In the US, putting the work in a tangible medium causes it to be fully protected. No "Copyright" required. [1]

And, in addition to circle-c, any of these are valid substitutes in the US for the symbol, according to [2] 2204.4(A):

• The letter c with a parenthesis over the top.

• The letter c with a parenthesis under the bottom.

• (c

• c)

• (c)

• The letter c with an unenclosed circle around it.

[1] https://www.copyright.gov/help/faq/faq-general.html#mywork

[2] https://www.copyright.gov/comp3/