Comment by attractivechaos | Hacker News Reader

Comment by attractivechaos | Hacker News Reader

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attractivechaos

This gets me interested. Link [1] below shows their implementation of strlcpy(). This is a questionable implementation. With strncpy, the source string "src" may not be NULL terminated IIRC. The git implementation requires "src" to be NULL terminated. If not, an invalid read. EDIT: according to the strlcpy manpage [2], "src" is required to be NULL terminated, so strlcpy imposes more restrictions and is not a proper replacement of strncpy.

Furthermore, imagine "src" has 1Mb characters but we only want to copy the first 3 chars. The git implementation would traverse the entire 1Mb to find the length first, but a proper implementation only needs to look at the first 3 chars. So, they banned strncpy and provided a worse solution to that.

[1]: https://github.com/git/git/blob/master/compat/strlcpy.c

[2]: https://linux.die.net/man/3/strlcpy

You have found the answer - strlcpy is not a replacement for strncpy at all (it's arguably a safer version of strcpy), and git people didn't invent this, it's the existing BSD strlcpy interface.

Thanks for the confirmation. But my concern remains: they banned strncpy without a proper replacement. In addition, I didn't know the extra restriction of strlcpy until today (I have never used it before because it is not conforming to C99/POSIX). I might have fallen into this trap.

The problem is the actually often the opposite, in the real world many treat strncpy as if it behaves like strlcpy. Note that strlcpy is equivalent to:

    snprintf(buf, sizeof(buf), "%s", string);

strlcpy is on track for future standardization in POSIX, for Issue 8, but even as a de facto standard, it exists in libc on *BSD, macOS, Android, Solaris, QNX, and even Linux using musl.

https://www.austingroupbugs.net/view.php?id=986#c5050

But you're correct in that it is not a replacement for strncpy because no code should be using strncpy.

Agreed. It's O(n) inefficient. I guess looping though chars up to `size` would perform better on average.

I see this `strlcpy` recommanded everywhere.

A few days ago people here were discussing the quadratic sscanf() behavior. strlcpy() has the same problem.

Take a step back and consider strlcpy isn't supposed to be a drop in replacement for strncpy (a function which already exists).

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