IP4 strings validator
&lt;stdbool.h&gt;

Unless you need compatibility with C89 for some reason, I would use bool for the return type of the function and true and false as the possible values.

It helps readability.



sscanf()

This one depends on your performance needs.  A solution using sscanf() will be much easier to understand with a simple look and also shorter in code, but may also be much slower (a benchmark would be appropriate).

Update: I would discard sscanf() because it would accept spaces before the numbers; better use inet_pton() as @vnp suggested.



&lt;stdint.h&gt;

Same as with bool: segs seems at first glance to be some string, but after some time I realized it's just an array of 8-bit unsigned integers.  You should use uint8_t for that.

In case of &lt;stdint.h&gt; I would say that if you don't have it, you should do the typedefs yourself (enclosed in some #if) anyway.  Don't do that with bool, however, which is very dangerous.



Unneeded cast

unsigned char *p;
int i;

...
*p = (unsigned char)i;


This cast is not needed.

Usually casts are very dangerous: they can hide bugs that otherwise the compiler would catch easily.  Don't ever cast, unless you know a very good reason to.



BUG! (No, I was wrong)

// Check numeric.
if (*str &lt; '0' || *str &gt; '9')
        return 0;


This line returns 0 at the first dot it finds.  The function doesn't work!

This if that comes later is always false due to that:

// Segment changeover.
if (*str == '.') {
        /// Unreachable code!!!
}




&lt;ctype.h&gt;

There are some functions in &lt;ctype.h&gt; that you should use:

// Check numeric.
if (*str &lt; '0' || *str &gt; '9')
        return 0;


That code above can be simplified to this one (and the comment is now even more obvious and therefore removed ;-):

if (!isdigit((unsigned char)*str))
        return 0;



  NOTES: The standards require that the argument c for these functions is either EOF or a value that is representable in the type unsigned char.  If the argument c is of type char, it must be cast to unsigned char.
  
  This is necessary because char may be the equivalent of signed char, in which case a byte where the top bit is set would be sign extended when converting to int, yielding a value that is outside the range of unsigned char.


See man isdigit.

If you're going to use these functions, I would create an unsigned char pointer to alias the char * parameter, so that you don't need to cast all the time.