const char *atoul(const char *src, size_t srclen,
int base, unsigned long *n);
size_t ultoa(unsigned long n, int base, char *dst,
size_t dstlen);
Numbers are specified in ASCII as decimal (e.g. 123), octal with a leading zero (e.g. 012, which has value 10), or hexadecimal with a leading 0x (e.g. 0x1f, which has value 31) in either upper or lower case.
The srclen parameter of atoul specifies the length of the ASCII string pointed to by src; it is an error for there to be anything else (e.g., a terminating NUL) within that length. As a convenience for cases where an entire NUL-terminated string is to be converted, a srclen value of 0 is taken to mean strlen(src).
The base parameter of atoul can be 8, 10, or 16, in which case the number supplied is assumed to be of that form (and in the case of 16, to lack any 0x prefix). It can also be 0, in which case the number is examined for a leading zero or a leading 0x to determine its base, or 13 (halfway between 10 and 16), which has the same effect as 0 except that a non-hexadecimal number is considered decimal regardless of any leading zero.
The dstlen parameter of ultoa specifies the size of the dst parameter; under no circumstances are more than dstlen bytes written to dst. A result which will not fit is truncated. Dstlen can be zero, in which case dst need not be valid and no result is written, but the return value is unaffected; in all other cases, the (possibly truncated) result is NUL-terminated.
The base parameter of ultoa must be 8, 10, or 16.
Atoul returns NULL for success and a pointer to a string-literal error message for failure; see DIAGNOSTICS. Ultoa returns the size of buffer which would be needed to accommodate the full conversion result, including terminating NUL; it is the caller's responsibility to check this against the size of the provided buffer to determine whether truncation has occurred.
The restriction of error reports to literal strings (so that callers don't need to worry about freeing them or copying them) does limit the precision of error reporting.
The error-reporting convention lends itself to slightly obscure code, because many readers will not think of NULL as signifying success. A good way to make it clearer is to write something like:
const char *error;
error = atoul( /* ... */ );
if (error != NULL) {
/* something went wrong */