Accessing Environment Variables In C++

Question

I'd like to have access to the $HOME environment variable in a C++ program that I'm writing. If I were writing code in C, I'd just use the getenv() function, but I was wondering if there was a better way to do it. Here's the code that I have so far:

std::string get_env_var( std::string const & key ) {                                 
  char * val;                                                                        
  val = getenv( key.c_str() );                                                       
  std::string retval = "";                                                           
  if (val != NULL) {                                                                 
    retval = val;                                                                    
  }                                                                                  
  return retval;                                                                        
}

Should I use getenv() to access environment variables in C++? Are there any problems that I'm likely to run into that I can avoid with a little bit of knowledge?

Answer 1

In the c++ you have to use std::getenv and to include <cstdlib>

Answer 2

• If you are on Windows you can use the Win32 API GetEnvironmentVariable
• On other linux/unix based systems use getenv

Why use GetEnvironmentVariable in Windows, from MSDN getenv:

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. Therefore, programs that use the envp argument to main or wmain may retrieve invalid information.

And from MSDN GetEnvironment:

This function can retrieve either a system environment variable or a user environment variable.

Answer 3

There is nothing wrong with using getenv() in C++. It is defined by stdlib.h, or if you prefer the standard library implementation, you can include cstdlib and access the function via the std:: namespace (i.e., std::getenv()). Absolutely nothing wrong with this. In fact, if you are concerned about portability, either of these two versions is preferred.

If you are not concerned about portability and you are using managed C++, you can use the .NET equivalent - System::Environment::GetEnvironmentVariable(). If you want the non-.NET equivalent for Windows, you can simply use the GetEnvironmentVariable() Win32 function.