64-bit Computing

This page contains notes on running on a 64-bit OS. For notes specific to Scientific Linux 5 (not just 64-bit SL5), please see ScientificLinux5.

When building on a 32-bit machine, you will produce a 32-bit executable. When building on a 64-bit machine, you can specify the target architecture to produce a 32-bit executable. Otherwise, you will produce a 64-bit executable if you build on a 64-bit machine and don't specify the target architecture.

Application Support


The command, setarch, can be used to change the reported architecture in new program environment. For example, setarch i386 program will cause "program" to see i686 instead of x86_64 as the machine type. See man setarch for more information.


With a 64-bit operating system, by default both firefox and its plugins are 64-bit. Some java applications (for example) require a 32-bit java plugin. To use these applications, you would need to start firefox in 32-bit mode. This can be done using setarch i386 firefox.


Totalview can be run from any linux computer using /nfs/opt/totalview/bin/totalview

If you're debugging a 32-bit (i386) executable on a 64-bit machine, you can use linux32 /nfs/opt/toolworks/totalview.8.9.2-2/linux-x86/bin/tv8

Please see the TotalView Debugger Documentation section of the Accelerator Physics Code Libraries wiki.


If you are on a 64-bit system, matlab will start in 64-bit mode. You can force 32-bit mode using the -glnx86 option.

If you are on SL5, matlab will start 2010a. Otherwise, matlab will start 2007b. You can force a specific version of matlab using, for example -r 2007b or -r 2010a. For example, Matlab 2007b can be run in 32-bit mode using matlab -r 2007b -glnx86

For more information, please see the Matlab wiki.


All native binary code that was written for a 32-bit VM must be recompiled for use in a 64-bit VM. For more information, please see Frequently Asked Questions About the Java HotSpot VM.

In order to work on both 32-bit and 64-bit systems, ~/.java/deployment/deployment.properties needs to be updated to support both 32 and 64-bit OS's. If you have trouble running javaws on a 32-bit or 64-bit OS, this can be fixed as follows:
  1. mv ~/.java/deployment/deployment.properties ~/.java/deployment/deployment.properties.bk
  2. create a new ~/.java/deployment/deployment.properties that contains the following (replace "dab66" with your username)
    #Tue Aug 31 17:23:01 EDT 2010
    #Java Deployment jre's
    #Tue Aug 31 17:23:01 EDT 2010
  3. In the third line of the new deployment.properties file, replace "dab66" with your LEPP Unix username


To build 32-bit apps on a 64-bit OS, the following lines will need to be added to the source Makefile (or possibly a more central location, such as CONFIG.Common.linux-x86):


To compile a 32-bit C/C++ application for Scientific Linux running on a 64-bit platform, use the gcc option -m32. According to the man page for gcc, the -m32 option sets int, long, and pointer variables to 32 bits (4 bytes). The -m64 option sets int to 32 bits (4 bytes) and long & pointer to 64 bits (8 bytes).

The following segment of code demonstrates this:
#include <stdio.h>

int main()
        int i;
        long l;
        char *p;

        printf("Integer size: %i bytes\\n", sizeof(i));
        printf("Long Integer size: %i bytes\\n", sizeof(l));
        printf("Pointer size: %i bytes\\n", sizeof(p));
        return 0;

The following is a comparison of the output when compiled with the -m32 and -m64 options:

# gcc -o ./test.64 -m64 test.c
# gcc -o ./test.32 -m32 test.c
# ./test.64
Integer size: 4 bytes
Long Integer size: 8 bytes
Pointer size: 8 bytes
# ./test.32
Integer size: 4 bytes
Long Integer size: 4 bytes
Pointer size: 4 bytes

ld (The GNU linker)

With a 64-bit OS, 32-bit libraries are stored in new directories that may need to be added to your library search path. This includes, at least, /usr/lib/gcc/x86_64-redhat-linux/3.4.6/32 and /usr/lib/gcc/x86_64-redhat-linux/4.1.1/32.

For some makefiles, this can be handled by adding the following to the beginning of your make file.
 LOCAL_LFLAGS   := -L /usr/lib/gcc/x86_64-redhat-linux/3.4.6/32 -L /usr/lib/gcc/x86_64-redhat-linux/4.1.1/32
Topic revision: r17 - 11 Feb 2015, AttilioDeFalco
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