The underlying problem isn't just setenv, because the string returned by getenv can be invalidated by another call to getenv. ISO C says:
"The getenv function returns a pointer to a string associated with the matched list member. The
string pointed to shall not be modified by the program, but can be overwritten by a subsequent call
to the getenv function."
In a single threaded virtual machine, you can immediately duplicate the string returned by getenv and stop using it, right there.
Under threads, getenv is not required to be safe.
I think that with some care, it may be; an environment implementation could guarantee that a non-mutating operation like getenv doesn't invalidate any previously returned strings.
I think POSIX does that. It allows getenv to reallocate the environ array, but not the strings themselves:
"Applications can change the entire environment in a single operation by assigning the environ variable to point to an array of character pointers to the new environment strings. After assigning a new value to environ, applications should not rely on the new environment strings remaining part of the environment, as a call to getenv(), secure_getenv(), [XSI] [Option Start] putenv(), [Option End] setenv(), unsetenv(), or any function that is dependent on an environment variable may, on noticing that environ has changed, copy the environment strings to a new array and assign environ to point to it."
environ is documented together with the exec family of functions; that's where this is found.
So whereas there are things not to like about environ, it can be the basis for thread safety of getenv in an application that doesn't mutate the environment.
"The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but can be overwritten by a subsequent call to the getenv function."
In a single threaded virtual machine, you can immediately duplicate the string returned by getenv and stop using it, right there.
Under threads, getenv is not required to be safe.
I think that with some care, it may be; an environment implementation could guarantee that a non-mutating operation like getenv doesn't invalidate any previously returned strings.
I think POSIX does that. It allows getenv to reallocate the environ array, but not the strings themselves:
"Applications can change the entire environment in a single operation by assigning the environ variable to point to an array of character pointers to the new environment strings. After assigning a new value to environ, applications should not rely on the new environment strings remaining part of the environment, as a call to getenv(), secure_getenv(), [XSI] [Option Start] putenv(), [Option End] setenv(), unsetenv(), or any function that is dependent on an environment variable may, on noticing that environ has changed, copy the environment strings to a new array and assign environ to point to it."
environ is documented together with the exec family of functions; that's where this is found.
So whereas there are things not to like about environ, it can be the basis for thread safety of getenv in an application that doesn't mutate the environment.