C ++ getenv总是返回null值

Question

I have just added the environment variable "DataDir", but the getenv function still returns null value.

Here is my code: const char *ret = getenv("DataDir");

I restarted my computer and it done.

Answer 1

did you remember to export the variable before running the program? If you are using bash shell on linux, for example, you generally should use export DataDir="..."

On windows, if you set the environment variables using the system settings window, it will not immediately propagate to all of the running programs. If "I restarted my computer and it done." means "restarting the computer resolved the issue", then I believe that explains the problem. After changing the environment variable, try closing all programs and then start a CMD session (or visual studio) and run the program again

Answer 2

Are you running on Windows? Did you set the environment variable through the control panel? If so, that only affects processes that you start (programs that you launch) after you changed the setting. If you're running from a command prompt, and the command prompt didn't inherit the new environment variable, then your program won't inherit it either.

After rebooting, all new processes inherit the new environment variable.

On the other hand, if you set the variable and then run the program:

C:\>set DataDir=blah
C:\>.\my_program

then your program will inherit the variable (but it won't persist across a reboot).

Similar considerations apply on Linux and other systems, but the details differ.

Note that I'm only guessing, based on the symptoms you reported, what system you're using. In the future, it would be helpful to provide that information in the question (if it's not relevant we can ignore it).