Objectives

1. Understand how to manipulate enviorn within a program.

Notes

• char *getenv(const char *name);
  • This returns
    • A pointer to the value if the value of name appears in the environment.
    • Null otherwise.
  • If you get a value back
    • YOU MUST NOT modify this.
    • It may change in future calls, something else might modify the environment.
    • The pointer may end up pointing to junk.
• int putenv(char *string);
  • Changes or creates an environment variable.
  • returns 0 on success or non-zero on error.
  • You need to be careful, the string you pass it becomes part of the environment.
    • IE It saves your pointer.
    • Do not change the value pointed at.
• C++ wants to make all string constants const char *
  • But the function wants an char*
  • SO
    1. Make a const char * and do a constant cast.

       const char* sillyVar{"SILLY=Now I Am"};
       ...
       if (putenv(const_cast<char *>(sillyVar)) == -1) { 

    2. Declare a c++ style string, use c_str(), and do a constant cast

       string  sillyVar{"SILLY=Now I am"};
       ...
       if (putenv(const_cast<char *>(sillyVar.c_str())) == -1) {

    3. Declare an array of characters.

       char sillyVar[] = "SILLY=Now I Am";
       ...
       if (putenv(sillyVar) == -1) { 

    4. Copy a string, but you need to free it later.

       char* sillyVar = strdup("SILLY=Now I Am");
       ...
       if (putenv(sillyVar) == -1) {
       ...
       free(sillyVar); 

• int unsetenv(const char *name);
  • Removes the name if it exists.
• int setenv(const char *name, const char *value, int overwrite);
  • Similar to putenv
  • But it makes a copy of the strings.
  • If the value exists, and overwrite is 0, it is not changed.