Are there names of environment variables that cannot be overwritten by the user/a call to
setenv? As I understand from POSIX1.2008, any process may edit the environment block, but must avoid overwriting such vars as LANG etc.
The environment is a list of strings of the form
var=value (by convention) that is passed as the 3rd argument to the execve() system call.
That list is put somewhere on the stack of the process when it starts to execute the new command just like for the list of arguments (another list of strings passed as the second argument to
In programs using the libc (most), an initialisation code called before the
main() function is invoked, makes those environment strings available as the
libc also provides with
setenv functions that can modify (a copy of) that list that the program received. That maintained, modified copy will then be passed to the next command executed by the process or any of its children via the
popen()... functions of the libc (which themselves end up calling the
execve() system call).
Now while when you build up a list of strings that you pass manually to the
execve() system call, you may pass strings like
foo (without a
= character) or
=bar (with an empty variable name),
setenv won't let you do that (
setenv("", "bar", 1) is rejected).
setenv("a=b", "c") will also be rejected. So the strings that will be defined by
setenv will always be of the format
x may not be empty.
That's about the only restriction (also applied by
putenv). Well those being NUL terminated strings, of course the NUL character cannot appear in the variable name or value.
setenv("*", "x", 1), or
setenv("\n\n", "", 1) are all OK as far as
setenv() or the kernel are concerned. Now, whether you're going to be able to do anything useful with those is another matter.
No, there are no restrictions on which environment variables can be changed by a process. However, keep in mind that each process has its own copy of the inherited environment, and a process cannot change any environment variable in any other process. The
setenv call can only modify the environment within the calling process.