Questions about putenv() and setenv()

Question

I have been thinking a little about environment variables and have a few questions/observations.

• putenv(char *string);

  This call seems

Answer 1

There is no special "the environment" space - setenv just dynamically allocates space for the strings (with malloc for example) as you would do normally. Because the environment doesn't contain any indication of where each string in it came from, it is impossible for setenv or unsetenv to free any space which may have been dynamically allocated by previous calls to setenv.

"Because it doesn't copy the passed string you can't call it with a local and there is no guarantee a heap allocated string won't be overwritten or accidentally deleted." The purpose of putenv is to make sure that if you have a heap-allocated string it's possible to delete it on purpose. That's what the rationale text means by "the only function available to add to the environment without permitting memory leaks." And yes, you can call it with a local, just remove the string from the environment (putenv("FOO=") or unsetenv) before you return from the function.

The point is that using putenv makes the process of removing a string from the environment entirely deterministic. Whereas setenv will on some existing implementations modify an existing string in the environment if the new value is shorter (to avoid always leaking memory), and since it made a copy when you called setenv you're not in control of the originally dynamically allocated string so you can't free it when it's removed.

Meanwhile, setenv itself (or unsetenv) can't free the previous string, since - even ignoring putenv - the string may have come from the original environment instead of being allocated by a previous invocation of setenv.

(This whole answer assumes a correctly implemented putenv, i.e. not the one in glibc 2.0-2.1.1 you mentioned.)