Environment Names
ANSI 4.10.4.4 The set of environment names and the method for altering the environment list used by the getenv function
The set of environment names is unlimited.
To change environment variables from within a C program, call the _putenv function. To change environment variables from the command line in Windows, use the SET command (for example, SET LIB = D:\ LIBS).
Environment variables set from within a C program exist only as long as their host copy of the operating system command shell is running (CMD.EXE or COMMAND.COM). For example, the line
system( SET LIB = D:\LIBS );
would run a copy of the command shell (CMD.EXE), set the environment variable LIB, and return to the C program, exiting the secondary copy of CMD.EXE. Exiting that copy of CMD.EXE removes the temporary environment variable LIB.
Likewise, changes made by the _putenv function last only until the program ends.