C++: Iterating over a string vector and using the index for putenv

Question

I have a vector of strings which was created from parsing a config file. All the strings should be in the format key=value . I'd like to iterate over the vector, and use the putenv function to set an environment variable to the key-value pair.

The code:

for(auto it = settings.begin(); it != settings.end(); it++) {
      try {
         auto i = it - settings.begin();
         cout << i << endl;
         putenv(settings.at(i));
      } catch (...) {
         cout << "Config is not in the format key=value ... please correct" << endl;
      }
   }

This throws the error:

cannot convert ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ to ‘char*’ for argument ‘1’ to ‘int putenv(char*)’

I'm very new to C++, and all these variable types and pointers are confusing me.

Answer 1

You're mixing C and C++ stuff.

• Your vector contains C++ strings, std::string .
• putenv is an "old" function expecting a pointer to a char buffer, ie a C-string.

Fortunately, std::string makes it easy to get one of those :

putenv(settings.at(i).c_str());
//                   ^^^^^^^^

However, there is still a problem there. putenv takes "ownership" of the buffer you give it, and expects it to last "forever". Yours won't; it'll only be there until the std::string is modified or destroyed. Not good enough!

Conventionally, we use C's strdup here to allocate a copy of the char buffer. It's then putenv 's (or the OS's) responsibility to free it later.

putenv(strdup(settings.at(i).c_str()));
//     ^^^^^^^                      ^

Since putenv is not in the C or C++ standard, but is provided by POSIX, you should check the manpages for your operating system to make sure you're using it correctly.

Answer 2

The error is caused by your call to putenv(), which expects a pointer to char 's. Your vector contains C++ strings (std::string)...

You can try this:

for (auto setting : settings) {

     // ... putenv() is inconsistent across compilers, so use setenv() instead...         
     std::string key = item.substr( 0, item.find_first_of( "=" ) ) ;
     std::string val = item.substr( key.length()+1 ) ;

     if ( setenv( key.c_str(), val.c_str(), 1 ) != 0 ) {
        cerr << "setenv(" << key << ',' << val << ") has failed: " << strerror( errno ) << endl;
     }
}

From what I've read, putenv should be avoided in new code, and setenv should be used, as it makes a copy of its arguments, regardless of compiler version.

( setenv is in stdlib.h)