Here’s what the C standard (n1256) says:
5.1.2.2.1 Program startup
…
2 If they are declared, the parameters to the main function shall obey the following
constraints:
- The value of argc shall be nonnegative.
- argv[argc] shall be a null pointer.
- If the value of argc is greater than zero, the array members argv[0] through
argv[argc-1] inclusive shall contain pointers to strings, which are given
implementation-defined values by the host environment prior to program startup. The
intent is to supply to the program information determined prior to program startup
from elsewhere in the hosted environment. If the host environment is not capable of
supplying strings with letters in both uppercase and lowercase, the implementation
shall ensure that the strings are received in lowercase.- If the value of argc is greater than zero, the string pointed to by argv[0]
represents the program name; argv[0][0] shall be the null character if the
program name is not available from the host environment. If the value of argc is
greater than one, the strings pointed to by argv[1] through argv[argc-1]
represent the program parameters.- The parameters argc and argv and the strings pointed to by the argv array shall
be modifiable by the program, and retain their last-stored values between program
startup and program termination.
The last bullet is the most interesting wrt where the string values are stored. It doesn’t specify heap or stack, but it does require that the strings be writable and have static extent, which places some limits on where the string contents may be located. As others have said, the exact details will depend on the implementation.