MATLAB: Does setenv not set variables for MEX functions

MATLABmexsetenvwindows

Hello,
I have noticed that when I use setenv in MATLAB on Windows, the change is not visible in MEX functions. The behavior is as I would expect on Linux. I tested the following MEX function on MATLAB 2011a and 2012a:
#include <mex.h>
void mexFunction(int nargout, mxArray *pargout [ ], int nargin, const mxArray *pargin[])
{
  printf("OS  %s\n", getenv("OS"));
  printf("AAA %s\n", getenv("AAA"));
}
When I run test from MATLAB comandline I get
    >> test
    OS  Windows_NT
    AAA (null)
I get exactly the same when I setenv('AAA') to some value:
>> setenv('AAA','BBB')
>> getenv('AAA')
ans =
BBB
>> test
OS  Windows_NT
AAA (null)
Is that a feature, or is it a bug?
Thanks a lot!
Marcin

Best Answer

Hi,
I think the following Microsoft statement should help here:
"getenv and _putenv use the copy of the environment pointed to by the global variable _environ to access the environment. getenv operates only on the data structures accessible to the run-time library and not on the environment "segment" created for the process by the operating system. "
http://msdn.microsoft.com/en-us/library/tehxacec(v=vs.80).aspx
I think you would need to use GetEnvironmentVariable instead
http://msdn.microsoft.com/en-us/library/windows/desktop/ms683188%28v=vs.85%29.aspx
To answer it pretty short: It seems like getenv only looks at the global variables, and GetEnvironmentVariable works for application specific too
I modified the code to (change extension to .cpp)
#include <mex.h>
#include "Windows.h"
void mexFunction(int nargout, mxArray *pargout [ ], int nargin, const mxArray *pargin[])
{
    const DWORD buff_size = 50;
    LPTSTR buff = new TCHAR[buff_size];
    DWORD var_size;
    buff[0] = '\0';
    printf("OS  %s\n", getenv("OS"));
    printf("AAA %s\n", getenv("AAA"));
    var_size = GetEnvironmentVariable("AAA",buff,buff_size);
    printf("AAA through GetEnvironmentVariable %s\n",buff);
}
And when you run it you see this:
>> test
OS  Windows_NT
AAA (null)
AAA through GetEnvironmentVariable 
>> setenv('AAA','BBB11')
>> test
OS  Windows_NT
AAA (null)
AAA through GetEnvironmentVariable BBB11
>>
Related Question