atoi() is a pretty easy-to-use function, but it's also not very informative. You get one integer back and that's it.
My man page for atoi recommended strtol() and strtod(), which provide greater functionality and error-checking. They're less useful than in libc, because errno doesn't exist, but regardless I thought they would be useful, so I implemented them in bg_lib.c
I'm about to attach a patch for:
long strtol( const char *nptr, const char **endptr, int base );
which skips initial whitespace in nptr and performs a conversion of the following string in the base given, which can be 2 to 36 or the special value 0, which is equivalent to 16 if the string starts with 0x, 8 if the string starts with 0, or 10 otherwise. The conversion will not overflow: it will be limited at LONG_MIN or LONG_MAX. *endptr is set to the character after the last used in the conversion, which will be the value of nptr if no conversion was possible.
double strtod( const char *nptr, const char **endptr );
similar to the above, parses a floating-point number in decimal, or hexadecimal if the string starts with 0x. Special string values like inf and nan(0x7fffff) are supported, along with exponents: in the decimal form, 5.2e8 means 5.2 times ten to the power of eight; in hexadecimal 0x7f8p20 means 0x7f8 times 2 to the power of 20 (oddly, the exponent is decimal, so this is twenty and not 32).

I tested this a little, but due to issues like bug 3718 it's not too easy to stress-test a floating-point parser. I realise the code style is a little bit strange at times, but I've been sitting on this for a while now and I think it needs feedback before I can really improve it further.