When is EVP_MD_CTX_reset() needed?

[Hal Murray <halmurray+openssl@xxxxxxxxx>]

I'm expecting the answer to be "not in your use case" but I'm fishing for 
something I might have overlooked.

The man page says:
       EVP_MD_CTX_reset()
           Resets the digest context ctx.  This can be used to reuse an
           already existing context.

I have a subroutine that does:
  EVP_DigestInit_ex
  EVP_DigestUpdate
  EVP_DigestFinal_ex

That code is reusing a ctx.  The init often gets a new digest.  It works 
without the reset.

What does "reuse" mean?  When would EVP_MD_CTX_reset() be necessary?


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