<stdbool.h>
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).
<stdint.h>
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 <stdint.h> 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!
// Check numeric.
if (*str < '0' || *str > '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!!!
}
<ctype.h>
There are some functions in <ctype.h> that you should use:
// Check numeric.
if (*str < '0' || *str > '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;
NoteNOTES: 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.
