The setenv() function adds the variable name to the environment with the value value, if name does not already exist. If name does exist in the environment, then its value is changed to value if overwrite is non-zero; if overwrite is zero, then the value of name is not changed.
The unsetenv() function deletes the variable name from the environment.
The setenv() function returns zero on success, or -1 if there was insufficient space in the environment.
clearenv(3), getenv(3), putenv(3), environ(5)