C doesn't even originally have true/false, I think you may be conflating the two concepts that "any nonzero int is truthy" and "boolean expressions evaluate to ints". The standard mandates that boolean expressions like equality always evaluate to 0/1.

The `true` constant is always 1. C11 §7.18 (3):

> true which expands to the integer constant 1,

And equality yields a 1 or 0. C11 §6.5.9 (3):

> The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence. Each of the operators yields 1 if the specified relation is true and 0 if it is false.

That is entirely unneccessary. An == expression will always evaluate to 1 or 0. The !!x trick can be useful in some other situations, though.

Here’s a thing you could do (but I don’t know why):

+= !(c-’s’) - !(c-’p’)

I'm sure people do that (even though it's not necessary per some year C standard) but generally the pattern is actually for converting things which are not already 0 or 1 into 0 or 1. For example, you might want to use it here:

    int num_empty_strings = !!(strlen(s1)) + !!(strlen(s2)) + !!(strlen(s3))

which is equivalent to:

    int num_empty_strings = (strlen(s1) != 0) + (strlen(s2) != 0) + (strlen(s3) != 0)

Which you use is really a matter of coding style.