Add support for --enable-assume-ram=SIZE.

INSTALL

@@ -237,6 +237,22 @@ XZ Utils Installation
                 to optimize for size. You need to add -Os or equivalent
                 flag(s) to CFLAGS manually.
 
+    --enable-assume-ram=SIZE
+                On the most common operating systems, XZ Utils is able to
+                detect the amount of physical memory on the system. This
+                information is used to set the default memory usage limit.
+
+                On some systems, there is no code to detect the amount of
+                RAM though. Using --enable-assume-ram one can set how much
+                memory to assume on these systems. SIZE is given as MiB.
+                The default is 32 MiB, which is probably too low for most
+                systems, but it is enough to allow decompressing .xz files
+                created with the default settings.
+
+                Feel free to send patches to add support for detecting
+                the amount of RAM on the operating system you use. See
+                src/common/tuklib_physmem.c for details.
+
     --disable-threads
                 Disable threading support. This makes some things
                 thread-unsafe, meaning that if multithreaded application

configure.ac

@@ -380,6 +380,30 @@ AC_MSG_RESULT([$enable_threads])
 # We use the actual result a little later.
 
 
+#########################
+# Assumed amount of RAM #
+#########################
+
+# We use 32 MiB as default, because it should be small enough for most
+# cases and allows decompressing files compressed with the default settings.
+# Probably it is too small for most systems, but it's safer to guess too low.
+AC_MSG_CHECKING([how much RAM to assume if the real amount is unknown])
+AC_ARG_ENABLE([assume-ram], AC_HELP_STRING([--enable-assume-ram=SIZE],
+		[If and only if the real amount of RAM cannot be determined,
+		assume SIZE MiB. The default is 32 MiB. This affects the
+		default memory usage limit.]),
+	[], [enable_assume_ram=32])
+assume_ram_check=`echo "$enable_assume_ram" | tr -d 0123456789`
+if test -z "$enable_assume_ram" || test -n "$assume_ram_check"; then
+	AC_MSG_RESULT([])
+	AC_MSG_ERROR([--enable-assume-ram accepts only an integer argument])
+fi
+AC_MSG_RESULT([$enable_assume_ram MiB])
+AC_DEFINE_UNQUOTED([ASSUME_RAM], [$enable_assume_ram],
+		[How many MiB of RAM to assume if the real amount cannot
+		be determined.])
+
+
 ############################################
 # xz/xzdec/lzmadec linkage against liblzma #
 ############################################

src/xz/hardware.c

@@ -68,11 +68,10 @@ hardware_memlimit_set_percentage(uint32_t percentage)
 
 	uint64_t mem = tuklib_physmem();
 
-	// If we cannot determine the amount of RAM, assume 32 MiB. Maybe
-	// even that is too much on some systems. But on most systems it's
-	// far too little, and can be annoying.
+	// If we cannot determine the amount of RAM, use the assumption
+	// defined by the configure script.
 	if (mem == 0)
-		mem = UINT64_C(32) * 1024 * 1024;
+		mem = (uint64_t)(ASSUME_RAM) * 1024 * 1024;
 
 	memlimit = percentage * mem / 100;
 	return;

src/xzdec/xzdec.c

@@ -106,9 +106,10 @@ memlimit_set_percentage(uint32_t percentage)
 {
 	uint64_t mem = tuklib_physmem();
 
-	// If we cannot determine the amount of RAM, assume 32 MiB.
+	// If we cannot determine the amount of RAM, use the assumption
+	// set by the configure script.
 	if (mem == 0)
-		mem = UINT64_C(32) * 1024 * 1024;
+		mem = (uint64_t)(ASSUME_RAM) * 1024 * 1024;
 
 	memlimit = percentage * mem / 100;
 	return;