如何为多线程程序设置环境变量？

Question

I would like to run a parallel Java threaded program and take advantage of multiprocessor execution. However I need to set the environment variable, to enable a multi-threaded environment. I understand that you can set the environment by issuing setenv PARALLEL 4 OR setenv OMP_NUM_THREADS 4 (for an OpenMP program).

This should enable 4 processors to run concurrently if you have 4 processor.

My Question is:

1. where do you issue the above command (SETENV) and how do you do it?

In java you can call System.getenv("NUM_THREADS") to get the NUM_THREADS . However there is no clear way of setting the environment. I am running AMD-x64 machine: OS: Windows 8, Processor: AMD E-300 APU Dual-Core processor, Ram: 4.00GB, System Type: 64-bit OS.

Answer 1

Below is the link which explains on how to set environment variable in a Windows machine manually :

http://www3.ntu.edu.sg/home/ehchua/programming/howto/Environment_Variables.html

A piece taken from above link (must read the link completely, its very rich in knowledge):

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Display Variables and their Values

To list all the variables and their values, start a CMD shell (Click "Start" button ⇒ Run ⇒ Enter "cmd") and issue the command "set". To display a particular variable, use command "set varname". For examples,

// Display all the variables (in NAME=VALUE pairs)
prompt> set
COMPUTERNAME=xxxxxxx
OS=xxxxxxx
PATH=xxxxxxx
.......

// Display a particular variable
prompt> set COMPUTERNAME
COMPUTERNAME=xxxxxx
// OR use echo command with variable enclosed within a pair of '%'s
prompt> echo %COMPUTERNAME%
COMPUTERNAME=xxxxxx

Try issuing a set command on your system, and study the environment variables listed. Pay particular attention to the variable called PATH.

Set/Change/Unset a Variable

To set (or change) a variable, use command "set varname=value". There shall be no spaces before and after the '=' sign. To unset an environment variable, use "set varname=", ie, set it to an empty string.

prompt> set varname
prompt> set varname=value
prompt> set varname=
prompt> set

Display the value of the variable

Set or change the value of the variable (Note: no space before and after '=') Delete the variable by setting to empty string (Note: nothing after '=') Display ALL the environment variables. For examples,

// Set an environment variable
prompt> set MY_VAR=hello

// Display
prompt> set MY_VAR
MY_VAR=hello

// Unset an environment variable
prompt> set MY_VAR=

// Display
prompt> set MY_VAR
Environment variable MY_VAR not defined

A variable set via the "set" command under CMD is a local variable, available to the current CMD session only.

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If you want to set the same using Java code, below is one example:

public static void main(String[] args) throws IOException {

ProcessBuilder pb = new ProcessBuilder("CMD", "/C", "SET");
Map<String, String> env = pb.environment();
env.put("MYVAR", "myValue");
Process p = pb.start();
InputStreamReader isr = new InputStreamReader(p.getInputStream());
char[] buf = new char[1024];
while (!isr.ready()) {
    ;
}
while (isr.read(buf) != -1) {
    System.out.println(buf);
 }
}

If you want to pass some value to your program, you could also do that in command line:

java -DMyVar=varValue <main program>

This value could be read as:

String myVar= System.getProperty("MyVar");

Answer 2

I believe setenv is a command for linux/unix.

In windows 7, you can use the setx command in command prompt to set a User Environment Variable. eg:

setx myvariablename myvariablevalue

Or you can do it through the GUI:

Right click My Computer -> Properties -> Advanced -> Environment Variables