glibc's malloc is very bad about returning memory from the kernel, add a call

to its malloc_trim to prompt it to do so. Reported by Sarunas Valaskevicius.

compat.h

@@ -27,6 +27,10 @@
 #include <termios.h>
 #include <wchar.h>
 
+#ifdef HAVE_MALLOC_TRIM
+#include <malloc.h>
+#endif
+
 #ifdef HAVE_UTF8PROC
 #include <utf8proc.h>
 #endif

configure.ac

@@ -328,6 +328,31 @@ AC_SEARCH_LIBS(inet_ntoa, nsl)
 AC_SEARCH_LIBS(socket, socket)
 AC_CHECK_LIB(xnet, socket)
 
+# Check if using glibc and have malloc_trim(3). The glibc free(3) is pretty bad
+# about returning memory to the kernel unless the application tells it when to
+# with malloc_trim(3).
+AC_MSG_CHECKING(if free doesn't work very well)
+AC_LINK_IFELSE([AC_LANG_SOURCE(
+	[
+		#include <stdlib.h>
+		#ifdef __GLIBC__
+		#include <malloc.h>
+		int main(void) {
+			malloc_trim (0);
+			exit(0);
+		}
+		#else
+		no
+		#endif
+	])],
+	found_malloc_trim=yes,
+	found_malloc_trim=no
+)
+AC_MSG_RESULT($found_malloc_trim)
+if test "x$found_malloc_trim" = xyes; then
+	AC_DEFINE(HAVE_MALLOC_TRIM)
+fi
+
 # Check for CMSG_DATA. On some platforms like HP-UX this requires UNIX 95
 # (_XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED) (see xopen_networking(7)). On
 # others, UNIX 03 (_XOPEN_SOURCE 600, see standards(7) on Solaris).

grid.c

@@ -265,6 +265,9 @@ grid_free_lines(struct grid *gd, u_int py, u_int ny)
 
 	for (yy = py; yy < py + ny; yy++)
 		grid_free_line(gd, yy);
+#ifdef HAVE_MALLOC_TRIM
+	malloc_trim(0);
+#endif
 }
 
 /* Create a new grid. */