space allocation for sprintf()

[Eric R Vook]

Harbison & Steele p.328 contains the following parenthetical comment
after a discussion of sprintf():

"(It is the programmer's responsibility to ensure that the destination
  string area is large enough to contain the output generated by the
  formatting operation.)"

	      ["C: A Reference Manual," 2nd ed, Harbison & Steele, p.328]

I have just a short question, two-part question:
    How do you know how much space to allocate?
Or
    How do you know when you didn't allocate enough?

My application has a varargs function which needs to eventually get to a
single string to pass to routines which do NOT take a varargs interface.

-Eric

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Eric R Vook                  |  Face reality as it is,
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