support_capture_subprocess.  */
+void support_capture_subprocess_free (struct support_capture_subprocess *);
+
+#endif /* SUPPORT_CAPTURE_SUBPROCESS_H */