Hacker News Reader

Comment by bboreham | Hacker News Reader

> 'strcpy' should usually be replaced by 'strncpy'

Sorry to butt in, but this is a bit of a trigger for me: I’ve had to fix a number of programs infected with this idea.

The main problems with strncpy are:

When the source string is shorter than n, strncpy will pad the target to n bytes, filling with zeros. This is bad for performance.

When the source string is longer than n, strncpy will copy n bytes but _not_ nul-terminate the target. So you need extra schenanigans every time you use it to cover this case.

So strncpy is hardly ever a good idea. Sadly there is no standard replacement that is widely accepted. More details at https://en.wikipedia.org/wiki/C_string_handling#Replacements

View on HN

I agree with you completely, and in general think the whole idea of using "safe" string functions with built-in buffer length checking is wrong because it is a solution to a symptom, not a cause.

Before writing to the buffer you should've ensured that it's big enough, and decided what to do if it's not, long before actually doing it. In other words, what happens if it's not big enough? These "always use $length_checking_function" proponents miss that point. Yes, you've avoided an overflow here, but chances are something was already too small long before the flow reached here, and the fix is not to replace an overflow with truncate/not copy/etc. here, but fix the check/sizing that came before elsewhere.

> Before writing to the buffer you should've ensured that it's big enough, and decided what to do if it's not, long before actually doing it. In other words, what happens if it's not big enough?

If you planned all this out, you're still making an assertion as to the length. The contract is "give me a string of this length" and if that's not enforced by the compiler, it ought to be enforced at runtime so that the error is detected and dealt with as soon as possible.

So maybe "safe string functions" should really be "fail fast string functions."

> Sadly there is no standard replacement that is widely accepted.

True, but practically: when having access to BSD extensions, strl* are used. If only C standard is available, snprintf is preferred. I have seen C libs that will check for strl* availability, and if not, reimplement them using snprintf.

So for portability, snprintf is the way to go. For correctness, and pushing for their extended use, strl* is nice.

The least bad options seem to be strl* or snprintf(..., “%s”, ...) but yeah nothing is perfect.

There's strlcpy, but it's not part of POSIX unfortunately.

  #define strlcpy(d, s, n) snprintf(d, n, "%s", s)

Not quite the same (different return type) but close.

Reply on news.ycombinator.com