Cygwin's heap is extensible. However, it does start out at a fixed size
and attempts to extend it may run into memory which has been previously
allocated by Windows. In some cases, this problem can be solved by
-adding an entry in the either the <literal>HKEY_LOCAL_MACHINE</literal>
-(to change the limit for all users) or
-<literal>HKEY_CURRENT_USER</literal> (for just the current user) section
-of the registry. </para>
+changing a field in the file header which is utilized by Cygwin since
+version 1.7.10 to keep the initial size of the application heap. If the
+field contains 0, which is the default, the application heap defaults to
+a size of 384 Megabyte. If the field is set to any other value between 4 and
+2048, Cygwin tries to reserve as much Megabytes for the application heap.
+The field used for this is the "LoaderFlags" field in the NT-specific
+PE header structure (<literal>(IMAGE_NT_HEADER)->OptionalHeader.LoaderFlags</literal>).</para>
<para>
-Add the <literal>DWORD</literal> value <literal>heap_chunk_in_mb</literal>
-and set it to the desired memory limit in decimal MB. It is preferred to do
-this in Cygwin using the <command>regtool</command> program included in the
-Cygwin package.
-(For more information about <command>regtool</command> or the other Cygwin
-utilities, see <xref linkend="using-utils"></xref> or use the
-<literal>--help</literal> option of each util.) You should always be careful
-when using <command>regtool</command> since damaging your system registry can
-result in an unusable system. This example sets memory limit to 1024 MB:
+This value can be changed for any executable by using a more recent version
+of the <command>peflags</command> tool from the <literal>rebase</literal>
+Cygwin package. Example:
<screen>
-regtool -i set /HKLM/Software/Cygwin/heap_chunk_in_mb 1024
-regtool -v list /HKLM/Software/Cygwin
+$ peflags --cygwin-heap foo.exe
+foo.exe: initial Cygwin heap size: 0 (0x0) MB
+$ peflags --cygwin-heap=500 foo.exe
+foo.exe: initial Cygwin heap size: 500 (0x1f4) MB
</screen>
</para>
<para>
-Exit all running Cygwin processes and restart them. Memory can be allocated up
-to the size of the system swap space minus any the size of any running
-processes. The system swap should be at least as large as the physically
-installed RAM and can be modified under the System category of the
-Control Panel.
+Heap memory can be allocated up to the size of the biggest available free
+block in the processes virtual memory (VM). By default, the VM per process
+is 2 GB for 32 processes. To get more VM for a process, the executable
+must have the "large address aware" flag set in the file header. You can
+use the aforementioned <command>peflags</command> tool to set this flag.
+On 64 bit systems this results in a 4 GB VM for a process started from that
+executable. On 32 bit systems you also have to prepare the system to allow
+up to 3 GB per process. See the Microsoft article
+<ulink url="http://msdn.microsoft.com/en-us/library/bb613473%28VS.85%29.aspx">4-Gigabyte Tuning</ulink>
+for more information.
</para>
+<note>
<para>
-Here is a small program written by DJ Delorie that tests the
-memory allocation limit on your system:
+Older Cygwin releases only supported a global registry setting to
+change the initial heap size for all Cygwin processes. This setting is
+not used anymore. However, if you're running an older Cygwin release
+than 1.7.10, you can add the <literal>DWORD</literal> value
+<literal>heap_chunk_in_mb</literal> and set it to the desired memory limit
+in decimal MB. You have to stop all Cygwin processes for this setting to
+have any effect. It is preferred to do this in Cygwin using the
+<command>regtool</command> program included in the Cygwin package.
+(see <xref linkend="regtool"></xref>) This example sets the memory limit
+to 1024 MB for all Cygwin processes (use HKCU instead of HKLM if you
+want to set this only for the current user):
<screen>
-main()
-{
- unsigned int bit=0x40000000, sum=0;
- char *x;
-
- while (bit > 4096)
- {
- x = malloc(bit);
- if (x)
- sum += bit;
- bit >>= 1;
- }
- printf("%08x bytes (%.1fMb)\n", sum, sum/1024.0/1024.0);
- return 0;
-}
+$ regtool -i set /HKLM/Software/Cygwin/heap_chunk_in_mb 1024
+$ regtool -v list /HKLM/Software/Cygwin
</screen>
-
-You can compile this program using:
-<screen>
-gcc max_memory.c -o max_memory.exe
-</screen>
-
-Run the program and it will output the maximum amount of allocatable memory.
</para>
+</note>
</sect1>
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